Sedimentary Geology 157 (2003) 253 – 276 www.elsevier.com/locate/sedgeo Vegetation-controlled modern anastomosing system of the upper Narew River (NE Poland) and its sediments Ryszard Gradziński a,*, Janusz Bary»a a, Marek Doktor a, Dariusz Gmur a, Micha» Gradziński b, Artur Ke˛dzior a, Mariusz Paszkowski a, Roman Soja c, Tomasz Zieliński d, S»awomir Żurek e a Institute of Geological Sciences (Cracow Research Centre), Polish Academy of Sciences, Senacka 1, 31-002 Cracow, Poland b Institute of Geological Sciences, Jagiellonian University, Oleandry 2a, 30-063 Cracow, Poland c Institute of Geography and Spatial Organization, Polish Academy of Sciences, św. Jana 22, 00-018 Cracow, Poland d Faculty of Earth Sciences, University of Silesia, Be˛dzińska 60, 41-200 Sosnowiec, Poland e Geographical Institute, Pedagogic University, Konopnickiej 15, 25-406 Kielce, Poland Received 5 September 2001; accepted 16 May 2002 Abstract The anastomosing system of the upper Narew River consists of a network of interconnected channels. The channels are relatively deep (width/depth ratio 2 – 10), straight to sinuous, and they lack natural levees. They are characterised by a low water slope and very low stream power. The river is distinctly bedload-dominated and the transport of suspended clastic fines is minimal. Channel deposits consist almost exclusively of medium- to coarse-grained sand. Laterally extensive interchannel areas are flat and covered with peat-forming vegetation. These stable wetlands are flooded for many weeks during high water stages. Except for the channels, the valley fill consists of peat layer reaching 4 m in thickness. The rate of vertical aggradation of the peat deposit is estimated at 1 – 1.5 mm/year. The radiocarbon dating indicates that the peat layer is predominantly late Holocene in age. The impact of vegetation on the system is overwhelming. Vegetation produces an erosion-resistant peat layer, stabilizes channel banks and slows down the water flow. Vegetation also stimulates aggradation of bedload material on the channel bottom, and contributes to avulsion by blocking the channels. The channel network owes its origin to repeated though infrequent avulsion. Avulsion in the studied system is a small-scale, gradational and slow process. New channels evolve very slowly because of unfavourable hydrologic conditions and the presence of a resistant peat substratum. A new channel eventually intercepts only a part of the flow, while the old channel is still active, though to a limited extent. Although newly formed channels might subsequently be abandoned, long-lasting ones predominate within the system. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Anastomosing river; Vegetation; Avulsion; Poland 1. Introduction * Corresponding author. Fax: +48-12-4221609. E-mail address: ndgradzi@cyf-kr.edu.pl (R. Gradziński). Anastomosing rivers, which are characterised by a network of interconnected low-gradient channels, were described more than half a century ago in Venezuela (Crist, 1932; Garner, 1966), central Africa 0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0037-0738(02)00236-1 254 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 and northern China (Garner, 1967) and central Australia (Whitehouse, 1944; fide Nadon, 1994; Garner, 1967). Later, anastomosing rivers were recognised as a separate group (see, e.g. Miall, 1977, 1996; Makaske, 1998, 2001) and distinguished from the three earlier recognized river types (Leopold and Woolman, 1957): braided, meandering and straight. Schumm (1968) was probably the first to distinguish anastomosing rivers as a separate type within the group of multichannel fluvial systems. According to him, besides a specific planform geometry, anastomosing rivers are characterised by suspended load channels of low gradient and low width/depth ratio, as well as by very slow aggradation rate and high sinuosity (Schumm, 1981). Smith and Smith (1980) and Smith (1983, 1986) developed the anastomosing river facies model based on investigations of several reaches in Canadian and Colombian rivers. The anastomosing river system, according to these authors, is characterised by lateral stability of channels, which is due to both—very low channel gradients and cohesive bank sediments; vertical aggradation rather than lateral accretion is the dominant sedimentation pattern of the anastomosing river system. The anastomosing river facies model presented by Smith and Smith (1980) was promptly used in environmental interpretations of ancient fluvial successions, especially the coal-bearing ones (e.g., Cairncross, 1980; Rust and Legun, 1983; Rust et al., 1984; Kirschbaum and McCabe, 1992). Knighton and Nanson (1993, p. 615) state that ‘‘an anastomosing river consists of multiple channels separated by islands which are usually excised from continuous floodplain and which are larger relative to the size of the channels’’. They state also that anastomosing rivers represent a separate type within the group of anabranching rivers (Nanson and Knighton, 1996). Nadon (1994) in his critical review of publications dealing with the anastomosing rivers emphasises the role of natural levees. His definition of a modern anastomosed reach reads: ‘‘a suspended-load system composed of multiple, interconnected, laterally stable, deep, sand-bed channels, confined by prominent levees, and separated by interchannel topographic lows’’ (Nadon, 1994, pp. 453– 454). A somewhat different definition has been given by Makaske (1998, p. 56): ‘‘a river [. . .] composed of several interconnected channels, which enclose floodbasins’’. This author also stressed that the floodbasins are saucer-like in shape. Publications on modern anastomosing systems are rather rare. Knighton and Nanson (1993) and Makaske (1998, 2001) in their reviews noted briefly the role of riparian vegetation in anastomosing river systems from various climatic zones. The most comprehensive studies on the impact of vegetation came from the Okavango anastomosing system of Botswana (McCarthy et al., 1986, 1992; Ellery et al., 1990b, 1993; Stanistreet et al., 1993). The purpose of this paper is to present characteristic features of the upper Narew River anastomosing system and the control exerted by vegetation on sediment features and fluvial processes. 2. Location and background The Narew River is a tributary of the Wis»a River and drains the NE Polish Lowland (Fig. 1, inset). An Fig. 1. Upper Narew River between Suraż and Tykocin. The channel network in the section downstream from Rze˛dziany is shown as it looked like before the drainage works. R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 Fig. 2. Geological map of the middle section of the upper Narew River valley (after Bałuk, 1973, simplified). (A) Area of the detailed study; (B) area of reconnaissance studies. 255 anastomosing segment of the river is preserved in its almost pristine form between Suraż and Rze˛dziany (Fig. 1). This segment of the river is about 35 km long and lies within the Narew National Park. The middle part of this area was the subject of detailed field work by the authors (Fig. 2). Until recently, the river was anastomosing for yet another 35 km downstream, but that part of the valley was strongly altered by drainage works some 20– 30 years ago; hence, this area is not taken under consideration here. The section of the valley situated within the Narew National Park is referred to as the Narew anastomosing system (NAS). The NAS developed within a flatbottom valley in an area where Pleistocene sediments, mostly glacial till and glacio-fluvial sands, are up to 200 m thick (Lindner and Astapowa, 2000). The river valley is occupied by wetlands and is bounded by low hills made up largely of Warthanian till (Fig. 2). The width of the valley varies markedly. Its narrow stretches are about 1 km in width, and the wide ones, referred to as ‘‘basins’’, are 2 – 4 km wide. The average longitudinal gradient of the valley bottom between Suraż and Rze˛dziany is ca. 0.00022. The gradients in transverse sections are minimal. The regional subsidence rate of the area is estimated at 2 mm/year (Wyrzykowski, 1985). The modern valley is developed along a depression in the top surface of glacial till (Fig. 3). The origin of this depression is not clear. It has been attributed to the presence of large blocks of dead ice in locations of the present-day ‘‘basins’’ (Falkowski, 1970). Old borehole data indicate that the depression is filled with the so-called basal sand series, 15 – 25 m thick, considered to represent older fluvial systems (Churski, 1973; Banaszuk, 1996). The exact age of the basal sand series is not known. It might be either late Pleistocene or early Holocene in age. Fig. 3. Schematic cross-section through the Narew River valley near Kurowo. No horizontal scale, valley is ca. 2.5 km wide. Note that the channels are filled with sand to various levels. 256 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 The basal sand series is overlain by a layer of peat and peat-like deposits, a few metres thick, largely of late Holocene age (Okruszko and Oświt, 1973; Banaszuk, 1996; Gradziński et al., 2000). This layer is referred to herein as the peat layer. The distribution and thickness of the peat layer are well documented by several hundred shallow boreholes drilled in the 1950s and 1960s for a drainage programme (Churski, 1973; Banaszuk, 1996). The area of the upper Narew River has a temperate – humid climate. The number of days with maximum temperature below 0 jC is usually between 50 and 60 per year. The mean annual rainfall is about 560 mm. The period of maximum precipitation is between June and August. The winter ice cover within the NAS is ca. 30 cm thick and usually lasts from the beginning of December until mid-March. As a rule, the NAS undergoes one high water stage in the spring, due to snow melting, and one low water stage from July to October. The mean annual discharge at the Suraż gauging station is 13.3 m3/s, while the maximum recorded discharge was 250 m3/s. The surface of the interchannel areas of the NAS is only slightly elevated above the mean water level. Therefore, the normal flow may be considered almost equal to the bankfull discharge. The interchannel areas are often flooded for a few weeks or more when the water level rises. The drainage basin of the Narew River has one of the lowest denudation rates in Poland (Branski and Banasik, 1996). The upper Narew River is distinctly bedload in character. The suspended clastic load is negligible. The river carries almost exclusively sand derived from glacio-fluvial deposits. The amount of sand feeding the system is relatively small. 3. Scope and methods Sedimentological aspects of the NAS and its vegetation have been the major subject of the study. The vegetation was studied by J. BarylAa, peat and peat-like deposits by S. Żurek and hydrology by R. Soja. Field work was carried out in June and September of 1998 and 1999, and in September of 2000. Inflatable boats were the main means of transport, enabling collection of core and box samples of bottom sediments. Underwater investigation was supported by a scuba diver and by a specially adapted TVequipment. Transverse sections of channels were usually surveyed using traditional methods. The longitudinal ones were surveyed utilising an electronic acoustic bottom profiler. Short-distance differences in ground elevation and in water surface level were measured with a custommade instrument of our design, which was accurate up to 0.5 cm over a distance about 35 m. The land elevation measurements were less accurate because of the presence of abundant plant debris on the ground surface. Airborne observations and photographs were made using a helicopter, flying at altitudes of 50 –200 m. About 160 cores up to 5 m long were taken from channel bottoms and from interchannel areas, using a self-designed Plexiglas-tube corer. Another 45 boreholes, up to 6 m deep, were drilled in interchannel areas using an Eikelkamp hand-auger set with peat sampler. Additional samples of sediments with undisturbed structure were taken using wedge-shaped samplers (cf. Chudzikiewicz et al., 1979). These samples have been subsequently impregnated with epoxy resin (see Fig. 14). Water level observations in the NAS came from the Kurowo watergauge. The water level referred to in this paper as normal corresponds approximately to the average level, which is close to the bankfull flow. Water level described as low is ca. 30 –40 cm lower than the normal. Finally, water level called here extremely low is ca. 75 cm lower than the normal. Current and archival maps have been used to study the changes in the channel pattern of the NAS. The oldest, 1:84,000 map (based on a survey made in 1886) has been compared with more recent and more detailed 1:100,000 and 1:25,000 maps. Much more useful for the comparisons appeared to be series of successive aerial photographs at a scale ca. 1:25,000 from 1966, 1980 and 1989 (all black and white), and at a scale ca. 1:10,000 from 1997 (infrared). Forty samples of peat and peat-like deposits were dated by radiocarbon method by the Institute of Geological Sciences of the Academy of Sciences of Belarus in Minsk. 4. Physiography The NAS consists of a channel network and wetland-type interchannel areas. The latter include not R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 only islands but also other flat parts of the valley bottom adjacent to the valley margins. 4.1. Network of channels The network of interconnected channels is rather irregular (Figs. 4 – 6). The distances between the nodes—i.e. the places of channel branching and the places of channels rejoining—vary from a few tens to several hundred metres along the valley. The angles between branching channels vary within a wide range, locally exceeding 90j. Individual channels vary also in size. The larger ones, which show higher discharges than the others, are considered the main channels (Fig. 7A). Some of the main channels lose most of their water to the smaller ones and they may gradually lose their rank. In contrast, some of the main channels form by convergence of the smaller ones. As a result, the anastomosing pattern of branching and rejoining channels consists of a different number of channels at different cross-sections of the valley. The main channels run approximately parallel to the valley margins. Most channels show relatively low sinuosity (below 1.3) and these may be classified as ‘‘straight’’. Such ‘‘straight’’ channels sporadically exhibit tight bends. High-sinuosity channels (up to 1.7), which consist of regular bends showing a meandering pattern, are subordinate (cf. Figs. 4 and 6). These meanders are inactive, which is indicated by the lack of both erosion on their concave banks and discernible ridge-andswale topography on the convex banks. Such meandering channels are locally accompanied by oxbows. The vast majority of the channels are active, even when the water level is low. Some of them, however, are in various phases of abandonment and are heavily overgrown with water plants (Fig. 7B). Fragments of abandoned channels of various length and shape are locally preserved. They are partly or completely isolated from the active channels. Some parts of the channel network are transformed into shallow lakes, amoeboid in shape (see Fig. 4). The slope of water surface in individual channels is generally low, usually from 0.0002 to 0.00012. Locally, over distances of up to few tens of metres, the slope of water surface attains 0.0014 (see Section 9.1), while in the moribund channels the slope is much lower than average. 257 At the normal water stage, the maximum velocity of flow in the main active channels is rather low and usually varies between 10 and 35 cm/s. The stream power values in these channels are very low as well, ranging between 2 and 3 W/m2. Exceptionally high velocities, up to 70 cm/s, were measured only over a few short reaches (see Section 9.1.). Active channels are usually 5 – 35 m wide and relatively deep. Large channels are typically 3 –4 m deep (locally up to 7 m). The smaller ones are usually 1.5 – 3 m deep. Moribund channels, with low discharge, are usually shallower. The width/depth ratio of the active channels usually falls within the range between 2 and 10. Some channels display considerable differences in depth along their thalwegs, even over short distances. This is particularly true in relation to the main channels. Active channels are usually canal-like in cross-section, i.e. they have steep sides and flat bottoms. Such outlines have been observed along both straight and bent reaches of many channels. Outlines of cross-sections of some bent reaches are distinctly asymmetric with either the convex or concave bank sloping more gently (see Fig. 15A). Only a few fragments of the channel network are artificial. These include shortcuts through the meander necks of main channels probably made with the aim of facilitating local navigation or wood rafting (Fig. 4). 4.2. In-channel accretionary macroforms Several types of modern accretionary macroforms can be distinguished within the NAS channels. These macroforms differ largely in shape and in their position within the channels. The following types have been distinguished (Fig. 8): (1) mid-channel bars, (2) linguoid bars, (3) side bars, (4) plug bars, (5) concavebank bars and (6) point bars). Only the first two types are conspicuous during normal water level because they are overgrown with relatively high semi-aquatic plants and they have rather distinctive shapes. The other forms are not distinguishable during normal water level because they are covered by aquatic and semi-aquatic plants. Their presence is detectable only by sounding, diver observations and drilling. The uppermost portions of these bars emerge during extremely low water stages. 258 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 Fig. 4. The Narew River valley between Waniewo and Kruszewo. Interchannel areas are white, elevations bordering the valley are grey. Lettered points (A – Z) correspond to selected boreholes (see Figs. 12 and 13). RI—Remiz Island. R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 259 Fig. 5. A fragment of valley near Waniewo. Upflow view from the north. Width of lower foreground is approximately 200 m. Only then could their morphology and sedimentary features be studied in some detail. Mid-channel bars are narrow, downstream-elongated ridges approximately parallel to the channel margins. The size of these forms varies. Some of them reach 150 m in length. Mid-channel bars are often segmented by secondary elevations and depressions which are clearly visible in longitudinal sections. Such Fig. 6. A fragment of highly sinuous channel near Kurowo composed of inactive meanders. Straight channel is visible in the upper left corner. PB—point bar. RI—Remiz Island; arrow shows the only place where a concave bank is subject of erosion. Downflow view. Width of lower foreground is approximately 150 m. 260 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 Fig. 7. (A) Upflow view of a main active reed-lined channel. Reeds are ca. 3 m high. (B) Inactive channel completely overgrown with water soldier (Straitres aloides); the 3 m high reed marks the original channel margins. (C) Grill-like margin devoid of floating plant mat (cf. Fig. 10B). Width of lower foreground is approximately 3 m. (D) Downstream part of the point bar opposite the Remiz Island (cf. Fig. 6), emerged at extreme low water stage. Upstream part of the bar is colonised by bur reed (Sparganium erectum). Flow to right. forms resemble a chain of vegetated isles separated by shoals (see Fig. 18). Less common are mid-channel bars forming single isles. The linguoid bars extend obliquely from the channel margins and resemble peninsulas (see Figs. 18 and 19). Individual bars may differ slightly in shape and may attain many tens of metres in length. The tops of the emerged parts of the linguoid bars rise approximately to the height of the adjacent interchannel areas. The bars extend downstream in the form of submerged sandy ridges of gradually decreasing height (see Fig. 15B, inset map). The growth of linguoid bars is initiated by protrusions of heavily vegetated channel banks in the places where plants prograde towards the channel axis. The linguoid bars occur within slow-current channels, usually near gently convex margins, less frequently within the straight reaches of channels. The linguoid bars seem to grow quite slowly, over decades or more. The slow Fig. 8. Scheme showing various types of in-channel accretionary macroforms. R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 growth of linguoid bars is suggested by a thin layer of peat present on their tops and by the fact that most of the currently existing linguoid bars were already visible on air photographs taken in 1966 (see Fig. 22A). The midchannel and linguoid bars are quite common within the NAS and many of them are clearly visible on oblique air photographs (see Fig. 18). All the mid-channel and linguoid bars are densely vegetated with plants rooted in soft substrate (see Section 5). The bars resemble the vegetation-shadow bars described by Gibling et al. (1998) from anastomosing systems of central Australia and vegetated ridges described by Wende and Nanson (1998) from anabranching rivers of tropical northern Australia. Side bars occur within small embayments in otherwise straight, grill-like channel margins (see Section 5). The bars are up to a few metres wide and up to 30 m long and they straighten the contour of the channel bank. Plug bars are similar to the side bars but they are located at the outlets of the abandoned channels to the main channel. Concave-bank bars occur on bends near the outer banks. They are slightly crescent-shaped in plan view. Their platforms are slightly convex up, several metres wide and separated from the bank by a shallow depression. The concave-bank bars usually do not reach the elevation of the adjacent interchannel areas (see Fig. 15A). These forms resemble concave benches described by Nanson and Page (1983). Point bars are rare within the NAS. Only one emergent point bar has been observed. It is situated 261 on the convex bank of the channel, opposite to the Remiz Island (Fig. 6). The bar is slightly convex, ca. 40 m long, and up to 10 m wide. It is elongated parallel to the bank, rises to the level of the interchannel area and terminates downstream with a rounded tip (Fig. 7D). The bar grows, both upwards ad downstream. The channel bank opposite to the point bar is one of few places where the peat layer is being eroded (Fig. 6). It should be emphasised, however, that the formation of the in-channel accretionary macroforms of NAS is, as a rule, not accompanied by erosion of the opposite bank of the channel. Emergent parts of the side bars, plug bars, concave-bank bars and the point bars are being rapidly colonised by plants rooted in the sandy substrate. The bur reed (Sparganium erectum) dominates. The only parts of the bars which are not yet vegetated are those accreted during the most recent flood event. This conclusion is substantiated by measurements conducted over a 2-year period on the point bar near the Remiz Island. The distal part of the bar has accreted vertically at least 60 cm and extended laterally downstream by ca. 20 m. The development of in-channel bars, rapidly stabilised by plants, is one of the processes which cause the narrowing of channels within the studied system. 4.3. Interchannel areas The interchannel areas are flat (cf. Figs. 3 and 9) and densely vegetated predominantly with non-arborescent, peat-forming plants. The vegetation is well Fig. 9. Transverse cross-section through a part of the Narew River valley. Note flatness of interchannel areas. For location, see Fig. 4. 262 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 rooted in relatively firm but wet, peaty ground. The surface of the interchannel areas is, in some places, so heavily waterlogged that it is not firm enough for walking. Such places, which we call quagmires, are best described as a mosaic of small pools of water and clumps of semi-aquatic and aquatic plants. The quagmires occur along the shores of some lakes and along moribund or abandoned channels. They constitute only a small fraction of the interchannel areas of the NAS. Natural levees or crevasse splays have not been found in the area of our exploration that is as far as 250 m off the channels. 5. Vegetation The interchannel areas are covered with a dynamic plant community which invades into the channels. The plant growth and peat accumulation is probably the most important depositional process within the NAS. Common reed (Phragmites australis) dominates the plant community of the interchannel areas. Sedges Carex elata and Carex acuta ( = Carex gracilis), canary grass (Phalaris arundinacea) and floating sweet grass (mostly Glyceria fluitans) occupy much smaller areas. Small patches of osier community (Salicetum pentadro-cinereae) and single arborescent willows occur locally. Alder carr (Ribo nigri-Alnetum) is sporadic. The quagmires are covered by reed mace (Typha angustifolia), great reed mace (Typha latifolia), water plantain (Alisma platago aquatica), bur reed (Sparganium erectum), yellow-cress (Rorippa amphibia), cowbane (Cicuta virosa), marsh woundwort (Stachys palustris) and mint (Mentha sp.). Similar plants grow on mid-channel and linguoid bars. Shallow lakes are inhabited by numerous angiosperm species. These plants are rooted in the soft bottom or float at water surface. Mass blooms of green algae, diatoms and blue-green algae take place during summer time and provide material for organic slurry at the lake bottom. The degree of colonisation of the channels by vegetation depends on the water depth and current velocity. Active channels deeper than 2 m are free of vegetation but shallower stretches are promptly colonised. The first species to appear is arrowhead (Saggittaria sagittifolia) followed by yellow water lily (Nuphar lutea f. submersa). Sluggish-water and inactive channels are rather densely covered with numerous species of aquatic plants. Especially abundant is water soldier (Stratiotes aloides), accompanied by frog’s-bit (Hydrocharis morsus-ranae) and duckweed (Spirodela polyrhiza). Banks of the active channels are largely vegetated with common reed (P. australis). Such banks are often very conspicuous and are referred to as the grill-like margins (Gradziński et al., 2000). The grill-like mar- Fig. 10. Scheme of various types of banks overgrown with reed. (A) Reed stems not submerged. (B) Grill-like margin. (C) Grill-like margin with floating plant mat. Not drawn to scale, reed is approximately 3 m high. R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 gins are vegetated by reed whose stems grow from the underwater channel bank (Figs. 7A,C and 10B and C). The vegetated banks with water flowing between the stems, may be several metres wide. In-channel sand bars are colonised by vegetation within a few months after emersion. In this case the pioneer plant is usually the bur reed (S. erectum). Another form of colonisation is due to plant mats. Most mats consist of great yellow-cress (R. amphibia), narrow-leaved water parsnip (Berula erecta) and cowbane (C. virosa). The plants forming a mat are not rooted and may float freely with the current. They may be laterally attached to plants rooted in the bank or in the grill-like margin of a channel (Fig. 10C). Spring flood events often facilitate mat detachment. The mats initially float away as ‘‘floating islands’’ and subsequently may form plant jams (Fig. 11B). Fig. 11. Example of small channel, ca. 8 – 10 m wide. (A) Without plant jam (June 1997). (B) With plant jam (June 1998). For location, see Fig. 4. 263 6. Sediments There are two main, lithologically distinct groups among the modern sediments of the NAS: (1) sandy sediments laid down in the channels, and (2) peaty sediments of the interchannel areas (Fig. 12). Both types of sediments overlie the basal sand series that covers the previous valley bottom (Figs. 3 and 9). It should be emphasised that the sediments of the basal sand series are in many cases very similar to the modern channel sediments, and it is often difficult to establish the boundary between them. 6.1. Channel deposits Channel deposits consist predominantly of sand, mostly medium- and coarse-grained. Locally, coarse sand contains granule-sized particles of quartz and crystalline rocks. Rip-up clasts of peat, varying in size (up to several decimetres in diameter) are observed sporadically. Clastic fines are subordinate. Usually, they occur together with plant detritus forming single laminae within sand, which enhances stratification. Less common are decimetre-thick layers composed of mud and coarse plant debris. Sporadically, thin mud layers rich in organic material alternate with fine-sand layers, forming together heterolithic sediment. A specific type of modern sediment is a dark slurry composed of organic ooze, which occasionally forms ‘‘layers’’ up to several decimetres thick. Such layers occur often at the bottom of sluggish or abandoned channels and lakes. The bottom sediments of active channels are formed mainly of coarse and medium sand, largescale cross-stratified, crudely horizontally laminated or massive (Figs. 13D and 14). The lower portions of the bars are built mainly of coarse and medium sand, usually showing large-scale cross-stratification with laminae commonly inclined downstream. The upper portions comprise mainly medium and fine sand with common ripple crosslamination, in places of B-type of climbing-ripple structures (cf. Allen, 1970). Semi-horizontal laminations and large-scale cross-stratification are subordinate. Intercalations, up to 20 cm thick, of sediments rich in plant debris occur locally. Sedimentary structures observed in older parts of the bar platforms are 264 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 Fig. 12. Simplified logs of selected boreholes from interchannel areas (A – J), lake (K), abandoned channels (L – M) and active channels (N – T). For location, see Fig. 4. strongly obliterated by penetration of plant rhizomes and roots. The whole sequence of the bars is generally fining-upwards and may be interpreted as formed at successive stages of bar evolution. The uppermost portion of the mid-channel and linguoid bars is composed usually of peat or peat-like deposit, underlain with heterolith (Fig. 15B). Numerous channels of the NAS are filled with a several metre-thick sandy deposit (Figs. 3, 9 and 14). In the moribund channels, the top of such sandy fill lies often 0.5 – 1.5 m below the normal water-stage level. 6.2. Interchannel deposits The peat layer consisting of peat and peat-like deposit covers almost the entire interchannel areas. The peat usually contains a significant admixture of dispersed mineral material, largely sand. The admixture content varies between 9% and 50% of dry mass. The sediment that mesoscopically resembles peat and contains more than 50% of mineral material (up to ca. 80%), is referred to as the peat-like deposit. Several types of peat have been found. They can be distinguished by the dominant peat-forming plants. Most common are sedge peat and reed peat, less abundant are sedge-reed peat and osier peat (Fig. 13). No regularity has been found in the vertical succession of the various types of peat and peat-like deposit. In some cores, the peat layer contains subordinate thin intercalations of sand or mud (Fig. 14A). The peat layer is usually 1 –2 m thick, only exceptionally it exceeds 4 m (Fig. 12, logs A –J). Its basal R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 265 Fig. 13. Examples of detailed borehole logs from interchannel areas (G, U and W), and the main-channel thalweg (Z). For location, see Fig. 4. surface is usually uneven and shows locally significant differences in depth over relatively short distances (cf. Figs. 9 and 15A). Few cores that were obtained from the quagmires suggest that the quagmires are usually underlain by semi-firm organic-rich mud. This, in turn, is in most places underlain by firmer peat-like deposit, peat or heterolithic sediment. 6.3. Deposits of the basal sand series Logs of a few boreholes (20 –30 m deep) given in Churski (1973) and Banaszuk (1996), indicate that the basal sand series filling the Narew River valley consists predominantly of medium and coarse sand. Gravel admixture is common in the lower part of the series. According to Churski (1973) and Banaszuk 266 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 Fig. 14. Resin-hardened samples showing various types of channel sediments. Dark laminae and layers are rich in plant detritus. Scale in cm. (A) Large-scale cross-stratification from the bottom of active channel. (B) Thalweg deposits with peat clasts in the upper part. (C) Small-scale cross-lamination from a plug bar. (D) Thalweg deposit. (E) Set of large-scale cross-strata with backflow-ripple structures in the lower part; small-scale cross-stratification at the bottom; sample from an accretionary bank within a main channel. R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 (1996), a several decimetre-thick mud layer can be found in some places at the top of the basal sand series. The lithology of the basal sand series suggests a general fining-upward trend of this unit. The basal sand series has been studied by the present authors in boreholes reaching down to 3 m below the base of the peat layer. The uppermost part of the series comprises mainly sands, generally similar to those observed in the modern channels. Only in some boreholes occur subordinate thin intercalations of mud, heterolith, peat, peat-like deposit or gyttja (Fig. 12, logs E – G). The most characteristic feature of the upper part of the sand series is its large lateral variability. 7. Sediment age and sedimentation rates The basal portions of the peat layer from the analysed sections of the NAS have been dated by the radiocarbon method. Most of the obtained results fall between 3200 and 1340 BP (cf. Fig. 12). Only a few dates are much older: 9727 F 294, 7080 F 80, 6858 F 180, 4800 F 100 BP, and some are slightly younger. All the dates indicate that the accumulation of the peat layer began at different times in various parts of the valley, but mainly in the late Subboreal and in the Subatlantic stages. It should be noted, however, that some radiocarbon dates may be slightly too young because the dated samples, though carefully prepared, could be contaminated by small fragments of younger rhizomes and roots that penetrated deep into the underlying older sediments. Long-term sedimentation rates for the peat layer range from 0.16 to 2.37 mm/year, though most are bracketed within 1 –1.5 mm/year. These differences seem attributable to varying proportions of peat matter to clastic sediment, various rates of peat accumulation and variable compaction rates of different peat types. 267 slow down the water current. As a result, lateral erosion is diminished or almost halted (cf. Ellery et al., 1995). Similarly protective, though to a lesser degree, are plants densely overgrowing other types of channel banks (cf. Hickin, 1984; Harwood and Brown, 1993). Moreover, steep channel banks are stabilised by peat layer. Such peat banks are reinforced by a mat of intergrown rhizomes and roots (Fig. 16), and they are usually very steep or nearly vertical and extremely resistant to erosion (cf. Smith, 1976). Similar peat banks have been described in the Okawango anastomosing system (Stanistreet et al., 1993; Ellery et al., 1995). On the other hand, local erosion of the peat banks is indicated by the presence of block-sized and smaller clasts of peat on channel bottoms and within channel deposits (Fig. 14B). Such erosion, however, have been proven to occur only exceptionally within the NAS in few several short sections of the banks. These sections are situated at the outer banks where the grill-like margins are poorly developed or absent. Submerged parts of such banks show erosional niches and peat blocks partly detached and leaning out from their original position, as well as peat blocks lying nearby on the channel slope and bottom (Fig. 17). The peat blocks may be transported along the bottom for at least several tens of metres and smaller fragments even farther. It is supposed that the erosional retreat of a bank may be compensated by the encroachment of reeds whose rhizomes grow horizontally. Vertical incision of the existing channels and the formation of new ones within the resistant peat substratum (see Section 9.1 and Fig. 21) is also very slow. It can be concluded that the protective nature of riparian vegetation, resistivity of the peat layer and low stream power are all responsible for the negligible rate of lateral erosion of the Narew River channels. Extremely low lateral erosion of the river banks seems to be the main cause of the high lateral stability of the internode reaches of the river, which is suggested by a comparison of the successive series of air photographs. 8. Erosion 9. Avulsion The most characteristic feature of the NAS is an almost complete lack of lateral erosion of the channel banks. Active channels often have grill-like margins covered with a belt of dense reed, which substantially The study of the successive series of air photographs taken over the last 36 years and of older topographic maps does not reveal any newly formed 268 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 Fig. 16. Lower part of vertical channel-bank built of peat. Photograph taken during extremely low water stage (September 2000). Width of lower foreground is approximately 1.2 m. long channel reaches on the interchannel areas. Nonetheless, some other observations shed some light on the processes leading to the development of the channel network of the NAS. 9.1. Description Most of the observations have been obtained at the study site referred to as the Zielona Budka area (Figs. 18 and 19). That area embraces a part of the NAS with an especially dense network of channels which are now in various stages of their evolution. The river flows through the centre of the Zielona Budka area, from the south to the northwest, in large channel A from which smaller active channels B, C, D and E branch off. Channels B, C and D currently intercept most of the water from the main channel A (Fig. 20), so that only a small part of the water flows down its lower reach. Channel A has features of a main channel up to the point of divergence of channel D. It is 4 –4.5 m deep and the water-surface slope between the points of divergence of channels B and D is ca. 0.00001. A farther reach of channel A is shallower (largely 1.5 – 2 m) due to: (1) sand accretion on the bottom (see Fig. 269 Fig. 17. Scheme of peat bank erosion, based on underwater observations. 15B), (2) aquatic plant overgrowth and (3) occurrence of three large vegetated linguoid bars (Figs. 18 and 19). It should be mentioned that the initial segment of channel B was artificially widened several decades ago to facilitate local navigation (Grygoruk, personal communication, 1999). Channels C and D are natural. They are narrow and branch from channel A at right angles. These channels, which have erosional bottoms and are in a phase of scouring, might be considered evolving crevasses. They feature exceptionally steep water-surface slopes (0.0009 and 0.0014, respectively), and high current velocities at normal water stage (ca. 50 and 70 cm/s, respectively). This results from a relatively high local difference in water-surface level (up to 5 cm) between their lower nodes and channel A (see Fig. 21). The depth and width of channels C and D vary. The channels are shallow and narrow as they flow over the peat substratum but become much deeper and wider when incised below the peat layer (Fig. 21). Channel E, situated far downstream along channel A, in the past also functioned as a ‘‘crevasse’’ much the same way that channels C and D do. At present, however, channel E is in a moribund phase and shows minimal flow. Its bottom is accretionary. The air photograph record suggests that the outline of the anabranches B – E has not changed much since Fig. 15. (A) Cross-section through the main channel and concave-bank bar; for location, see Fig. 4. (B) Cross-section through the lower reach of channel A and vegetated linguoid bar within it; for location see Fig. 19. Vegetation marked schematically, not to scale. (1) Common reed (Phragmites australis). (2) Great reed mace (Typha latifolia). (3) Lesser reed mace (T. angustifolia). (4) Bur reed (Sparganium erectum). (5) Arrowhead (Sagittaria sagittifolia). (6) Marsh woundwort (Stachys palustris) and great yellow-cress (Rorippa amphibia). (7) Grasses (Gramineae), sedge (Carex sp.), bur marigold (Bidens sp.), etc. (8) Common sallow (Salix cinerea). ELWS—extremely low water stage; NWS— normal water stage. 270 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 Fig. 18. Fragment of the dense channel network at the Zielona Budka area, view eastward (cf. Fig. 19). (L) Vegetated linguoid bars. (M) Midchannel bars. (A – E and R) Channels described in text. Flow to left. 1966 (Fig. 22A). It appears that they are rather old and develop slowly, which should be attributed to the high resistance of the peat layer. The Zielona Budka area also provides an example of gradual reoccupation of an old channel. Channel R in the east part of this area is now almost stagnant. An air photograph taken in 1966 (Fig. 22A) shows that only the upstream fragment of this channel is not vegetated and the remaining part of the area is occupied by quagmires with small ponds, possibly formed in place of an old, completely overgrown channel. Since then, channel R has gradually merged into the active channel network (Fig. 22B and C). At present time (1998 – 2000) it is in a moribund phase again—i.e. it is shallow and largely overgrown, especially in its upstream reach (Fig. 19). Other areas of the NAS are not significantly different from the above described channels (A – E and R) of the Zielona Budka area. 9.2. Interpretation and discussion The term avulsion is understood here in the sense of Makaske (2001, p. 158) as: ‘‘the diversion of flow from an existing channel onto the floodplain, even- tually resulting in a new channel belt’’. Channel avulsion has been postulated for many modern anastomosing river systems (Smith and Smith, 1980; Smith et al., 1989; Schumann, 1989; McCarthy et al., 1992; Schumm et al., 1996; Gibling et al., 1998; Makaske, 1998; Morozova and Smith, 1999, 2000; see also review by Makaske, 2001). The observations presented here seem to confirm that the interconnected multiple channel system of the Narew River also resulted from avulsion. The anastomosing pattern of the Narew River channel network strongly suggests that avulsion was the principal process which led to the flow diversion and ultimately produced a system of interconnected channels. It is our contention that the anabranches C and D of the Zielona Budka area are avulsion channels in statu nascendi. This hypothesis is based on the hydrologic parameters and geometry of the channels, and on the fact that they are not fully incised into the peat layer. It will take decades before they take over much of the flow of the channel A, unless channel B takes over the main flow before that. Our view is further substantiated by a tendency, common within the NAS, to develop high-angle channel diversions (Figs. 4 and 19). R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 271 Fig. 19. Fragment of a dense channel network at the Zielona Budka area. For location, see Fig. 4. (A – E and R) Channels described in text. (L) Vegetated linguoid bars. (M) Mid-channel bars. The process of avulsion is attributed by many authors to the presence of alluvial ridges (see review by Jones and Schumm, 1999, and references therein). There are, however, known examples of modern avulsions in the Okavango system which is devoid of alluvial ridges (McCarthy et al., 1992). In the case of the NAS, one of the causes of avulsion might have been small local difference in the water-surface level between adjacent channels. It seems that the local rise of water level is due to increased aggradation in the lower reach of one channel accompanied by invasion of plants and drop in current velocity. These interrelated processes initiated a feedback mechanism, similar to that described from the Okavango system (McCarthy et al., 1992). 272 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 Fig. 20. Scheme of discharge distribution in channels A – D at the Zielona Budka area; percent of discharge during normal and low (in brackets) water stage. Not drawn to scale. In the NAS, avulsion can be triggered by a sudden blockage of channel by plant jams (Fig. 11) or ice jams, or a combination of both. The location of new channels in the interchannel areas was most likely predisposed by elk or beaver trails. The classic concept of avulsion concerns a process which leads to the entire takeover of the flow from the old channel to a new one (see Allen, 1965; Miall, 1996; Jones and Schumm, 1999). A process of avul- sion, however, may or may not be accomplished. In some instances, the new channel never fully intercepts all the discharge from the old one. An example of such failed avulsion on a large scale has been described from the Mississippi Valley (Guccione et al., 1999). Many anastomosing systems consist of several active channels of various size. NAS is one of them. The occurrence of many coexisting active channels suggests that partial avulsion, which produces a new channel that partially takes over the flow from the main channel, is characteristic of the NAS. Morozova and Smith (1999, 2000) quote well-documented examples of coexistence of long-lasting channel belts. These belts developed in result of repeated avulsion in Cumberland Marshes of Canada. It is possible that the NAS is of analogous origin. In the case of NAS, the process of channel development by avulsion is slow. It has been taking place for many decades. This is indicated by our observations in the Zielona Budka area. The avulsion process is very slow because of the hydrologic conditions and the presence of resistant peat substratum. Small-scale and long-lasting avulsion processes predominated over large-scale avulsions. The former probably took place with a frequency of hundreds or thousands of years. Although the newly formed channels might subsequently be abandoned, the lasting channels predominate in the modern anastomosing system of the Narew River. Fig. 21. Longitudinal section and transverse cross-sections (1 – 3) of channel D (for location see Fig. 19). Measured difference in water level surface with respect to channel A. Normal water stage. R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 273 10. Characteristics and development of the Narew anastomosing system Fig. 22. Successive air photographs of the Zielona Budka area (cf. Figs. 18 and 19). Black reflex comes only from plant-free water surfaces. Photographs A and B taken at low water stage, C taken at water stage slightly higher then normal. The NAS displays most features typical of modern anastomosing systems studied in detail from a sedimentological point of view (cf. Smith and Smith, 1980; Rust, 1981; Smith, 1983, 1986; Rust and Nanson, 1986; Smith et al., 1989; McCarthy et al., 1991, 1992; Makaske, 1998; Gibling et al., 1998; Pérez-Arculea and Smith, 1999; Morozova and Smith, 1999, 2000). These features are: (1) a pattern of interconnected channels and stable islands; (2) a lithological contrast between the channel and interchannel deposit; and (3) a low gradient of channels and low width/depth ratio. The NAS also displays several distinctive features. Its channels are strictly bedload-dominated and can be characterised by the lack of natural levees built of clastic sediment, and by the lack of alluvial ridges and crevasse splays. These characteristics, however, are not unique to the Narew anastomosing system. The Okavango system in Botswana shows similar characteristics (cf. McCarthy et al., 1991; Stanistreet et al., 1993; Smith et al., 1997). Another characteristic which NAS shares with the Okavango is the common presence of peat in the interchannel areas. The impact of vegetation is crucial for both the NAS and the Okavango system (McCarthy et al., 1986, 1988a,b; Ellery et al., 1990a,b, 1993, 1995). The following are the most important aspects of the NAS vegetation: (1) plants provide indigenous material for the peat and peat-like deposit that accumulate in the interchannel areas; (2) presence of riparian vegetation and resistance of the peat layer prevent erosion of channel banks (cf. Smith, 1976; Smith and Smith, 1980; Hickin, 1984; Ellery et al., 1995); (3) encroachment of riparian vegetation into channels results in their gradual narrowing (cf. Ellery et al., 1995); and (4) colonisation of active channels slows down the current and facilitates accretion of mineral sediment, thus initiating a feedback mechanism which leads to local shallowing and waning-flow conditions. Rising water levels in the upstream reaches of some active channels may cause avulsion. The impact of vegetation and the presence of the peat layer in the NAS suggest a causal and temporal relationship between the evolution of the anastomosing system and the growth of the peat layer on the 274 R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 valley bottom. Radiocarbon dates from the basal parts of this layer indicate that peat accumulation commenced at different times in various parts of the valley. These dates suggest that development of the anastomosing system progressed gradually. It was in the late Holocene when the network of interconnected channels, similar to the present one, was formed and covered the whole valley bottom. The sedimentological characteristics of the basal sand series suggest deposition within bedload-dominated channels. It is likely that the Narew River was initially of braided type and subsequently, by the end of the basal sand series deposition, the river channels turned into meandering ones, as was suggested by Churski (1973) and Banaszuk (1996). The river channels finally evolved into the anastomosing ones. The present-day, highly sinuous outline of some Narew River channels consisting of inactive ‘‘meanders’’ and associated oxbows, strongly suggests a meandering nature of the fluvial system that preceded the NAS. In our opinion (see Gradziński et al., 2000), the relics of the older meandering fluvial system are still partly preserved within the existing anastomosing system due to protection by riparian vegetation and development of the peat layer. A direct transformation of the system from braided into an anastomosing one is rather difficult to accept considering the irregular pattern of channels with high angle interconnections, and the presence of the meander relics. Vertical accretion of the peat layer seems to be the main factor controlling the gradual rise of the depositional surface in the whole system—i.e. in the interchannel areas and in the channels as well. The rate of vertical accretion is of the same order of magnitude as the subsidence rate in this part of Poland, as determined by surveying (see Wyrzykowski, 1985). The system is moderately aggrading and its preservation potential is rather good. If preserved in the geological record, the NAS would produce a dense network of interconnected ribbon sand bodies built up of channel facies separated laterally by lithologically contrasting, organic-rich, interchannel deposits. The interchannel peat layer would ultimately be transformed into carbonaceous sandy sediments rather than coal (cf. McCabe, 1984). The NAS is not unique within the vast lowlands of Poland. Similar late Holocene anastomosing fluvial systems were common both in Poland and in neigh- bouring countries before their transformation by drainage and regulation works during the last two centuries. 11. Conclusions (1) The upper Narew River may be considered as a typical anastomosing fluvial system of temperate – humid climate, although it stands out with its distinctly bedload-dominated character, lack of natural levees, alluvial ridges and crevasse splays. (2) The impact of vegetation is crucial for the evolution of the NAS. It is the most important factor controlling sedimentation processes. Vegetation forms layer of peat which covers the valley bottom. It stabilises channel margins preventing their erosion. It overgrows shallower stretches of the active channels, thus slowing down the current and facilitating accretion. And finally, it locally raises the water level causing avulsion. (3) Avulsion in the studied system is a small-scale, gradational, long-term and infrequent process; it usually does not result in complete interception of the whole discharge by a new channel. This is why the network of interconnected channels consists of several coexisting, active channels. Plant jams and ice jams or combination of both may trigger avulsion. (4) Vertical accretion of the peat layer that covers the interchannel areas controls the rise of the depositional surface in the whole NAS system. Acknowledgements The study has been funded by State Committee for Scientific Research (project No 6 PO4E 020 14, 1998 – 2000). Bogusław Deptuła, the Director of the Narew National Park, and other members of the Park staff have rendered great help during the field work. Extremely helpful was the cooperation of Maciej Tomaszek as the scuba diver. The authors also thank Piotr Banaszuk, Michał Banaś, Iwona Bieleś, Władysław Danowski, Józef Halama, Marek Sielicki and other colleagues for their participation in the field work. Our technician Piotr Wal made the necessary self-designed equipment. Terence S. McCarthy kindly sent copies of all publications by the Okavango Research Group. We owe a particular debt to R. Gradziński et al. / Sedimentary Geology 157 (2003) 253–276 Grzegorz Haczewski, Krzysztof Birkenmajer, Szczepan J. Pore˛bski and Elżbieta Turnau for their help in the preparation of the final version of this paper. We thank L.S. Jones and K.S. Davies-Vollum for reviews of the manuscript and useful comments. References Allen, J.R.L., 1965. A review of the origin and characteristics of recent alluvial sediments. Sedimentology 5, 89 – 191. Allen, J.R.L., 1970. A quantitative model of climbing-ripples and their cross-laminated deposits. Sedimentology 14, 5 – 26. Ba»uk, A., 1973. 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