In A River, What Happens To The Channel Size And Discharge Downstream?

Channelization

Channelization, nonexistent riparian encompass, silt, depository financial institution reinforcement with concrete and canvass piling, alterations of littoral areas and shorelines, and dredging are contributors to the impairment of fish and wildlife in the AOC.

From: Paradigms Lost , 2006

Ecogeomorphology

A.R. Pierce , S.L. Male monarch , in Treatise on Geomorphology, 2013

12.xiv.i.ii Channelization

Channelization of fluvial systems includes widening and deepening the stream channel, which increases the channel capacity, shortening the stream channel length, and increasing the stream gradient ( Figure 4). These factors combine to typically move greater volumes of water through the system at a much more rapid charge per unit compared to pre-channelization weather. As a result of increased channel capacities and increased send efficiency, channelization of streams causes the channels to be hydrologically disconnected from the side by side floodplain and alters functional processes of fluvial systems (Kroes and Hupp, 2010). There has been no official cess of the number of streams that have been channelized, merely this type of alteration has been recognized as being more extensive than damming (National Research Council, 1992), which has occurred on virtually of the world's largest rivers (Nilsson et al., 2005). Throughout the southeastern United States, stream channelization has been used extensively to reduce flooding and facilitate agronomics product on floodplain areas (Simon and Hupp, 1992; Hupp and Bazemore, 1993; Shankman, 1993).

Channelization can typically reduce flooding in upstream reaches of a system; meanwhile, lower reaches usually experience an increment in peak flood levels and have a college frequency of flooding (Shankman and Pugh, 1992). Channelization, forth with dredging activities used in maintenance of channelized systems, has as well been plant to influence water-table levels within the adjacent floodplain (Tucci and Hileman, 1992). Stream channelization can besides produce conditions that initiate connected degradation of the stream channel, including headcutting and aqueduct erosion that can produce all-encompassing banking company failures (Robbins and Simon, 1983; Simon and Hupp, 1987; Simon, 1994).

The increased stream power that characterizes channelized streams also facilitates increased rates of sediment send within these altered systems. Typically, channelized systems will accumulate big amounts of sediment in the lower reaches of the system because of decreased stream gradients and channel obstructions that occur in these areas, which reduce stream velocities and initiate sediment deposition (Schumm et al., 1984; Simon and Hupp, 1987). Greater degradation rates in these lower reaches reduce the channel chapters and cause a widening of the stream channel; this is recognized as a critical stage of geomorphic recovery of channelized streams (Schumm et al., 1984; Simon and Hupp, 1987). Notwithstanding, in some cases, where stream channelization combines with unique geological conditions and country-employ practices, severe aggregating occurs that completely blocks the channels with sediment, forming valley plugs (Happ et al., 1940; Fryirs and Brierley, 1998; Brierley and Fryirs, 1999; Pierce and King, 2007a, 2008).

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Fluvial Changes in the Anthropocene: A European Perspective

Nicola Surian , in Reference Module in Earth Systems and Environmental Sciences, 2021

iii.1 Anthropogenic disturbance at the reach scale

Channelization and sediment mining were identified equally main causes of fluvial changes acting at the reach scale. Both interventions were carried out in several rivers, although in different time periods. Channelization, which includes a range of interventions (e.g., straightening, embankments, banking company protections), was carried out since the 19th century and even before in the largest rivers (east.g., the Rhine and Danube; run across ( Hudson et al., 2008; Arnaud et al., 2019; Hohensinner et al., 2014), and then it intensified during the 20th century.

In-channel sediment mining was carried out mainly in the second half of the 20th century and, commonly, for relatively brusque periods (i.east., 15–xx years). Since sediment mining was very intense, that is at rates greatly exceeding the sediment replenishment rate (Rinaldi et al., 2005), it was considered in several cases every bit the main gene of channel adjustments (Bravard et al., 1997; Surian et al., 2009b). The major role of mining is conspicuously non only from the strong temporal relation betwixt channel aligning (due east.g., narrowing and incision) and mining activity, but likewise from adjustments that occurred later abeyance of mining. For instance, in several Italian rivers narrowing and incision significantly slowed down and were replaced, in some cases, by widening and aggradation once in-channel mining was forbidden or significantly reduced (Surian et al., 2009b; Bollati et al., 2014) (Fig. 9).

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Fluvial Geomorphology

A. Simon , M. Rinaldi , in Treatise on Geomorphology, 2013

ix.29.six.2 Comparing with a Channelized System in a Littoral Plain Surround

Channelization of the sand-bed streams in western Tennessee created upstream migrating knick zones that resulted in hundreds of kilometers of failed banks and millions of tonnes of sediment erosion ( Simon, 1989b; Simon and Hupp, 1992). Though the Obion-Forked Deer River system of western Tennessee and the Toutle River organization of Washington were drastically different, trends of vertical adjustment were almost identical (Effigy iii). Differences in the magnitude of specific adjustment processes are explained in terms of the resistance and grapheme of the boundary sediments, with the western Tennessee streams having moderately cohesive, fine-grained banks.

Average rates of widening in the Toutle River organization were 10–xx 1000 twelvemonth⁻¹ compared to 0–three.5 thousand yr⁻ⁱ in western Tennessee. This is because the cohesive banks of the Obion River system were more constructive at resisting mass failure in eye and upstream reaches, yet when eroded, the fine depository financial institution sediment was transported through the system and did not contribute to downstream aggradation. By dissimilarity, fibroid-grained depository financial institution sediments in the Toutle River system readily were eroded and provided an important source of hydraulically controlled sediment for downstream aggradation.

Changes in aqueduct width divided by changes in depth differed by an social club of magnitude for the two systems, with ways of 59 and 5.4 for the Toutle (n=16) and Obion-Forked Deer (due north=25) systems, respectively. This once again, reflects the greater resistance of the cohesive banks in western Tennessee and the importance of separating and accounting for differences between bed- and bank-material backdrop. When plotted against disturbed channel gradient, this index of channel alter (Simon, 1992) provides a conceptual motion picture of the relative magnitudes of vertical and lateral adjustment following a disturbance resulting from excess flow energy in unlike fluvial environments (Figure 17).

The above discussion is an case of how different disturbance events tin pb to similar spatial and temporal trajectories of deposition, aggradation, and widening, although the varying magnitudes of response effect in different channel morphologies. Although the two systems were exposed to physically different disturbances, both correspond conditions of excess energy and stream power relative to sediment supply.

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NELSON AND CHURCHILL RIVER BASINS

DAVID M. ROSENBERG , ... ROBERT West. NEWBURY , in Rivers of N America, 2005

Human being Impacts and Special Features

Channelization, ditching, and water-level control dams are the major alterations that affect all the rivers. The largest dams are Shellmouth Dam (congenital in 1969) on the upper Assiniboine, which created Lake of the Prairies; Portage la Prairie Dam and spillway (built in 1970); and Lockport Dam on the lower Scarlet River (built in 1922), which enhances navigation betwixt Winnipeg and Lake Winnipeg. The major hydrologic affect on the Cherry-red River chief stem is the continued operation of the Lockport Dam, which maintains water levels within the city of Winnipeg ∼2m to 3m in a higher place normal summer levels, mainly for recreational purposes.

Operation of the Shellmouth Dam on the Assiniboine River greatly affects the hydrograph of the river. Prior to the installation of dams the lowest flows typically occurred in midwinter; lowest flows at present tend to be in late autumn, prior to releasing stored water from the reservoir to brand room for jump inflows. The flow authorities during winter frequently is manipulated to reflect the latest predictions for bound flooding. Wintertime flows tin can be quite stable for long periods under depression snow conditions, but if a major snowstorm brings significant precipitation to the upper office of the subcatchment flows may increase by every bit much as four to six times over a period of a few weeks. The effects of these menses variations on biota are unknown and virtually unstudied.

Other major dams in the subcatchment include Rafferty (built in 1991) and Alameda (built in 1994) dams on the upper Souris River in Saskatchewan, built for alluvion command, irrigation, and recreation. Additional low-caput weirs are located at major towns in the U.S. portion of the main stem, but several were modified in the late 1990s to improve fish and pocket-sized-gunkhole passage (Luther Aadland, personal communication). There also are two depression-caput weirs on the Assiniboine River in Brandon ∼200km west of Winnipeg. In add-on to these chief-stalk structures, there are reservoir control structures on the Red Lakes, Otter Tail, and Sheyenne rivers and at the outlet of Lake Traverse (Stoner et al. 1993; see Fig. 19.13) and many water-level command weirs on tributary streams throughout the subcatchment (<350 in the U.South. portion of the subcatchment).

Some of the existing dams and weirs are barriers to fish passage, particularly at low flows. Fish distributions testify, for instance, that the Portage la Prairie dam on the Assiniboine is a barrier to the ongoing postglacial range expansions of recently colonizing species, such as the golden redhorse and the bigmouth buffalo, even though the dam is opened each fall.

The Manitoba government has initiated a study of instream catamenia needs for fish and river processes (designed according to recommendations of the Instream Flow Council 2002, www.instreamflowcouncil.org), merely the analyses are not yet available. The impetus for this study was increasing demands for irrigation h2o for potato agriculture. A parallel written report on h2o-quality issues in the river also is underway.

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River Ecosystems

K.E. Limburg , ... D.50. Strayer , in Encyclopedia of Biodiversity, 2001

Half-dozen.B. Stream Channelization

Stream channelization is designed to ameliorate the navigability of rivers or to reduce flooding potential in streams. In the former case, the river channel is deepened and widened past dredging, which destroys benthic habitat. In the latter, the streambed is sometimes straightened and "paved" to increment the capacity of the stream to send water downstream. These processes change the flow authorities of the stream, favoring species that tolerate faster, turbulent currents, and excluding others. Changes to the streambed tin affect the conductivity of h2o through the hyporheic zone, which affects nutrient processing (see Section 4).

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Turning a River Into Infrastructure to Accommodate the Rising of the Megacity Chosen Los Angeles, California

Kat Superfisky , Jessica Chiliad. Henson , in Reference Module in Earth Systems and Environmental Sciences, 2021

Jurisdictional

The channelization of the LA River has as well contributed to an increase in jurisdictional and managerial complexity. The river right-of-fashion includes over 2300 acres of primarily publicly endemic state with a complex web of ownership and easements ( Los Angeles County, 2021). Farther, many agencies have jurisdiction over different parts of the river, making it difficult to manage and maintain every bit ane contiguous organisation: the Metropolis of LA and artist/activist Lauren Bon have water rights for water falling within the watershed upstream of downtown LA; the Upper LA River Area Watermaster manages much of the water in the upper basin, including groundwater in the San Fernando Groundwater bowl and sub-basins; the major groundwater basins in the Lower LA River Area, the Central and West Coast Basins, are managed past the Water Replenishment District of Southern California; LA County Public Works operates and maintains roughly half of aqueduct and is responsible for flood safety on behalf of the LA County Overflowing Control District, which was formed in 1915; the USACE maintains and operates the other half of the aqueduct and ensures the structural integrity and chapters of the channel to manage flows and issues all permits for changes to the channel shape or design; the State of California Water Board certifies that the h2o meets the requirements of the United states of america Clean Water Act; the air rights are owned by a variety of owners; and the California Section of Transportation owns the overhead air space (Los Angeles County, 2021). The LA County Board of Supervisors requested a feasibility study examining divestiture of the USACE from the river's mainstem and tributaries in 2019, requesting to take on operations and maintenance, likewise as permitting, for all channels (Hahn et al., 2019). This chat is ongoing and complex (Fig. 8).

The rivers' channelization is demonstrative of the mid-20th century's focus on creating single-purpose solutions to regulate such hazards as floods. This approach to controlling nature finer allowed for the region to develop into the second largest urban expanse in the U.S., simply caused an assortment of concrete and ecological alterations, and socio-political and jurisdictional complexities.

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Biogeography

Timothy D. Schowalter , in Insect Environmental (Second Edition), 2006

B Disturbances to aquatic ecosystems

Stream channelization and impoundment accept reduced heterogeneity in channel morphology and flow characteristics. Channelization constrains channel morphology, removes obstacles to menses, and shortens stream length. These modifications eliminate habitats in overflow areas (such as wetlands and side channels) and in logs and other impediments and accelerate drainage in the channeled sections. Impoundments replace a sequence of turbulent sections and pools backside logs and other obstacles (characterized by rocky substrates and high oxygen contents) with deep reservoirs (characterized by silty substrates and stratification of oxygen content and temperature). These changes in stream atmospheric condition eliminate habitat for some species (such every bit species associated with high flow rate and oxygen concentrations) and increase habitat availability for others (such every bit species associated with lotic status and depression oxygen concentrations).

The linear configuration of stream systems (i.e., the stream continuum concept; Vannote et al. 1980) makes them particularly vulnerable to disturbances that occur upstream. For example, heavy precipitation in the watershed is concentrated in the stream aqueduct, scouring the channel and redistributing materials and organisms downstream. Burn or harvest of riparian vegetation exposes streams or wetlands to increased sunlight, raising temperatures and increasing principal production, altering habitat and resource conditions downstream, often for long fourth dimension periods (Batzer et al. 2000a, Haggerty et al. 2004). Industrial effluents, runoff of agronomical materials (e.grand., fertilizers), or accidental inputs of toxic materials (e.chiliad., pesticides) touch on habitat suitability downstream until sufficient dilution has occurred (S. Smith et al. 1983, Southwick et al. 1995). Eutrophication, resulting from addition of limiting nutrients, substantially alters the biological and chemical conditions of aquatic systems.

Lake Balaton (Europe'due south largest lake) in Hungary has experienced incremental eutrophication since the early 1960s, when lake chemistry was relatively compatible (Somlyódy and van Straten 1986). Since that time, phosphorus inputs from agricultural runoff and urban development have increased, starting at the west end where the Zala River enters the lake. The division of Lake Balaton into four relatively distinct basins draining distinct subwatersheds facilitated documentation of the progression of eutrophication from west to e (Somlyódy and van Straten 1986). Dévai and Moldován (1983) and Ponyi et al. (1983) found that the abundance and species composition of chironomid larvae were correlated with this longitudinal gradient in water quality. The original species characterizing oligomesotrophic conditions have been replaced by species characterizing eutrophic weather in a due west-to-east direction. Similarly, sedimentation resulting from erosion of croplands or clearcut forests or from trampling of streambanks past livestock alters substrate conditions and habitat suitability for organisms downstream.

Pringle (1997) reported that disturbances and anthropogenic modification of downstream areas (due east.g., urbanization, channelization, impoundment, etc.) too affect conditions for organisms upstream. Degraded downstream areas may be more vulnerable to establishment of exotic species that are tolerant of stream degradation. These species subsequently invade upstream habitats. Degradation of downstream areas may restrict move of upstream species within the watershed, thereby isolating headwater populations and limiting gene flow betwixt watersheds. Finally, deposition of downstream zones may prevent movement of anadromous or catadromous species.

Disturbances to adjacent terrestrial ecosystems affect aquatic species. Davies and Nelson (1994) compared aquatic invertebrate responses to woods harvest inside ten one thousand of streams, ten-30 thou of streams, xxx-l m of streams, or unharvested in Tasmania. Densities of aquatic invertebrates were measured at a site upstream of the treatment and at a second site immediately downstream from the treatment. Differences in mayfly (Ephemeroptera) and stonefly (Plecoptera) densities betwixt the two sites were significantly, negatively correlated with width of the riparian forest buffer. Overall, mayfly density declined 62% and stonefly density declined 34% at sites with <xxx g of buffer, demonstrating the importance of riparian forest buffers to aquatic species.

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River Ecosystems

K.E. Limburg , ... D.L. Strayer , in Encyclopedia of Biodiversity (Second Edition), 2013

Stream Channelization and Interbasin Connection

Stream channelization is designed to improve the navigability of rivers or to reduce the flooding potential in streams. In the former case, the river channel is deepened and widened by dredging, which destroys the benthic habitat. In the latter, the streambed is sometimes straightened and paved to increment the capacity of the stream to ship h2o downstream. These processes alter the flow regime of the stream, favoring species that tolerate faster, turbulent currents, and excluding others. Changes to the streambed can affect the conductivity of water through the hyporheic zone, which affects nutrient processing ( run across Different Conceptual Models of Riverine Ecosystems).

Additionally, many waterways have been engineered to raise transportation and trade. Such activities have been undertaken for millennia, such that many alterations have become part of the cultural landscape and heritage. The Mississippi River is a case case, where locks permit barges to modify elevation and canalways connect across basins. Similarly, the Saint Lawrence Seaway raised h2o levels above the natural rapids so that commercial shipping vessels could travel from the body of water into the Nifty Lakes. The furnishings in both systems have included habitat loss, ecosystem alteration, major changes in the hydrograph, and species invasions (come across Nutrient Loading).

Damming for water supply purposes (see Dams) can besides outcome in interbasin transfers of water, nutrients, and biota. In periods of high water demand, which often coincide with periods of low h2o in rivers, riverine h2o withdrawal or detention and transfer to municipal h2o supply networks or irrigation systems can significantly modify flow regimes. For case, the New York Metropolis water reservoir system relies in part on dams in the Upper Delaware bowl, effectively transferring water from the Delaware to the Lower Hudson River basin via the Urban center's distribution network. In Red china, the South–Northward Water Transfer Project is the largest water-works project ever developed at that place. Plans are to transfer nearly 45×10⁹ m³ of h2o annually from the Yangtze River in the southward to the Xanthous River via several routes. This volition event in massive changes in water supply, habitat, and fisheries, and will affect communities both downstream and in headwater areas (i.e., beyond international borders).

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Hydromorphology: Case Studies***

Gwendolin Porst , in Reference Module in Globe Systems and Environmental Sciences, 2021

Instance written report 3: Dams and weirs

Together with channelization of rivers and streams, damming is one of the most common hydromorphological alterations constitute in river systems around the earth ( McCartney, 2009; Van Looy et al., 2014). Worldwide, more than than 45,000 large dams exist and more are meant to be congenital in the hereafter (McCartney, 2009).

Dams and weirs are unremarkably built to support water supply, particularly for irrigational purposes, to generate hydropower, control floods or for navigational purposes. Nevertheless, the construction of firm, solid walls built across a river valley or catchment to cake the flow of the river, leads to a neat diverseness of negative furnishings, such as landslides, a not bad bargain of ecological problems, together with a general pass up in water quality within the river systems and floodplains. While each dam'south environmental impact is very unique and dependent on the dam's structure, the characteristics of the resident biota, the local climate and geomorphology, a variety of negative furnishings of the construction of these very sturdy structures inside a river system cannot be denied (McCartney, 2009). Dam construction has severely contradistinct rivers and their wetlands effectually the world, which has global implications on river and, especially, wetland biodiversity (Nilsson et al., 2005). Dams always modify the flow government of a river greatly. The robust structure prevents the movement of sediments and nutrients downstream and causes slower flows upstream, which in plough leads to increased settling of sediments in this part of the river and is accompanied past more unpredictable flows downstream of the dam (Wu et al., 2019). Additionally, annual overflowing-pulses are too disturbed by dams or weirs, which severely interrupt the substitution of nutrients betwixt the river and its floodplains (Benke et al., 2000; Wu et al., 2019). Other negative effects of dams are the interruption of fish migration inside the river system, the loss of many foraging and spawning habitats within the river floodplains and a general change in plant and animal communities, often resulting in lower biodiversity below the dam (McCartney, 2009; Wu et al., 2019).

The Yangtze River, the largest river in East Asia and the third longest in the globe, is the most important river in China, on which more than than 400 million people depend (Kaifeng et al., 2013). To facilitate irrigation, to generate hydropower, to store water and to manage flooding in the expanse, between 1950 and 2010 more than l,000 dams have been built in the Yangtze River basin (CCYRA, 2006, Yang et al., 2011). Since 2012, the Iii-Gorges-Dam (TGD), located in the Yangtze River, is presently the largest dam and hydropower project in the world (Fig. three). The benefits and costs of this huge dam have been under debate since the planning stage and have continued since the dam became fully functional (Zhang et al., 2012). Especially the environmental and ecological effects of the TGD on the ecosystem of the Yangtze River and here specially its biodiversity, accept been of increasing concern in contempo years (Wu et al., 2004, 2019). The structure of the dam and its reservoir has implications on the entire river system, both upstream and downstream (Fu et al., 2010), also every bit associated wetlands, floodplains and terrestrial systems (Wu et al., 2004). Quite a number of studies have been carried out looking at the ecology furnishings of the TGD on its faunal communities. Research observing the implications of the changed hydrological regime and movement of sediments and nutrients along the river in the TGD reservoir region, for example indicated an increase in densities/biomass together with a decrease in biodiversity of many macroinvertebrate taxa downstream of the TGD (Wu et al., 2019), similar to the furnishings of morphological lake shore alterations.

Very devastating furnishings were, furthermore, institute for many fish species inhabiting the Yangtze River. The main reasons for the observed dramatic changes in different fish populations are explained past the obstruction of migratory routes, with no effective mitigation through structure of fish passes (Shi et al., 2015), habitat fragmentation, irresolute from lotic to lentic water in impounded areas, and changes in hydrological flow regimes, especially in downstream reaches (Wu et al., 2004, 2019; Zhang et al., 2012). An example for these adverse effects of the construction of the TGD on fish populations is the drastic pass up of catches of the 4 "major carps" (known as grass carp Ctenopharyngodon idella, argent carp Hypophthalmichthys molitrix, black bother Mylopharyngodon piceus and bighead carp Aristichthys nobilis), which naturally inhabited the eye and lower stretches of the Yangtze River. These commercially very of import species grow and mature in the Yangtze River floodplains and migrate into the river between April and July to spawn, when temperatures increment and water levels rising during the flooding season. Larvae and juveniles enter the nursery areas in the inundation plains again when transported downstream by the river period (Zhang et al., 2012). Through the structure of the TGD, about of the natural spawning areas of the carps were lost and temperature and flow weather changed, which resulted in a dramatic decline of the juvenile bother that had drifted into the floodplain beneath the TGD (Xie et al., 2007; Zhang et al., 2012). Negative effects of the changed hydrological regimes associated with the construction of the TGD were furthermore described for plant communities in the drawdown area of the dam (Wang et al., 2014). Every bit a upshot of the hydrological alteration in the area, the number of tracheophyte species in the Three Gorges reservoir declined noticeably from 175 to 127 later the full impoundment (Wang et al., 2014). The Yangtze River Dolphin (baiji; Lipotes vexillifer) was declared extinct in 2007 probably because of pollution, unsustainable fishing and a high shipping pressure in the river. The Three Gorges Dam is assumed to accept added an additional pressure to the already highly endangered species, by farther reducing its natural living space through the alteration of river flow downstream, and facilitating an increase in send traffic along the Yangtze River (Wu et al., 2019).

Additionally, the impoundment of the 660 km long reservoir behind the TGD has almost certainly led to an increase of seismic activity in the region (Tang et al., 2019). Later the filling of the reservoir in 2003, the number of earthquakes per yr increased from approximately two per year to approximately 14 earthquakes per year after total impoundment of the reservoir in 2008, together with hundreds of landslides (Tang et al., 2019).

Despite providing water for drinking, irrigation and electricity, the TGD shows impressively the implications the constructions of dams and weirs can have on rivers, streams and associated floodplains. Dams are 1 of the well-nigh noteworthy anthropogenic interventions in the hydrological cycle (McCartney, 2009) and accept (had) severe impacts on freshwater systems through the disruption of physicochemical and biological processes worldwide. Plans for new large dams continue, especially in tropical regions (Vörösmarty et al., 2010; Zarfl et al., 2015). In the planning and blueprint of most already existing dams, potential environmental problems did non play a major role, but ecology sensation has led to more holistic planning and management of dams in recent decades in order to decrease the most dissentious effects of dams. These should include, among other things, the implementation of effective fish passages in the blueprint of all dams (big or small) to ensure river connectivity and migratory fish conservation.

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Baltic and Eastern Continental Rivers

Henn Timm , ... Marina 1000. Mel'nik , in Rivers of Europe, 2009

xvi.3.11.ii Damming and Channelisation

Intensive regulation and channelisation of the Vistula started at the end of the 19th century. The construction of groynes that narrowed and deepened the channel was most intensive in the mid 20th century. Damming started in 1930 with most dams used for water supply, power production, overflowing protection and navigation. Co-ordinate to the Polish Geological Plant, the accumulated sediments in Włocławek reservoir comprise 36 600 tons of mineral oils, fifteen 000 tons of zinc, 7600 tons of chromium, 4200 tons of copper, 2200 tons of pb, 2100 tons of nickel, 190 tons of cadmium, 51 tons of polyaromatic hydrocarbons, 46 tons of mercury, 1.5 tons of chloroorganic pesticides and 0.55 tons of pesticides (Bojakowska 1999).

Damming has caused a progressive decline in migratory fishes. Virtually all anadromous and catadromous fishes in the Vistula River are considered equally threatened, (Bontemps 1976; Backiel 1985; Woźniewski 1999). Effective fish-passes are indispensable for potamodromous migratory species (e.g. nase, barbel and grayling). The nase, C. nasus 50., was one of the nearly abundant rheophilic species in the Vistula until the 1960s. Today, it is i of the near threatened rheophilic fish in Poland (Witkowski et al. 1999; Heese 2000; Kaczkowski 2004) because Włocławek dam has stopped its migration. Similarly, the migration of Vimba vimba was ceased because of the dam (Backiel & Bontems 1996).

Sturgeon (Acipenser sturio), razorfish (P. cultratus) and natural populations of salmon (S. salar) disappeared in the Vistula almost a century ago. Body of water trout, eel, reintroduced Atlantic salmon and lampreys are seriously endangered (Backiel & Penczak 1989). Before the construction of Włocławek dam, several fishes were commercially important, e.g. bream (41% of total catch), ocean trout (xi%), vimba (11 %), barbel (6.5%), pike (v%), zander (4%) and asp (3%). Today, bream, roach and argent bream contribute 95% of the full commercial take hold of (Wiśniewolski 2002).

Changes in the fish customs besides occured in dammed tributaries. The abundance of obligatory riverine species (B. barbus, L. cephalus, L. leuciscus, Gobio gobio, C. nasus) was drastically decreased in the Pilica River (Penczak & Kruk 2000). Nase, barbel and dace became almost extinct; chub and gudgeon are at present considered vulnerable (Kruk & Penczak 2003). The pregnant influence of Siemianówka and Zegrzyński (Dębe) reservoirs on the Narew River fish customs is as well documented (Penczak et al. 1990a). There are several projects, concepts and scenarios, how to avoid harmful furnishings of Włocławek dam. These incorporate building a new large dam downstream (at Nieszawa) that considers the mitigation of environmental impacts (Hydroprojekt-Warszawa 2004; Romanowski et al. 2005; Van der Sluis et al. 2007; Zalewski et al. 2005), constructing the Lower Vistula Cascade that will include 7 new dams, or removal of the existing dam (KERM 2000; Fiedler-Krukowicz & Żelazo, 2000; WWF 2001; Majewski 2002).

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