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sequences in the wetland (see generally many <br />papers in KUSler 1987). Riverine wetlands <br />subject to normal flood flows have larger <br />flushing rates than those on rivers or streams <br />controlled by dams. vegetation "successional" <br />sequences for shrubs and even trees on riverine <br />wetlands subject to normal flood flows and low <br />water fluctuations are also interrupted by such <br />flows and fluctuations. <br />Sediment regime. <br />A river or stream's sediment regime is <br />determined by the the total flow of water, <br />gradient of the river, its bed materials, flood <br />characteristics including water velocity, the <br />erodibility of its floodplain and watershed <br />materials, and total sediment loading to the <br />river. The sediment regime, in turn, determines <br />the erosion and depositional characteristics of <br />the river including channel, bank, and <br />floodplain characteristics. All of these affect <br />the location, size, width, depth and other <br />characteristics of channel and floodplain <br />wetlands. Sediment regime is affected by dams <br />and reservoirs, channelization, dredging, and <br />land uses in on the floodplain and watersheds <br />(Klimas, this volume; Gagliano, this volume). <br />Contrary to popular belief, sediment is not <br />necessarily the "enemy" of riverine wetland and <br />riparian habitat systems although excessive <br />sediment loadings may destroy habitat values <br />and fill individual wetlands. Studies over the <br />last decade in the Mississippi delta document <br />the importance of continued sediment supply to <br />maintenance of delta wetlands. (e.g., Klimas, <br />this volume; Gagliano, this volume). Equally <br />important, the loss of sediment in headwater <br />streams may increase streambed erosion and <br />cause down-cutting, dropping floodplain water <br />tables and destroying floodplain wetlands. <br />Connections between water/wetland systems. <br />Many of the functions of riverine wetlands <br />depend upon the exchange of water and other <br />materials to and from these wetlands to the <br />rivers and streams and other wetlands. These <br />functions include flood storage and conveyance, <br />fish spawning, food chain support, pollution <br />control, and recreational values. Dams, levees, <br />dikes, water diversions and channelization <br />projects often partially or wholly cut off the <br />movement of water from one portion of a system <br />to another. <br />Why Hydrologic Changes in Rivers Have <br />Severe Impacts Upon Riverine Wetlands <br />The characteristics and functions of all <br />surface water bodies -- lakes, rivers, estuaries, <br />oceans, wetlands -- are dependent upon hydrology <br />including all inflows and outflows, water depths, <br />etc. However, unlike most open water systems where <br />changes in water depth of a foot may make <br />relatively little difference, wetlands are <br />particularly sensitive to small changes in <br />hydrology. The difference in mean surface water <br />elevation of a foot often makes the difference <br />between wetland or no wetland or between marsh and <br />forested wetland. <br />Because river wetlands are so sensitive to <br />changes in hydrology, they are also severely <br />affected by the common alterations to river <br />hydrology. <br />Activities Impacting <br />Rivers and Streams and Adjacent Wetlands <br />Principal activities with major impacts upon <br />rivers and stream hydrology and the hydrology of <br />associated floodplain and watershed lands include <br />(See also Table 1): <br />Dams. <br />Flood control, navigation, water supply and <br />recreational dams have been constructed at <br />periodic intervals in most major rivers and <br />streams in the U.S. Dams and their accompanying <br />reservoirs have often directly displaced large <br />acreages of riverine wetland (see papers <br />throughout this volume). They have also changed <br />flood peaks and hydroperiods in the downstream <br />wetlands, reduced flooding, and reduced <br />sediment loadings (sediment is deposited in the <br />reservoirs). Dams also cause water quality <br />problems by reducing oxygen levels and <br />increasing water temperatures. <br />Dikes and Levees. <br />Flood control dikes and levees have been <br />constructed along many reaches of larger rivers <br />and streams and some smaller rivers and streams <br />across the nation. Dikes and levees directly <br />displace some riverine wetlands and cut off <br />water, sediment and nutrient flows to and from <br />many others. <br />Dredging. <br />Dredging for navigation and flood control <br />purposes has been carried out for many of the <br />larger rivers. Dredging directly destroys wet- <br />lands. Sediments from the dredging operations <br />adversely affects others. Dredge spoil has <br />often been placed in wetlands. <br />Channelization. <br />Channelization has been carried out for many <br />thousands of miles of streams and small creeks <br />in agricultural and urban areas to reduce <br />flooding, reduce groundwater water levels, and <br />reduce streambank erosion and meander. Channel- <br />ization involves channel deepening and, in some <br />instances, construction of concrete channels or <br />installation of bank stabilization measures. <br />Channelization directly destroys some wetlands <br />and results in lowered water levels in others. <br />Hydroperiod and sediment regimes are affected. <br />Agriculture. <br />Agricultural activities are the major cause of <br />stream and floodplain alterations along smaller <br />streams in rural areas of the nation. <br />Alterations are caused by water extractions and <br />diversions for irrigation purposes, channel- <br />9