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-
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