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<br />Table 1. Physical responses to altered flow regimes.
<br />Source(s) of alteration. Hydrologic change(s) Geomorphic response(s) Reference(s)
<br />Dam Capture sediment moving Downstream channel erosion and Chien 1985, Petts 1984, 1985,
<br /> downstream tributary headcutting Williams and Wolman 1984
<br /> Bed armoring (coarsening) Chien 1985
<br />Dam, diversion Reduce magnitude and frequency Deposition of fines in gravel Sear 1995, Stevens et al. 1995
<br /> of high flows
<br /> Channel stabilization and Johnson 1994, Williams and
<br /> narrowing Wolman 1984
<br /> Reduced formation of point bars, Chien 1985, Copp 1989,
<br /> secondary channels, oxbows, Fenner et al. 1985
<br /> and changes in channel planform
<br />Urbanization, tiling, drainage Increase magnitude and frequency Bank erosion and channel widening Hammer 1972
<br /> of high flows
<br /> Downward incision and floodplain Prestegaard 1988
<br /> disconnection
<br /> Reduced infiltration into soil Reduced baseflows Leopold 1968
<br />Levees and channelization Reduce overbank flows Channel restriction causing Daniels 1960, Prestegaard
<br /> downcutting et al. 1994
<br /> Fooodplain deposition and Sparks 1992
<br /> erosion prevented
<br /> Reduced channel migration and Shankman and Drake 1990
<br /> formation of secondary channels
<br />Groundwater pumping Lowered water table levels Streambank erosion and channel Kondolf and Curry 1986
<br /> downcutting after loss of vegetation
<br /> stability
<br />such a disruption, it may take centu-
<br />ries for a new dynamic equilibrium
<br />to be attained by channel and flood-
<br />plain adjustments to the new flow
<br />regime (Petts 1985); in some cases, a
<br />new equilibrium is never attained,
<br />and the channel remains in a state of
<br />continuous recovery from the most
<br />recent flood event (Wolman and
<br />Gerson 1978). These channel and
<br />floodplain adjustments are some-
<br />times overlooked because they can
<br />be confounded with long-term re-
<br />sponses of the channel to changing
<br />climates (e.g., Knox 1972). Recogni-
<br />tion of human-caused physical
<br />changes and associated biological
<br />consequences may require many
<br />years, and physical restoration of
<br />the river ecosystem may call for dra-
<br />matic action (see box on the Grand
<br />Canyon flood, page 774).
<br />Dams, which are the most obvi-
<br />ous direct modifiers of river flow,
<br />capture both low and high flows for
<br />flood control, electrical power gen-
<br />eration, irrigation and municipal
<br />water needs, maintenance of recre-
<br />ational reservoir levels, and naviga-
<br />tion. More than 85% of the inland
<br />waterways within the continental
<br />United States are now artificially
<br />controlled (NRC 1992), including
<br />nearly 1 million km of rivers that are
<br />affected by dams (Echeverria et al.
<br />1989). Dams capture all but the fin-
<br />est sediments moving down a river,
<br />with many severe downstream con-
<br />sequences. For example, sediment-
<br />depleted water released from dams
<br />can erode finer sediments from the
<br />receiving channel. The coarsening of
<br />the streambed can, in turn, reduce
<br />habitat availability for the many
<br />aquatic species living in or using
<br />interstitial spaces. In addition, chan-
<br />nels may erode, or downcut, trigger-
<br />ing rejuvenation of tributaries, which
<br />themselves begin eroding and mi-
<br />grating headward (Chien 1985, Petts
<br />1984). Fine sediments that are con-
<br />tributed by tributaries downstream
<br />of a dam may be deposited between
<br />the coarse particles of the streambed
<br />(e.g., Sear 1995). In the absence of
<br />high flushing flows, species with life
<br />stages that are sensitive to sedimen-
<br />tation, such as the eggs and larvae of
<br />many invertebrates and fish, can suf-
<br />fer high mortality rates.
<br />For many rivers, it is land-use
<br />activities, including timber harvest,
<br />livestock grazing, agriculture, and
<br />urbanization, rather than dams, that
<br />are the primary causes of altered
<br />flow regimes. For example, logging
<br />and the associated building of roads
<br />have contributed greatly to degrada-
<br />tion of salmon streams in the Pacific
<br />Northwest, mainly through effects
<br />on runoff and sediment delivery
<br />(NRC 1996). Converting forest or
<br />prairie lands to agricultural lands
<br />generally decreases soil infiltration
<br />and results in increased overland
<br />flow, channel incision, floodplain iso-
<br />lation, and headward erosion of
<br />stream channels (Prestegaard 1988).
<br />Many agricultural areas were drained
<br />by the construction of ditches or ttle-
<br />and-drain systems, with the result
<br />that many channels have become en-
<br />trenched (Brookes 1988).
<br />These land-use practices, com-
<br />bined with extensive draining of
<br />wetlands or overgrazing, reduce re-
<br />tention of water in watersheds and,
<br />December 1997
<br />773
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