NonBasicSpecificComments04

• Title: Comments 4
• Date: 10/23/2003
• SWSI - Doc Type: Comments

instead" ro ute i t quickly downstreamJ
increasing the size and frequency of
floods and reducing baseflqw levels
during dry periods (Figure 3b; Leo-
pold 1968). 'Over time, these prac-
tices degrade in-channel habitat for
aquatic species. They may also iso-
late the floodplain from overbank
flows I thereby degrading habitat for
riparian species. Similarly, urban~
ization and sub~rbanization associ-
ated with human population expan-
sion across the landscape create
impermeable surfaces that direct,
water away from subsurface path-
ways to overland flow (and often
into storm drains). Consequently,
floods increase in frequency and in-
tensity (Beven 1986), banks erode,
and channels widen (Hammer 1972),

774

and baseflow declines during dry pe~
riods (Figure 3c). .
Whereas dams and diversions af-
fect rivers of virtually all sizes, and
land-use impacts are particularly evi-
dent in headwaterst lowland rivers
are greatly influenced by efforts to
sever channel-floqdplain linkages.
Flood control projects have short-
ened t narrowed, straightened t and
leveed many river systems and cut
the main channels.offfrom their flood-
plains (NRC 1-992). For example t
channelization of the Kissimmee River
above Lake Okeechobee, Florida, by
the US Army Corps of Engineers
transformed a historical 166 km
meandering river with a 1.5 to 3 km
wide floodplain into a 90 km long
canal flowing. through a series of five

impoundments, resulting in great loss
of river channel habitat and adjacent
floodplain wetlands (Toth 1995).
Because levees are designed to pre-
vent increases in the width of fl ow,
rivers respond by cutting deeper
channels, reaching'higher velocities,
or both.
Channelization and wetland
drainage can actually increase the
magnitude of extreme floodsJ be-
cause reduction in upstream storage
capacity res ults in accelerated water
delivery downstream. Much of the
damage caused by the extensive
flooding along the Mississippi River
in 1993 resulted from levee failure as
the river reestablished historic con-
nections to the floodplain. Thus, al-
though elaborate storage dam and
levee systems can "reclaim U the
floodplain for agriculture and hu-
man settlement in most years, the
occasional but inevitable large flo ods
will impose increasingly high disas-
ter costs to society (Faber 1996). The
severing of floodplains from rivers
also stops the processes of sediment
'erosion and deposition that regulate
the topographic diversity of flood-
plains. This - diversity is essential for
'maintaining specie~ diversity on
floodplains J where relatively small
differences in land elevation result in
large differences in annual inunda-
tion and soil moisture regimes, which
regulate plant distribution and abun-
dance (Sparks 1992).

~cological functions of the
natural flow regime

Naturally variable flows create and
maintain the dynamics of in-channel
and floodplain conditions and habi-
tats that are essential to aquatic and
riparian speciesJ as shown schemati-
cally in Figure 4. For purposes of
ill ustrationt we treat the components
of a flow regime individually, al-
though in reality they interact in
complex ways to regulate geomor-
phic and ecolo gical processes. In de-
scribing .the ecological functions as-
sociated with the components of a
flow regimet we pay particular at-
tention to high- and low-flow events,
because they often serve as ecologi-
cal r, bottlenecks" that present criti-
cal stresses and opportunities for a
wide array of riverine species (Poff
and Ward 1989).

BioScience Vol. 47 No. 11