Wetlands have many important hydrological functions. They effect
both ground and surface water supplies. They recharge aquifers;
serve as surface water sources for wildlife, human consumption,
recreation, agricultural irrigation, and industrial processes;
and act as cleaning filters for the water that passes through
them. Of all the hydrological functions wetlands perform, however,
the effects they have on the hydrology of the surface waters in
their specific watershed are perhaps the most pronounced.
help reduce the effects of both flood and drought conditions in
a watershed. They function like sponges, storing and releasing
water relative to the amount of water around them. Some wetlands
which have a relatively stable hydrology may routinely retain
a specific amount of water. During dry periods they may lose some
of this water to surrounding parched areas and through evaporation
and transpiration. During wet periods they may also have the capacity
to store more water than they usually contain (Demissie and Kahn
associated with Riverine systems serve as floodways, transporting
flood pulses from upstream to downstream locations while lessening
flood peaks. As flow rates increase in a given stream and the
water level rises, wetlands adjacent to a stream "soak up" some
of the overflow. This occurs through the filling of backwater
lakes or other low lying depressions in the floodplain and by
the saturation of soil in otherwise dry regions in the floodplain.
This absorption and storage of excess water lowers the overall
amount of flow that would otherwise be carried by a stream (Illinois
Department of Natural Resources 1994).
the flow of a stream begins to drop below its normal level, adjacent
wetlands drain back into the main stream and augment its flow.
These floodplain wetlands work cooperatively with wetlands outside
of the floodplain which store flood waters at higher elevations
and slowly deliver it downstream. Consequently the water that
enters the stream through this process is much cleaner than when
it entered the wetland due to the settling out of sediments and
the biological uptake of certain constituents by the hydrophytic
plants and micro-organisms contained in the wetlands (Illinois
Department of Natural Resources 1994 and Mitsch and Gosselink
of the most thorough studies on the effects wetlands have on streamflows
was released by the Illinois State Water Survey in 1993. This
study was conducted in 30 watersheds throughout Illinois over
10 years and included sample sites on streams of various sizes.
The report revealed how wetlands affect the peakflows, lowflows,
and floodflows of streams. For this study,
peakflow was defined as the maximum flow of a stream
for average precipitation;
lowflow was defined as the average minimum flow of
a stream; and floodflow
was defined as the flow of a stream at peak precipitation (Demissie
and Kahn 1993).
study concluded on average for every one percent of a watershed that existed
as wetlands in the northern region of the state, the peakflow of streams
in that watershed was decreased by 7.9 percent; the floodflow was decreased
by 2.3 percent; and the lowflow was increased by 15.0 percent. In the
central region of the state, the peakflow was decreased by 5.9 percent;
the floodflow was decreased by 4.5 percent; and the lowflow was increased
by 5.5 percent. In the southern region of the state, the peakflow was
decreased by 0.8 percent; there was no significant change in the floodflow;
and the lowflow was increased by 15.9 percent. The study also concluded
for every one percentage of a watershed that existed as wetlands, there
was a statewide average 3.7 percent decrease in peakflow; 1.4 percent
decrease in floodflow; and 7.9 percent increase in lowflow. These results
are outlined in Table 2-1 below (Demissie and Kahn 1993).
change in flow rates of streams for
every one percent of watershed present as wetland.
and Kahn 1993).
Introduction | Biological
| Threatened & Endangered Species
| Hydrological Functions | Water
Quality | Ground Water Recharge
| Terrestrial Functions | Aesthetics
& Recreation | Economics
| Conclusion ]
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