HYDROLOGICAL FUNCTIONS

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.

Streamflow

Wetlands 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 1993).

Wetlands 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).

When 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 1986).

One 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).

The 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).