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Sedimentation of Prairie Wetlands

Effects on Hydrologic Functions

The effects of wetland sediments on groundwater hydrology is unknown, but the alteration of the ratio of surface water to ground water hydrology in prairie wetlands is obvious. As the native prairie landscape was converted to cropland, the runoff dynamics of the entire landscape was changed. Surface runoff from snowmelt and storms during presettlement times was moderated by native vegetation, dampening the effect of runoff and increasing the time available for infiltration. Conversion of native prairie grassland to cropland has likely increased the intensity of runoff events and decreased the time available for infiltration. The unusually high variance in water level fluctuations in PPR wetlands in agricultural landscapes found by Euliss and Mushet (1996) was attributed to higher runoff potential of cropland versus grassland. Further, modifications to presettlement surface runoff dynamics result from an extensive road system in the PPR, with roads occurring in both north-south and east-west orientation at roughly 1.61 km (1 mile) intervals. Most of these roads are elevated and many lack adequate culvert systems to pass water through traditional paths of conveyance. As a result, the sheet flow dynamics have been severely altered since settlement of the PPR and the importance of surface flow has greatly increased in recent times. Increased surface flow can exacerbate flooding as was noted by Miller and Nudds (1996) who related intensity of floods in the Mississippi River Valley to landscape change involving conversion of grassland to cropland in the prairies.

Aside from altering the natural ratio of ground to surface water input into wetlands, wetland sedimentation may have altered local groundwater flow patterns. Precipitation that was once lost through evapotranspiration or infiltration to groundwater before entering wetlands in grassland catchments, may now enter wetlands via spates of surface runoff from tilled catchments. These surface runoff spates may transport sediments, nutrients, and other pollutants into wetlands (see Goldsborough and Crumpton 1998). In addition to the alteration of hydrologic inputs, the loss of basin volume from siltation reduces the water storage capacity and flood attenuation benefits of wetlands (Brun et al. 1981; Ludden et al. 1983).

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