TRENDS IN RECENT RESERVOIR SEDIMENTATION RATES IN SOUTHWESTERN OHIO

Soil erosion rates on U.S. cropland are declining. This decline can be expected to have two effects on sediment yield: i) an overall decrease in sediment yield; and 2) an increasing importance of channel erosion relative to upland sediment sources. Here we examine recent sedimentation rates in 12 reservoirs in two southwest Ohio counties to determine whether sedimentation rates are declining, and whether there is evidence for a shift to channel sediment sources. One of the two counties is primarily urban with high local relief, while the other is primarily agricultural with low relief. In each case a large range of reservoir sizes was studied ( 70 km 2 drainage area or 24 mi 2 ). Previous sediment surveys are available for some of the reservoirs studied. Measured sedimentation rates range from 1200 m 3 km -2 yr -1 ( 2.5 mi -2 yr -2 ). Sedimentation rates are generally higher in the urban county than the agricultural one. Comparison of recent (since ∼1980) sedimentation rates with those from the mid-20th Century indicate that sedimentation rates are declining in the agricultural county, presumably as a result of improved soil conservation practices, while those in the urban county remain high. In the agricultural county sedimentation rates follow the normal pattern of decreasing sedimentation per unit drainage area with increasing drainage area, indicating that the channel systems in the area are likely functioning as sediment sinks. In contrast, in the urban county there is no decrease in sedimentation rate with increasing drainage area, indicating that the channels in that landscape are functioning as efficient conduits and/or net sediment sources. These trends highlight the need to update our reservoir sedimentation databases to determine whether the trends are local or widespread.