Suspended-sediment yields in the Big Eau Pleine River basin are low to moderate in comparison with other drainage basins in Wisconsin. Average annual yield in the Big Eau Pleine River near Stratford is 32.tons per square mile, with an annual yield ranging from 1.0 to 64 tons per square mile. Fenwood Creek at Bradley and Freeman Creek at Halder, two smaller tributary basins, have average annual yields of 3.3 and 7.9 tons per square mile, respectively. Suspended-sediment concentrations in the basin ranged from 0 to 960 milligrams per liter, with a median concentration at the Stratford site of 13 milligrams per liter. Ninety percent of the material transported by the streams of the Big Eau Pleine basin is finer than sand and is made up of about equal percentages of silt and clay.
A l3-year average of about 9,400 tons per year of suspended sediment is transported to the Big Eau Pleine Reservoir by streamflow and overland flow, whereas about 2,500 tons per year leaves the reservoir in its outflow. Considering only sediment inflow and outflow by streamflow and overland flow, and assuming the 2,500 tons per year is approximately the long-term average, the reservoir trap efficiency would be greater than 70 percent. The actual trap efficiency of the Big Eau Pleine Reservoir probably is somewhat higher than 70 percent because bedload transport of the streams and sediment inflow from shoreline erosion were not measured.
