Big Eau Pleine Reservoir is a 6348 acre lake located in Marathon County. It was formed in 1937 with the Wisconsin and Eau Pleine Rivers. It has a maximum depth of 46 feet. Visitors have access to the lake from public boat landings, one public land or park within 1000 feet of the lake (Big Eau Pleine Woods State Natural Area). Fish include Musky, Smallmouth Bass, Northern Pike and Walleye. The lake’s water clarity is low.
The reservoir is classified as a drainage lake, with a flowage environment. Its water clarity is low, but contains abundant fish habitat such as stump fields, heavily vegetated bays and backwaters, man-made rip-rap, expansive sand bars and rock flats. Due to its weedgrowth and darker water color, it is classified as a eutrophic lake. Fish kills on the Big Eau Pleine flowage have been a problem since the reservoir was built in 1937, but the fishery is resilient and always seems to recover. The watershed is about 75% agricultural and a number of studies over the years have shown that unpredictable excess nutrient runoff can use up the oxygen and cause the fish to die in the winter.
The Wisconsin River flowage of Big Eau Pleine Reservoir offers multi species anglers great North Central Wisconsin fishing opportunities.