Clear Lake, a 257 ha body of water located in south-central Minnesota, is a heavily utilized recreational lake which has become eutrophic because of the inflow of nutrient-rich runoff water from the adjacent City of Waseca. In 1981, 50 percent of the hydraulic load and 55 percent of the phosphorus load to the lake was diverted into a 21.4 ha marsh. The marsh system reduced the annual phosphorus load to Clear lake by 34 percent (768 kg). In 1986 construction was completed on a second marsh system which will filter urban and agricultural runoff carrying 20 percent of the phosphorus load into Clear Lake. The mean total phosphorus concentration in Clear lake has been reduced 31 percent, from 158 ug/L to 109 ug/L, since the diversion in 1981. The total nitrogen:total phosphorus ratio increased from 10:1 to 18:1 since the diversion began. Neither the Secchi disk depth nor the chlorophyllaconcentration was correlated with the reduced total phosphorus concentration in the lake. The control of internal loading will be necessary to eliminate nuisance algae conditions in Clear Lake.