Excessive inflow of sediment and nutrients has contributed to eutrophication of Delavan Lake in southeastern Wisconsin. A wetland restoration project at the mouth of the lake's principal inflowing stream system was implemented in 1992, primarily for the purpose of reducing sediment and nutrient loading to the lake. This function of the wetland system, which includes three retention ponds, was examined in a 32-month study (February 1993-September 1995) that included continuous monitoring of discharge of two inflowing streams and the wetland outflow, intermittent monitoring of discharge of a third inflowing stream, monthly and storm-runoff sampling to determine sediment and nutrient concentrations, and measurement of sediment accumulation in three constructed sediment retention ponds. Inflow and outflow loads of sediment and nutrients were computed from the discharge and concentration data.

The wetland system consistently retained suspended sediments during the study period. Volumetric surveys of the ponds showed sediment accumulation each year, and during the 32 months, 46 percent of the inflow sediment load was retained in the wetland and the retention ponds. Retention of nutrients in the wetland system, however, was of lesser magnitude and much greater seasonal variability. Over time periods of one year or more, most nutrient forms were retained in the wetland system. Nutrient retention, relative to the 32-month inflow loads was: 19 percent of total phosphorus, 11 percent of dissolved orthophosphate, 8 percent of total ammonia plus organic nitrogen, and 0.4 percent of dissolved nitrite plus nitrate nitrogen. Of the nutrient forms measured, only dissolved ammonia, whose 32-month outflow from the wetland system was 22 percent greater than inflow, showed a long-term net release from the system. However, net releases over shorter time periods were commonly observed for all nutrients, and these occurred frequently during the growing season, suggesting probable availability to algal and macrophyte communities downstream. Awareness of the variability and complexity of the nutrient-trapping function of a wetland system can help maintain realistic expectations of the benefits of wetland restoration projects and can be valuable in developing more effective management practices.