A CASE STUDY IN LOCAL MANAGEMENT PRACTICES: SEDIMENT AND NUTRIENT MOBILIZATION DURING RESERVOIR DRAWDOWN IN DOOR COUNTY, WISCONSIN

The Forestville Millpond is located west of the Village of Forestville in Door County, Wisconsin. The Millpond was formed in 1877 when the Ahnapee River was dammed to provide mechanical power for the mill and later served for recreation. However, eutrophication of the reservoir lead to calls for action at the local level to improve water quality. After consultation with state and county stakeholders, it was decided that the water level in the Millpond would be drawn down between November 2019 and September 2021 - effectively returning the river to channelized flow in the reach with the aim of compacting the accumulated sediment and removing undesirable aquatic species. However, compaction did not occur as expected because the single sluice valve on the dam was inundated during high intensity rain events. At least 12 such events occurred during the drawdown period causing the Millpond to refill and mobilize sediments and other contaminants. In response, we monitored suspended sediments (SS) and total phosphorus (TP) concentrations monthly between October 2020 and September 2021 at five sites along the Ahnapee River both above and below the Forestville dam. Suspended sediment and TP concentrations both increased sharply below the Forestville Dam during our study and vary temporally related to precipitation and water level in the Millpond. 32% of all monthly samples analyzed had SS concentrations in excess of the average monthly level generally permitted in wastewater effluent (20 mg/L) and 39% of TP samples were in excess of the Wisconsin’s Phosphorus Water Quality Standards for streams (0.075 mg/L). However, TP concentrations over the course of the study were within the range of values found previous to the drawdown while SS concentrations in the downstream reach are much higher than previously reported values indicating a change occurred within the Ahnapee-Millpond system due to the drawdown. Rain event sampling indicates that mobilization occurs during the rising and falling of water levels in the Millpond.