Silver Lake TMDL


Silver Lake, located in Manitowoc Rapids Township, Manitowoc County, Wisconsin, in the Manitowoc River Basin, was listed as impaired due to excess phosphorus. The Lake is nutrient (phosphorus (P)) impaired as a result of agriculture, internal loading and local land use, is listed on the 1998 303(d) list as a high priority water and external load sources are nonpoint source (NPS) dominated. The designated use for Silver Lake is a full recreation, warm water sport fishery. Pollutant export data and information on the soils, topography, and other background information on the Silver Lake watershed is included in the Nonpoint Source Pollution Control Plan for the Sevenmile-Silver Creek Priority Watershed, dated February 1987.


The water quality of Silver Lake, at the beginning of the water quality improvements, was generally poor to very poor, falling into the eutrophic to hypereutrophic category (see “Nonpoint source control Plan for the Sevenmile – Silver Creek Priority Watershed Plan” (PWP) report. Summer surface water column total phosphorus levels averaged 184 ug/l. Mats of filamentous algae covered a large portion of the lake bottom and summer algal blooms result in foul odors and an unsightly build-up of algae biomass on the shoreline. In addition, trophic conditions in the lake limit rooting depth for emergent vegetation used by the resident fish populations. As a result, impairments affected the recreational/aesthetic value of the lake and stressed sport fish populations.


The site-specific epilimnetic total phosphorus concentration goal was identified as 80 ug/L. Silver Lake is part of a larger priority watershed project, Sevenmile – Silver Creek Priority Watershed Project. As part of a financing plan for priority watershed and priority lake projects, long-term cost sharing and local staff funding is committed to the Sevenmile – Silver Creek Priority Watershed Project. The following activities were conducted:

- The restoration of a former farm to wetlands and natural prairie.
- The completion in the fall of 2001 of a complete diversion of Silver Creek from Silver Lake.
- The eradication and restocking of the fishery budgeted for the summer of 2003.
- The completion of a whole lake alum treatment in the fall of 2003.
- The recent allocation of funds by the Manitowoc County Board to continue implementation.

Study Design

Silver Lake has received considerable best management practices including diverting Silver Creek from the lake, eradication and restocking with desirable fish species, and an alum treatment to seal the deep water sediments from phosphorus release. These management activities have improved the lake to a point where it can now be removed from the impaired waters list. Water quality monitoring has confirmed that the lake has improved since the completion of the restoration efforts. For instance, summer average secchi depth is 3.9 feet based on 32 samples collected between 7/15/2005 and 9/15/2009. Average chlorophyll TSI based on Carlson’s TSI calculations is 55 (‘Fair’ for Deep Seepage Lakes). This is below the natural community impairment threshold for TSI of 63 for deep seepage lakes. (Note: Using the Wisconsin chlorophyll TSI calculation, TSI-chl is 52, and average TSI-secchi is 58.) The average total phosphorus value is .035 mg/l based on May-Oct, 2004-2008 (or 0.034 mg/l based on 25 summer samples collected between 7/15/2005 and 9/15/2009). Although these TP data still exceed the impairment threshold of .02 mg/l for deep seepage lakes, Silver Lake showed improving TP levels from the 2004 alum treatment through 2007 (the 2008 values spiked due to extreme flooding). This lake has undergone extensive treatment and is greatly improved; as evidenced by the TSI-chl. values of 'Fair', it should no longer be considered impaired.

QA Measures

Run Project Summary Report

TMDL/303d Projects
Implement TMDL
Silver Lake TMDL
Reports and Documents
'After a full and complete review, EPA finds that the TMDL for Silver Lake satisfies all of the elements of an approvable TMDL. This document addresses a total of 1 TMDL for 1 waterboy with a total of 2 impairments from the 2002 Wisconsin 303d list.'
Silver Lake Manitowoc Co
Silver Lake Manitowoc County, Silver Island
Final TMDL Report
Activities & Recommendations
TMDL Actions in Wisconsin