Hydrologic & Hydraulic Evaluation of the Proposed Potato Lake Dam

The structure at the outlet of Potato Lake was built in 1938/39 under the WPA program. It consists of a road bridge owned by Hubbard County, and a stop-log dam owned by Minnesota DNR. On April 15, 1947, the DNR directed that all of the stop-logs in the dam be fixed at one height, and that no further operation of the dam be conducted. Over the years DNR acquiesced to continued operation of the dam stop-logs by private parties. In 1999, dam landowner Rich Rothermel decided to quit operating the dam. DNR subsequently installed steel stop-logs to a height of 3.2 feet about the dam sill, and intended to disallow any further operation of the dam. On October 9, 2000, then DNR Deputy Commissioner Steve Morse wrote a letter to State Senator Tony Kinkel and other interested parties concerning Hubbard County's plans to replace the road bridge, and DNR's plans for future management of the outlet dam. Deputy Commissioner Morse said the DNR would allow minimal local operation of the dam for the purpose of a winter drawdown until the dam is replaced. The letter prescribed limits on operation of the dam during this interim period. Senator Kinkel and others expressed concern about DNR's plan to replace the dam with a non-operable structure. DNR Waters' Director Kent Lokkesmoe agreed to provide technical information to the Potato Lake Association comparing the existing dam with a fixed-crest dam design. This report compares lake levels associated with the existing dam to lake levels that would result from three alternative fixed-crest dam designs. The report compares water levels for two time periods – high flows during the summer of 1993, and the more recent period of August 1999 through July 2002. This final version of the report incorporates changes to the design of the proposed dam, in part due to recent discussions with the county engineer and bridge design consultant. DNR Waters staff also met with the board of directors for the Potato Lake Association; additional analyses were requested by the board and are incorporated into this document.