Mequon-Thiensville Fishway

Construction Update:

The Ozaukee County Planning and Parks Department (Department) is completing a project to improve the form and function of the Milwaukee River Mequon-Thiensville Dam Fishway, located at river mile 20 in the Village of Thiensville, Ozaukee County, WI. The fishway was initially constructed in 2010 with support from multiple partners (e.g., Department, Wisconsin Department of Natural Resources (WDNR), City of Mequon (City), Village of Thiensville (Village), U.S. Fish and Wildlife Service (USFWS), National Oceanic and Atmospheric Administration (NOAA), and is operated and maintained by the Village and the Department. The original fishway design consisted of a series of pools and weirs within a meandering stream-like channel in the former dam millpond area. Water to the fishway was supplied and controlled at the dam’s sluice gate where it flowed for approximately 1,100 ft. before exiting at the river near the dam’s north spillway abutment under the pedestrian bridge. Fish entered the fishway near the base of the dam and swam through 8 weirs and small resting pools before re-entering the Milwaukee River above the dam. The fishway has been very successful in meeting original project goals, as over 37 species of fish have been documented passing through the fishway to bypass the 6.5’ high dam.

The fishway was reconstructed in 2021-2022 to accommodate larger-bodied fish and to reduce ongoing erosion and maintenance needs. The revised fishway design removed the weirs and now consists of approximately 650 feet of rock-lined riffles and pools (resting areas). Water depths throughout the fishway have been increased along with a reduction in water velocities to make fish passage conditions more suitable for a variety of species, especially for Lake Sturgeon, as they continue to return to the Milwaukee River to spawn.

Construction activities were completed by the Ozaukee County Highway Department in cooperation with the Planning and Parks Department and Village. The project is supported by grants from the USFWS, National Fish and Wildlife Sustain Our Great Lakes Program, WDNR, and the Fund For Lake Michigan in addition to in-kind support from the Department and the Village. For more information and to view the live underwater fishway camera, visit ozaukeefishway.org.

The Mequon-Thiensville Dam was constructed in 1882 on the border of the City of Mequon and Village of Thiensville. The M-T Dam was constructed to provide hydropower for a grist mill. Historic records indicate the dam was breached and rebuilt multiple times. A WDNR dam inspection found multiple structural deficiencies with the M-T Dam. The current dam has a structural height of 13 feet and a hydraulic height (difference between water elevation upstream and downstream of the structure) of 6.5 feet. The dam forms a 700 acre impoundment, the largest in Ozaukee County. The Fish Passage Program partnered with the City of Mequon, Village of Thiensville, WDNR, U.S. Fish and Wildlife Service, and others to construct a nature-like fishway in the abandoned millrace adjacent to the dam in 2010.

Fish and other aquatic life are now able to navigate upstream past the 6.5 foot high dam through a series of
pools (resting areas), runs and riffles constructed in an 800 linear foot, nature-like meandering stream channel. The fishway entrance is located downstream of the north dam abutment and the fishway exit is immediately upstream of the dam in the impoundment.

The original settlers to the area were drawn, in large part, to the Milwaukee River. The river provided a rich source of fish and game, transportation, and water power. The actual construction of the dam and flour mill started after John Weston sold his property to John Henry Thien in the summer of 1842. In all likelihood, the dam and foundation for the flour mill were built simultaneously during the low water of late summer of 1842. Recognizing the power offered by the 6.5 foot tall raceway impoundment, Thien added a sawmill between the dam and his flour mill in 1843.

The first flour mill, a three story wooden structure, burned to the ground in 1874 during one of Thiensville’s frequent bog fires. A second flour mill, a five story stone structure, was built in 1876 and served the farming community until just after World War II. The building fell into disrepair and was eventually razed in 1956. The dam also fell into disrepair following the failure of the Thiensville Milling Company in 1939. Recognizing the importance of the impoundment to recreation in the area and the value of over 200 properties upstream, ownership of the dam was transferred to the Mequon Township Advancement Association in 1941. In 1962, the group now known as the Mequon-Thiensville Advancement Association transferred ownership of the dam to the City of Mequon (75%) and the Village of Thiensville (25%). That shared ownership of the dam and raceway (which became a fishway in 2011) remains in effect today.

The village of Thiensville and U.S. Fish and Wildlife Service installed an underwater camera and two passive integrated transponder (PIT) tag readers in 2011. The camera and tag readers, along with extensive fish tagging by Ozaukee County and WDNR, have indicated successful passage of thousands of fish from dozens of species, including many tagged fish.

Anticipated long-term results include access to historic spawning and rearing habitat, increased probability of restoring sustainable populations of remnant and/or imperiled fish species, elimination of larval fish mortality below the dam spillway, increased genetic diversity of fisheries populations, and improved recreational opportunities for local anglers.

Mequon-Thiensville Fishway

Construction Update:

Before

After

Mequon-Thiensville Dam 1844

Mequon-Thiensville Dam 1910

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