Changes in the Habitat and Fish Community of the Milwaukee River, Wisconsin, Following Removal of the Woolen Mills Dam

We evaluated the response of fish habitat, biotic integrity, and populations of smallmouth bass Micropterus dolomieu and common carp Cyprinus carpio to the 1988 removal of the low-head Woolen Mills Dam from the Milwaukee River. After dam removal, instream habitat improvement work was done on a portion of river within the former impoundment, and a short reach just below the dam site was rerouted for bridge construction. Before dam removal in 1988, the two stations in the impounded reach of the river had fair habitat quality, few smallmouth bass, abundant common carp, and poor biotic integrity. Five years after dam removal, habitat quality was good to excellent, smallmouth bass abundance and biomass had increased substantially, common carp abundance and biomass had declined dramatically, and biotic integrity was good. A reference station on the nearby North Branch of the Milwaukee River had few major changes over the same time period, although common carp abundance and biomass did decline somewhat. Stations upstream and downstream of the impounded reach also experienced moderate declines in common carp populations, and the upstream station had a major increase in smallmouth bass abundance and biomass. Changes in size structure suggested that the smallmouth bass increases in the impounded and upstream stations were caused by increased recruitment rather than by permanent migration of fish from other areas. Habitat quality within the impounded reach increased more and faster in the station with instream improvement work, but the station without work also showed substantial gains in habitat quality through natural channel recovery processes. The station downstream from the dam experienced major declines in its smallmouth bass population and biotic integrity during channel rerouting in 1990. However, by 1993, both attributes had returned to 1988–1989 levels. We conclude that dam removal benefited habitat, fisheries potential, and biotic integrity in the Milwaukee River. Direct habitat improvement activities enhanced these benefits, but natural habitat recovery and removal of the dam as a barrier to fish movement accounted for many of the improvements we observed.