Special MSD Projects
Mitigation of Two Low-Head Dams on the Mill Creek
Cincinnati's Office of Environment & Sustainability (OES) and the Mill Creek Watershed Council of Communities, in partnership with MSD, mitigated two low-head dams on the Mill Creek in fall 2017.
The dams are located adjacent to the 4300 and 4600 blocks of Spring Grove Avenue respectively near Salway Park in the Cincinnati neighborhood of Spring Grove Village. The work began in November 2017 and was completed in December 2017.
The project was funded by a Section 319 Grant awarded to OES through the Ohio EPA, with a local match from MSD.
What is a low-head dam? A low-head dam is generally a concrete structure constructed across the width of a river or stream. These dams normally produce vertical water surface drops of 1-15 feet.
The two low-head dams mitigated by this project were created to protect sewer pipe crossings. Two other low-head dams were previously mitigated along the Mill Creek, one near Center Hill Avenue and another at Hopple Street. There are still several remaining dams along Mill Creek.
What are the impacts of a low-head dam? Low-head dams alter natural habitat and impair a stream’s ability to support a functional ecosystem. Adverse effects of low-head dams include the following:
- Low-head dams block the upstream movement of fish and other species, impacting their reproductive cycle. They change free-flowing river habitat and turn it into pond-like habitat, an environment where fish adapted to free-flowing conditions do not fare well. This usually leads to substantial decreases in the number and types of fish behind a dam.
- Water quality is impaired by low-head dams. Dams create conditions favorable to algal growth by slowing water and trapping sediment and nutrients. This can also significantly deplete the oxygen in the water behind a dam, leading to fish kills.
- Sediment transport is interrupted by low-head dams, aggravating undesirable sedimentation processes. Low-head dams in the Lower Mill Creek can create barriers to bedload transport, inducing aggradation upstream, while simultaneously encouraging erosion immediately downstream of the dam.
- Low-head dams can make recreation hazardous. Dozens of people are injured or killed each year from drowning at low-head dams. Dams block safe navigation on rivers for boaters, and degrade the quality of fish for anglers. Dam failure becomes a possibility as structures age. This can result in flooding, property damage, and even loss of life downstream. Maintenance and repairs for low-head dams can be costly.
How were the low-head dams mitigated? Rock riffles were constructed downstream of each low-head dam to raise the water level and submerge the dam during most flow conditions (see conceptual graphics below). In addition, boulders were placed on the downslope side of the riffle area to introduce additional habitat patches and serve as a fish ladder of sorts, allowing fish to “hide, or rest, while swimming upstream, over the riffle.