Maintaining Reservoirs

All, Nature Notes

As described in February’s blog, “What’s a Reservoir?,” Winton Lake, Miami Whitewater Forest Lake and Sharon Lake are man-made bodies of water called reservoirs that were constructed in the mid-20th century for recreation and flood mitigation. Although reservoirs (also called ‘impoundments’) and natural lakes share many of the same problems, certain concerns may be amplified in reservoirs and must be successfully managed to maintain boating, fishing and wildlife viewing opportunities.

According to the North American Lake Management Society (NALMS), impoundments are planned and situated to meet specific needs, such as water supply or flood storage. To accommodate these needs, they are often constructed within large watersheds (the land area that drains to the lake, stream or impoundment), because large watersheds can deliver high volumes of water to the reservoir (NALMS 2001). However, these large volumes of water can carry high and harmful amounts of sediment and nutrients, especially when land use in the area is urban or is undergoing rapid development.

High sediment and nutrient delivery rates into a reservoir can have profound impacts on the water chemistry, the aquatic ecosystem and the reservoir depth. For instance, excess nutrients like nitrogen and phosphorus that are carried in sediments eroding from streambanks can produce Harmful Algal Blooms (HABs) and unsightly algal mats that cover the water’s surface. Increased sedimentation in impoundments from the surrounding watershed can quickly – and dramatically – decrease the depth of the body of water, degrade fish habitat, decrease boat maneuverability and diminish water clarity. Aquatic vegetation like duckweed might also increase, reducing species diversity and making recreation difficult (As seen in figure 1).

Figure 1. Common duckweed (Lemna minor) as seen floating on Sharon Lake.

Historically, as one tactic to address accumulated sediment in reservoirs, Great Parks has utilized dredging. Dredging is the challenging task of removing large amounts of sediment from the bottom of a lake or impoundment, which, in turn, can produce benefits such as increased biodiversity. Depending on equipment availability and cost considerations, dredging can occur when the reservoir is filled or empty and may employ backhoes, excavators or suction pumps. Winton Lake was dredged in 1995-1996, when 930,000 cubic yards of silt were removed, and Sharon Lake was dredged in 1988.

In order to improve water quality and the aquatic habitat, deepen the reservoir and reduce aquatic vegetation like duckweed, Sharon Lake needs to be dredged again, and Great Parks has received funding support from federal, state and local sources in order to undertake this effort. Most recently, Great Parks was conditionally awarded $500,000 in funding through the Land & Water Conservation Fund program for the project’s construction phase. This project was also funded in part by a grant from the Greater Cincinnati Foundation and through State Capital Funds. Project design and engineering is beginning this year, and dredging is expected to begin at the end of 2020. You can review Great Parks’ 2018 feasibility study for more information on this project.

Figure 2. An example of dry dredging, which involves exposing the lake bed and using heavy equipment to remove sediment (photo from Great Parks’ 2018 feasibility study).

Although dredging can help mitigate many problems arising from nutrient and sediment loads into reservoirs, the best way to resolve the issue is to decrease or eliminate inputs from the watershed. Some ways you can help decrease the amount of nutrients and sediment entering Hamilton County reservoirs is by reducing fertilizer use on your lawn, using phosphate-free fertilizers, planting native trees and vegetation to prevent erosion, installing a rain garden, and properly maintaining your septic system.

For questions about Great Parks of Hamilton County reservoirs, please contact Watershed Specialist Amanda Nurre at  513-728-3551, ext. 274.


Amanda Nurre
Watershed Specialist

 

Citations
North American Lake Management Society (NALMS) and Terrene Institute. 2001. Managing Lakes and Reservoirs, 3rd ed. North American Lake Management Society, Madison, WI, and Terrene Institute, Alexandria, VA.