Constructed in 1920 – 1922 by impounding the Sangamon River, Lake Decatur has become a centerpiece of the community and essential to both the local and regional economies. It sustains and attracts industry and provides drinking water to over 39,000 metered connections. It is a popular resource for recreation, including fishing and boating. Two water treatment plants on the lake, one operated by the City of Decatur and the other by Archer Daniels Midland (ADM), withdraw approximately 34 million gallons per day. The area of land draining to the lake, or its watershed, is over 590,000-acres in size, touches seven counties, and consists primarily of row crop agriculture.
Since the construction of Lake Decatur, natural-and human-caused erosion severely reduced the lake’s storage capacity. By the time it turned 24, it had lost over 25%. By 1983, the lake had lost almost one third of its capacity. Fortunately, the entire reservoir was dredged from the mid-1990s through 2018, reclaiming and increasing its capacity by 47%. The most recent dredging program removed approximately 10,700,000 cubic yards of sediment at a cost of $92M and reclaimed 2.24 billion gallons of capacity, or 30%. This sediment is stored at the Oakley Sediment Basin that was constructed to dewater dredged material and is located one mile west of the unincorporated Village of Oakley. This 523-acre site was farmland before it was purchased by the city in 1991. Earthen perimeter dikes, almost twenty feet in height, form a roughly 390-acre interior storage basin, which is one and one-half miles long and one-half mile wide.
It is estimated that up to $1M of sediment enters the lake annually based on a dredging cost of $10.23/ton and that most of this is coming from a relatively small portion of the larger watershed. In addition, periodic spikes in nutrients, primarily nitrogen, result in additional water treatment costs of roughly $200,000 per year. To protect this investment and reduce costs, the City of Decatur embarked on an ambitious program in 2021 to supersize existing conservation efforts and measurably reduce sediment and nutrient loading to the reservoir. This includes strategic watershed and in-lake treatments to maximize reductions at the lowest possible cost. These treatments are largely funded through grant programs, most notably a $9.8M United States Department of Agriculture Regional Conservation Partnership Program (RCPP) grant.
In 2022, over 50 partners, including the Economic Development Corporation of Decatur-Macon County collaborated to secure the RCPP funding by leveraging an additional $16M of local resources. Currently underway, this grant is providing generous, targeted financial incentives to watershed farmers that voluntarily adopt practices that help conserve sediment and nutrients and keep them from entering Lake Decatur. The program incentivizes three key practices including cover crops, tillage management, and nutrient management. A cover crop is a temporary vegetative cover planted after harvest that is grown to provide protection for the soil and improve soil conditions. Tillage management focusses on cost-share payments for leaving soil relatively undisturbed from harvest to planting. Finally, nutrient management or the practice of using nutrients essential for plant growth such as nitrogen and phosphorus fertilizers in proper quantities and at appropriate times for optimal economic and environmental benefits. Other program elements include incentives for structural practices to filter and treat nutrients before entering streams, harvestable grasses used for bioenergy and promotion of alternative crops such as hemp and for the transition to organic farming.
Once funds are fully distributed over the next five years, the RCPP will have measurably improved water quality and positively impacted the local and regional economies. The City of Decatur is still seeking partnerships in its efforts to protect Lake Decatur. If you are a business, individual, or area farmer or landowner and would like to participate or learn more about the incentives be offered, as well as the larger watershed program, contact Jennifer Gunter, Watershed and Lake Manager with the City of Decatur at 217-450-2136 or by email at firstname.lastname@example.org.
Additional information on the RCPP and management of Lake Decatur can also be found on the city’s website at https://decaturil.gov/departments/water/lake-decatur-watershed-management-plan/.