Spotlight on Mansfield Hollow Lake, a Different Kind of Lake Mansfield Hollow Lake is an engineered impoundment of the Natchaug, Mount Hope and Fenton Rivers that resulted from the installation of a flood control project. The main function of the basin behind the dam is for flood control, although the lake is also used for many popular recreational activities. The CT DEEP Fisheries Division manages a northern pike nursery near one of the lake inlets and a boat launch for public use. The floodplain behind the dam is managed by the CT DEEP as Mansfield Hollow State Park. Swimming in the lake prohibited in the park by Connecticut State Statute section 25-43 due to its proximity to a downstream surface water drinking water intake. Landing of pontoon planes on the lake is also a prohibited activity under this legislation. Construction of the Mansfield Hollow Dam by the US Army Corp of Engineers was completed in 1952. The lake that formed behind the dam in Mansfield Hollow stretches over 450 acres through Mansfield, Windham, and Chaplin. It has a maximum depth of 16.5 feet. The flood storage area of the project, which is normally empty and is only utilized to store floodwaters, totals 1,880 acres and extends about three miles up the Natchaug River, 2.3 miles up the Mount Hope River, and 3.2 miles up the Fenton River. The project and associated lands cover 2,581 acres. Mansfield Hollow Lake can store up to 8.3 billion gallons of water for flood control purposes. This is equivalent to 6.1 inches of water covering its drainage area of 159 square miles. (Source: US Army Corp on Engineers Flood-Risk-Management, Mansfield-Hollow, Connecticut.) As the receiving body of water for 159 square miles of upstream stormwater runoff, the water quality of the lake appears to be good condition when compared to a similar flood control project in nearby Thompson, CT that went into service in 1965. The drainage area of the Quinebaug River upstream of West Thompson Lake is 173.5 square miles. (Source: US Army Corp on Engineers Flood-Risk-Management, West Thompson, Connecticut. Two major differences between the watersheds for Mansfield Hollow Lake and West Thompson Lake are: 1. The drainage area upstream of West Thompson Lake includes more urban development, and 2. The Quinebaug River is a waste receiving stream with multiple industrial and wastewater treatment plant effluent permitted point source discharges outletting into the river. Waste water discharges into the rivers are prohibited upstream of Mansfield Hollow Lake because the lake is upstream of a surface water intake for drinking water. West Thompson Lake has been impacted by cyanobacteria blooms in late summer. This has been studied by the USGSi. There have been no reports of similar issues to date in Mansfield Hollow Lake, but there is a lack of data on the current water quality conditions in Mansfield Hollow Lake. As a receiving body of water for stormwater runoff from a predominantly forested watershed, the natural aging of this lake is ongoing, but at a considerably slower pace than in West Thompson Lake. Water quality monitoring in Mansfield Hollow Lake would be a useful measurement of the watershed health of the Natchaug River system. Non-point source pollution is the main source of nutrients and other pollutants impacting water quality in Mansfield Hollow Lake. Efforts to manage stormwater runoff from developed areas, agriculture and construction areas in the upstream communities is critically important for protecting the water quality conditions in Mansfield Hollow Lake. For historical perspective, below are aerial views of the watershed in the Mansfield Hollow area before and after construction of the Mansfield Hollow Dam. Imperfect composite of the 1934 aerial Aerial view of Mansfield Hollow Lake in 2018. images of the Mansfield Hollow Area from Source Connecticut Environmental Conditions the UCONN Magic website online map service. The Mansfield Hollow Dam, and the downstream dam for the Willimantic Reservoir, act to separate the upper and lower Natchaug watersheds into isolated fish habitats, restricting fish migration from Long Island Sound to the upper reaches of the Natchaug watershed. The United States Geological Survey (USGS) has been maintaining a stream flow gage in the Natchaug River downstream of where the Mansfield Hollow Dam is located since 1931. The highest peak flow recorded for that gage was on September 31, 1938 following the devastating “Hurricane of ‘38”. The gage height read 16.39 feet and the stream flow was recorded at 32,000 cubic feet per second. On August 27, 1955, after back to back hurricanes deluged Connecticut with heavy rainfall that caused catastrophic flooding in many parts of Connecticut, the highest peak flow at that same gage, now influenced of the Mansfield Hollow Flood Control Dam was 8.58 feet and the stream flow was recorded at 3,250 cubic feet per second. This indicates that the dam at Mansfield Hollow was able to mitigate the peak stream flow from Natchaug River at Willimantic. (Email communication from Jon Morrison, USGS Senior Hydrologist dated September 23, 2020.) i Nutrient Loading and Algal Response in West Thompson Lake, Thompson, Connecticut, 2003-2005 Scientific Investigations Report 2008-5072 Prepared in cooperation with the Connecticut Department of Environmental Protection By: Jonathan Morrison and Michael J. Colombo .