Toronto Lake
Introduction
Toronto Lake is a Corps of Engineers reservoir located in southeastern Kansas, in the Verdigris River Basin (figure 1). The Toronto Reservoir watershed encompasses an area of 730 square miles and includes parts of Chase, Coffey, Greenwood, Lyon, and Woodson Counties. (Powell, 2011b)(figure 2). Land use in the Toronto watershed consists of, 71% grasslands, 19% crop production, with the remaining 10% split between woodland, water, and urban areas (Homer et al., 2001) (figure 3).
Toronto Reservoir was authorized by Congress in August 1941, for the purpose of flood control (Kansas Biological Survey, 2011). Construction of the reservoir began in November 1954, and it was deemed fully operational for flood control in March 1960 (Kansas Biological Survey, 2011). Aside from flood control, the lake is also federally authorized for water conservation, water supply, water quality, fish and wildlife conservation, and recreation (Powell, 2011b).
Figure 2. Map of the Verdigris River Basin (KWO, 2012b)
Figure 3. Map of the Toronto Reservoir Watershed (Nejadhashemi, 2009)
Figure 4. Map of Toronto Reservoir watershed land use (Homer et al., 2007)
Sedimentation
Toronto Lake is relatively shallow with high inorganic turbidity and high levels of sedimentation (Kansas Department of Health and Environment (a)). Sedimentation of the reservoir is exacerbated during large runoff events, when water is held in the reservoir to prevent flooding downstream (KDHE(a)). These runoff events cause large deposits of silt within the lake and the inflowing river channel (KDHE(a)). Later runoff events then transport the deposited sediments into the lake where it settles out (KDHE(a)).
Toronto Lake was surveyed by the Corps of Engineers in 1960. This survey revealed the area of the conservation pool at 901.5 feet to be 2,879 acres, and the storage capacity to be 27,320 acre-feet (KWO, 2012b). A Bathymetric survey of the lake was carried out in May 2010 by the Kansas Biological Survey (KBS, 2011)(figure 4). This survey revealed the area of the conservation pool at 901.5 feet to be 2224 acres, with a volume of 16,507 acre-feet (KBS, 2011). Comparison of these surveys indicate a loss of 10,813 acre-feet of storage and 655 acres in area at a conservation pool of 901.5 feet. (KBS, 2011). This represents a total capacity loss of 39.50% over the 50 year time span.
Figure 5. Bathymetric map of water depth in feet for Toronto Reservoir (KBS, 2011)
As part of the 2010 KBS survey, core samples were taken at 14 sites throughout the reservoir to determine the areas of highest sediment accumulation (KBS, 2011). The survey revealed that the area of highest sediment accumulation occurred at two sites north of an island that may serve as a constriction point where slower water velocity encourages sediment deposition (KBS, 2011)(figure 5). High sediment thickness was also recorded near the dam, which is typical for large reservoirs in the area (KBS, 2011). Sediment thickness was relatively low throughout the rest of the reservoir (KBS, 2011).
Figure 6. Sediment thickness, in centimeters, at 14 coring sites in Toronto Reservoir (KBS, 2011)