On Tuesday, July 20th, Seneca Lake’s lake-level climbed to a recent high of 446.68 feet above sea level, almost five inches above the Summer upper target of 446.3 feet. Since then, the lake level has fallen to the Gordon Hansen Seneca Lake Rule Curve target range of 446 +/- .3 feet.
The Seneca Lake Rule Curve, developed in the mid-70s by Gordon Hansen of the Navy’s Underwater System Center at Dresden, defines the lake’s highest and lowest tolerable levels. Rule curves establish a target range for the water level, helping to prevent severe flooding or severe drought conditions, and ensuring a stable, reliable water supply throughout low water summer months and protecting damage to delicate natural resources during high water months.
For reference, the Rule Curve for Seneca Lake predicts minor flood damage at about 447.8 feet, and major flood damage at 448.5 feet. The 100-year flood level for Seneca Lake is 449.9 feet, which has not been reached since the first Seneca Lake level record in October 1 of 1956. After the Dundee/Lodi storms in 2018, the lake rose about 12 inches in 12 hours topping out at 447 feet. Seneca Lake levels have reached to 447+ feet eight times since USGS began recording lake level data.
Each finger lake has a Rule Curve to guide its water level management, but there is only voluntary coordination among the lakes to manage both lake level and water discharge from each lake. Water can flow into Seneca Lake faster than it can flow out, since the downstream area is relatively flat and the outflow is regulated; therefore, the lake level rises after a rapid snow melt or big rain event.
The outflow of Keuka Lake, which drains directly into Seneca via the Keuka Outlet, is controlled by the Keuka Lake Outlet Compact (KLOC)—an intermunicipal organization representing the eight municipalities around Keuka Lake. Its primary purpose is to regulate Keuka Lake level using six outlet gates located at the Main Street bridge in Penn Yan, NY.
To minimize the risk of downstream flooding, the "Reservoir Regulation Manual-Keuka Lake Outlet" (developed by the Army Corps of Engineers), limits the discharge rate to 1,000 cubic feet per second (cfs). To achieve this maximum discharge rate, the Keuka Lake level must be at least 716.03 feet with all six gates open. On July 17th and 18th, the Keuka Outlet gage at Dresden, NY recorded two consecutive hours of more than 1,000 cfs (1,020 cfs to 1,300 cfs) which is due to both Keuka Lake outflow and precipitation in the Keuka Lake Outlet subwatershed. Although the Keuka Outlet was flowing high on these dates, there were no reports of significant damage due to the controlled outflow from Keuka Lake (Both residents and businesses on the west side of Seneca Lake near Himrod saw property damage during and after the recent rainfall event. Most reported damage was due to small stream spillover).
Visit the Pure Waters’ lake level webpage to see daily updates on Seneca Lake level (https://senecalake.org/Lake-Level) and to learn more about lake level management.