News

Understanding Seneca Lake Water Levels

By Tim Johnson, Pure Waters Board Member, Chair of the Lake Level and Education Committees

HIMORD, N.Y. – Water levels in Seneca Lake are a recurring concern for residents, boaters, farmers, and shoreline property owners. Keep reading to explore the main factors affecting the lake’s level and how rainfall, runoff, and human control interact to shape the lake’s daily and seasonal changes.

The most direct and visible input is rainfall. Rainwater either soaks into the ground or flows over it, eventually reaching the lake through tributaries and streams. The Seneca Lake Watershed covers about 457 square miles, collecting precipitation from as far west as Keuka Lake and from the highlands south of Watkins Glen. A widespread rainfall of just 1.5 inches can deliver tens of billions of gallons into the system, depending on how much becomes runoff. Since the lake itself is about 67 square miles, there’s a simple multiplication factor of nearly 7:1 between watershed rainfall and lake rise—if all of it reaches the lake without outflow.

Not all rainfall ends up in the lake. The runoff coefficient—ranging from about 0.1 for dry, forested land to over 0.6 for saturated or developed areas—determines how much flows directly into the streams and, ultimately, the lake. During dry spells, much of the rain is absorbed by soil or taken up by vegetation. After a series of storms or snowmelt, however, even modest rainfall can generate rapid runoff and a noticeable lake rise. Soil type, slope, and vegetation also influence runoff. Urbanized areas with paved surfaces produce far more runoff than forested or agricultural zones. In spring and fall, when plant uptake is lower, more water makes it to the lake.

Several major tributaries feed Seneca Lake, including Catharine Creek from the south and the Keuka Lake Outlet from the west. These inflows serve as highways for rainfall from their sub-watersheds. Their flow can vary by the hour and is often monitored by USGS stream gauges. The Keuka Lake Outlet is regulated by gates in Penn Yan to control Keuka Lake levels. Sudden inflows from these tributaries—especially when paired with rainfall directly over the lake—can raise water levels quickly, as seen on the east side in May 2011.

Lake level is naturally seasonal. Spring tends to bring higher levels from snowmelt and rain. Summer often sees stabilization or mild decline due to evaporation and lower runoff. Fall rains may cause temporary bumps, while winter levels are often held steady for flood control. Long-term climate trends—such as wetter springs or more intense summer storms—can gradually shift these patterns. The Hansen Curve shown depicts target seasonal lake levels alongside actual levels.

Pure Waters offers the Seneca Lake Alert text messaging system to notify subscribers about possible hazardous conditions, including Harmful Algal Blooms (HABs) and lake level changes. Alerts help you stay informed so you can monitor conditions and take action if needed. You’ll receive just one text per day to keep things manageable.

We’re working on improving predictive tools for Seneca Lake—stay tuned! If you have ideas, we’re all ears.

Learn more about Pure Waters

To learn more about lake level: https://senecalake.org/about/seneca-lake/lake-level/
To learn more about donating: https://senecalake.org/giving/

Seneca Lake Pure Waters Association is a 501(c)3 organization committed to preserving, protecting, and promoting the abundant resources of the largest Finger Lake. Seneca Lake is a source of drinking water, recreation, and economic opportunity for the greater community and its visitors. Our programs and partnerships make it possible to monitor water quality, educate the public, and evaluate threats to ensure the a healthy future for Seneca Lake and those who depend on it.