Why Do Lake Levels Fluctuate?

Many people wonder why water levels at their favorite lake tend to rise and fall over time. Here's an overview of some factors that influence lake water levels.

The Importance of Precipitation: During periods of heavy precipitation (rainfall, snow, etc.) within the Yadkin watershed, the lakes receive an above average amount of incoming water - primarily from the Yadkin River. As a result, lake levels tend to stay higher.

But lake levels may drop when the amount of water flowing into them begins to decline. Since the Yadkin watershed covers about 4,400 square miles, there may be areas of the watershed that receive rain while other areas do not. Corresponding lake levels respond to precipitation over the entire watershed.

The amount of precipitation generally follows a seasonal cycle, with the greatest amount of water flowing into the lakes in late winter and early spring, and short-term rains in the late summer and fall resulting from tropical storms and hurricanes.

Sending Water Downstream: Alcoa-Yadkin operates its lakes under a license issued by the Federal Energy Regulatory Commission (FERC). Based on the time of year, the license requires Alcoa-Yadkin to send a certain amount of water downstream each week — this provides drinking water for residents of South Carolina, among other uses. (Other FERC licensees have similar requirements. Depending on license conditions, the amount of the required flow may be higher or lower.)

Generating Hydroelectricity: When water is released downstream, Alcoa-Yadkin can generate hydroelectricity as water passes through the generators in the powerhouses. During periods of high precipitation, water may be released downstream through generators or flood gates. Hydroelectricity is one of the cleanest forms of electricity and provides approximately 7 percent of the nation's power. The Yadkin lakes were originally designed and created by Alcoa-Yadkin for the purpose of generating electricity — and that remains an important aspect of the lake operations.

Alcoa-Yadkin works under FERC's direction to balance the needs of all lake users — providing recreational opportunities for boaters and fisherman on the lake; drinking water for those who rely on the Yadkin-Pee Dee River for their water supply; and healthy habitats for fish and wildlife who live in the lakes.

The balance tends to work well most of the time. But unexpected or extended drops in precipitation levels can mean there's not enough water to go around. In those instances, everyone feels the pinch. Lake water levels drop lower, less water is sent downstream, and hydropower generation is reduced.

 

The Concept of a River Basin

A river basin, or watershed, is the entire area of land that is drained by a river. Any water, or pollutant, that occurs in a basin (even if it occurs far away from the river itself) can eventually wind up in the river. There are 17 major river basins across the state of North Carolina. Of those, 13 basins cover land in states other than North Carolina.

The Yadkin Project is contained within the Yadkin River Basin, part of the Yadkin-Pee Dee River Basin. The Yadkin-Pee Dee basin contains many cities — North Wilkesboro, Blowing Rock, Mt. Airy, Mt. Pilot, Winston-Salem, Lexington, Salisbury, Albemarle, Norwood, Troy, Cheraw, Bennetsville, Rockingham, Florence, Darlington and Myrtle Beach.

Roughly 4,000 square miles of land in North Carolina and Virginia drain into High Rock naturally, or unregulated. By comparison, the entire watershed for all Yadkin lakes includes about 4,400 square miles. The entire Yadkin-Pee Dee River Basin (which drains into the Atlantic Ocean south of Myrtle Beach, SC) is 14,989 square miles.