Water School

Why do lake levels fluctuate?

The level of water in all lakes fluctuates, raising during years of higher amounts of rainfall and snowmelt and falling during periods of drought. The Environmental Protection Agency has been measuring the fluctuations of the Great Lakes, the largest natural lakes in the United States, since the 1860s. Similarly, water levels in reservoirs, human-made lakes, fluctuate because of evaporation, a lack of rainfall, water supply use, and water flow requirements. Levels tend to be relatively lower in years of drought and extreme heat because there is no way for the reservoirs to be filled without rain or water being released from a reservoir dam upstream.

“We’ve had some very serious droughts here in Texas over the last 10 years, and it’s because of the water supply reservoirs that we actually had water to keep our faucets running, to keep our power plants running for electricity in our homes and to provide water to crops for food,” said  Brad Brunett, BRA’s lower/central basin regional manager.

Surprisingly, evaporation is the number one cause of a drop in lake levels in the Brazos River basin. The larger the size of the body of water, the more evaporation is likely to occur.

“If lake levels drop, it’s because we have to use them for what they are intended: to ensure that all the people depending on our water supply get the water they need to get through a drought,” said Chris Higgins, a senior hydrologist at the BRA. “The BRA wants its reservoirs to be full just like everyone else. A full reservoir is an indicator of a healthy water supply. We wouldn’t intentionally drop the level of a reservoir just because.” 

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The information provided on this site is intended as background on water within the Brazos River basin. There should be no expectation that this information is all encompassing, complete or in any way examines every aspect of this very complex natural resource.

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