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Why the lake level varies

Why the lake level varies

Not all dams are the same.

And not all lakes are the same – but not for the reason you might think.

Texas is notorious for its hot, humid and dry summers, as well as its thunderstorms and flooding. Everything is bigger in Texas, right? The combination of the extremes creates an interesting and complex situation for the Brazos River Authority’s three reservoirs. 

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Photo of Lake Limestone courtesy of Dennis McCain

Possum Kingdom Lake, Lake Granbury, and Lake Limestone are water supply reservoirs. This means the water stored in each lake is there to supply water to thirsty Texans year-round. 

Yes, each lake draws its fair share of recreationists fishing, boating, swimming, camping, and so forth. But recreation has always been a secondary benefit of the three reservoirs owned and operated by the BRA.

A water supply reservoir’s primary purpose is to store water during wet times for use during dry periods -- be it for municipal, mining, irrigation, or industrial use, said Chris Higgins, a senior hydrologist at the BRA. The water is used by those living on or near the reservoir via an intake from the lake, or downstream of the reservoir, and along tributaries and the river.

But the Brazos River stretches from Lubbock all the way to the Gulf?

That’s right, which is why those three reservoirs are part of a larger system that covers the entire Brazos River basin. That system includes water stored in eight federal flood control reservoirs within the basin.  

So, when a city, business or farmer needs to access water through their water supply contract but is located too far from a reservoir to access stored water directly from the lake, BRA hydrologists work to determine which reservoir can or should release water, Higgins said. 

But it’s not as simple as releasing X amount of water. Evaluations are made of current lake levels, the amount of water flowing into the lakes, the amount of water being used locally from the lakes, the travel time for released water to reach its destination, and the amount of water that would be lost to evaporation or absorbed by the stream channel, Higgins said. Ultimately, these release decisions are made to maximize the use of the water supplies while balancing adverse impacts.

“Reservoir elevation is an indicator of the amount of water that BRA has in storage to meet the contractual needs of its water supply customers; so, naturally, the BRA desires for all of the reservoir to be full just like everyone else,” Higgins said. “However, because our reservoirs were built to supply water, elevations will decline during dry times as the reservoir is utilized for its originally intended purpose.”

Because as Texans know, once the rain stops for the summer, it can be a while before enough falls to refill the lakes. As temperatures increase, water usage and evaporation rates increase, making the water in the reservoir vital. But this also means the lake level begins to drop in the summer as the reservoir gives up its storage to meet supply needs. 
“Because of these factors, very few man-made reservoirs have the ability to be maintained at a constant level,” Higgins said.

Higgins said that a lack of inflows and evaporation from the reservoir is typically the biggest reason for a reservoir to decline. But each lake is different. 

Lake Somerville, for instance, only has one customer, so there’s only one straw in the reservoir. Therefore, its decline is hardly due to local demands. On the other hand, Lake Georgetown and Lake Granbury each have relatively high local demands on the reservoirs, Higgins said. These and other reservoirs that have similarly high demands are more highly impacted by local water use. 

But it’s worth noting that even with high local demand, evaporation continues to be a significant component for water loss in a reservoir, Higgins said.

The three BRA reservoirs also don’t have something the Corps of Engineers dams have: flood control.

The Brazos River’s history, like that of other waterways around the state, was periodically marked by devastating floods that took numerous lives and destroyed homes, businesses, and agricultural land.

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Possum Kingdom Lake's Morris Sheppard Dam

So in 1944, Congress passed the Flood Control Act, which tasked the U.S. Army Corps of Engineers to build reservoirs around the nation to alleviate flooding. Among other places, the Corps went to work on the Brazos and its tributaries. They now own and operate Aquilla, Whitney, Georgetown, Stillhouse Hollow, Belton, Granger, Proctor, Somerville, and Waco lakes. Lake Waco is not part of the BRA water supply system. 

These lakes are typically kept at a level that allows the dam to hold an extended capacity during heavy rains that might otherwise flood the area. The added room in this flood pool allows excess water to be captured and stored until it may be released downstream in a safe manner.

The BRA’s water supply reservoirs don’t have flood control capacities, which means when the lake is full, any water that enters the reservoir must be passed along downstream, Higgins said. They can’t physically hold more water.

If there are any doubts the placement of these water supply reservoirs does not benefit Texas during dry times, check out the current streamflow north of Possum Kingdom Lake. Little water is flowing. If the reservoirs did not exist, the supply would also not exist for all those who’ve become reliant in their daily lives on a limited resource across the Brazos River basin.

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