Each reservoir within the Brazos River basin was built to serve a purpose.
It wasn't until 1941 – merely 81 years ago – that these lakes existed at all. Constructed to either serve as a water supply source, to help with flood control, or to cool the power plants that allow us to turn on the lights each day, each one has a goal.
Because of that, the water levels of the 11 reservoirs within the Brazos River Authority water supply system will always fluctuate.
The three BRA owned-and-operated reservoirs exist to supply water for municipal, industrial, agricultural, and mining purposes. Each lake also attracts its fair share of recreationists, from those who are fishing, boating, swimming, camping or just enjoying the sun. But recreation has always been a secondary benefit of the three lakes, which means when the lake level drops due to water supply use, evaporation and a lack of rainfall, there must be some adjustment.
A water supply reservoir's primary purpose is to store water during wet times for use during dry periods, 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.
When summer rolls around each year, water use increases as people aim to stay cool, fill their pools, attempt to keep their yards from completely turning brown, and stay hydrated under triple-digit heat. And the hotter it gets, the higher the lake's evaporation rate increases, further adding to the drop in a lake level.
"Water supply releases are really driven by the climate and can fluctuate significantly from one year to the next depending on how severe the drought is," Higgins said.
With 95% of the Brazos River basin experiencing some level of drought as of May 5, lake levels feel the effect.
There are only two ways to fill up a reservoir – it either rains right on top of the lake, or water enters from runoff upstream, Higgins said. Meanwhile, there are many reasons why water leaves.
A water supply reservoir's water level can drop due to evaporation, lakeside water use, to meet water supply needs downstream, leakage, releases for water right requirements, environmental flow requirements, or minimum flow requirements.
The reservoirs also release water to pass inflows when full, but that's not as much an issue during times of drought.
To many people's surprise, evaporation is by far the biggest contributor to a drop in the levels in a reservoir, Higgins said. Typically, the total amount of water use is about half the amount of evaporation lost in a given year, Higgins said.
Sometimes the biggest swing in fluctuation rates comes when river levels become so low that groups with senior water rights can no longer pull the water they need from the flow within the river. Releases from a reservoir must be made to supplement their need, Higgins said. Those types of releases during drought are often needed in the lower portion of the basin.
The proposed Allens Creek Reservoir will help alleviate some of the stress during droughts on the upper basin reservoirs in meeting the needs of the lower basin. The BRA recently purchased full ownership of the long-planned reservoir, which will provide an estimated 100,000 acre-feet of additional water supply for the basin. The reservoir is planned to be "off-channel," meaning it will be built near the Brazos River on Allens Creek, a tributary of the Brazos, near the cities of Sealy and Wallis in Austin County.
"The BRA wants its reservoirs to be full just like everyone else," Higgins said. "A full reservoir is an indicator of a healthy water supply. We wouldn't intentionally drop the level of a reservoir just because. 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."
The BRA has statutory responsibility for conserving and developing the water resources of the Brazos River Basin in Texas and making them available for beneficial use.