What happens when Lake Travis fills up?

AUSTIN (KXAN) -- Recent excessive rainfall and flooding caused Lake Travis and Lake Buchanan to rise significantly over the last several weeks.

Lake Buchanan and Lake Travis are the two major water supply reservoirs for Central Texas.

Lake Buchanan has already reached "full" with any excess water into Lake Buchanan being passed through and downstream into Lake Travis, which still has room for more water.

At the time of this writing, Lake Travis is 91% full, roughly 6.5 feet below full. Lake Travis is considered full when the water rises to an elevation of 681 feet above sea level.

But what happens if Lake Travis reaches full?

Lake Travis forecast levels determines releases

When Lake Travis isn't full, meaning the water levels are below 681 feet above sea level, LCRA doesn't need to release water from Mansfield Dam, except to maintain water level interests (ecological etc.) along the Colorado River below Lake Travis.

When Lake Travis is full, that means the "conservation pool" has reached its limit, but there's still a lot of room for additional water until the lake can't hold any more water. The additional space where runoff is contained within Lake Travis is known as the "flood pool," which is everything on top of the conservation pool.

LCRA has planned flood operations for Mansfield Dam that stipulate how much water would be released under what circumstances when Lake Travis is forecast to rise above 681 feet full.

• 681-683 feet: When Lake Travis is forecast to reach levels between 681-683 feet above sea level, LCRA are allowed to release up to 7,500 cubic feet per second (cfs.) of water downstream as long as the water level and flow rate remain below certain specified amounts at three sites: Austin, Bastrop and Columbus.
• 683-710 feet: When Lake Travis is forecast to reach levels between 683-710 feet above sea level, LCRA are allowed to release water more quickly. These releases may reach up to 30,000 cfs. of water downstream as long as the water stages and the stream flow remain below certain specific levels at certain locations along the Colorado River in Austin, Bastrop and Columbus.
• 710-714 feet: When Lake Travis is forecast to reach levels between 710-714 feet above sea level, LCRA are allowed to release water even more quickly. These releases may reach up to 50,000 cfs. of water downstream as long as the water stages and the stream flow remain below 50,000 cfs. at certain locations along the Colorado River in Austin, Bastrop and Columbus.

The top of the Mansfield Dam spillway is 714 feet above sea level. Once the water level is forecast to reach above the level of the spillway, it becomes more critical to release water to protect the safety of the dam.

• 714-722 feet: When Lake Travis is forecast to reach levels between 714-722 feet above sea level, LCRA are allowed to release up to 90,000 cfs. regardless of the impact to the water stage or stream flow at those downstream sites in Austin, Bastrop and Columbus.
• 722-754 feet: When Lake Travis is forecast to reach levels from 722-754 feet (top of Mansfield Dam), LCRA may release as much water as is needed to "maintain the safety of the dam."

When might Lake Travis reach full?

Back on July 1, before heavy rain caused historic flooding and runoff to significantly boost our lake levels, LCRA released their lake level forecasts for Lake Travis based on a variety of scenarios.

Clearly, reality exceeded even the 25th percentile of wet conditions with current levels of Lake Travis close to 675 feet above sea level. A new Lake Travis water level projection will be released in the beginning of August.

What we can learn from their July 1 forecast, however, is that median conditions during the summer cause Lake Travis to dip slightly or plateau followed by a rise in the fall and winter. This rise is due to less irrigation demand in the cooler months and the expected return of our second wet season from late summer into the fall.

We can use this original forecast to suggest that under median conditions for the rest of the summer into the fall would cause Lake Travis to rise to "full" by the end of December. Anything wetter than median conditions (50th percentile) would cause Lake Travis to reach "full" earlier than that.

In-Depth: The Lake Travis flood pool

Unlike Lake Travis, Lake Buchanan does not have a flood pool, so water is released downstream when Lake Buchanan is forecast to reach full, which is 1020 feet of elevation above sea level.

The flood pool of Lake Travis isn't there to store water ahead of drought, but to protect Austin and other communities downstream by slowing down the release of water into the Colorado River.

If Lake Travis didn't have a flood pool and was already full during another flooding event, then water would head down the Colorado River in a less controlled way and put a significant risk to life and property downstream including in Austin and other cities that sit along the Colorado River.