When storms drop heavy rain in the Highland Lakes watershed, the Colorado River and its tributaries swell, sending the runoff into the Highland Lakes. Rain upstream of Buchanan Dam flows into Lake Buchanan, where LCRA can store it for water supply. LCRA can send floodwaters downstream to Lake Travis through a chain of pass-through lakes and dams.
While all the dams were built to help manage floods, only Mansfield Dam was designed to hold back floodwaters and temporarily store them in Lake Travis.
On the map below, see the estimated time it takes for water to flow to Mansfield Dam from upper basin locations, and how long it takes water to reach downstream locations when it is released from Mansfield Dam. The flow times are estimates. Water travel times can be shorter during floods and longer during times of low flow. Please note the map is for illustrative purposes; it is not drawn to scale.
- Heavy rains falling anywhere in the upper Colorado River basin can impact Lake Buchanan. However, runoff from heavy rain in San Saba and Mills counties, as well as portions of Lampasas, Burnet and Llano counties, will reach Lake Buchanan relatively quickly. This rainfall normally replenishes the water supply used from the lake. When the lake is full, floodwaters are released downstream.
- The Hill Country and Central Texas is known as Flash Flood Alley. This is one of the nation's most flood-prone areas. Runoff from rainfall in Kimble, Mason, Llano and portions of other surrounding counties will send floodwaters through Lake LBJ. These floodwaters are passed downstream into Lake Travis.
- The four smaller lakes -Inks, LBJ, Marble Falls and Austin- are pass-through lakes. They pass through releases made from lakes Buchanan and Travis downstream.
- The Pedernales River collects floodwaters primarily from Gillespie and Blanco counties as it winds through the Hill County to Lake Travis. This river can send a large amount of floodwater into Lake Travis in a short time.
- As the only Highland Lake designed for flood control, Lake Travis has a flood pool to store floodwaters until LCRA can safely release them downstream.
- Flooding near Austin usually occurs from rain falling in Travis County. As the Colorado River flows toward the Gulf of Mexico, it collects runoff from Bastrop, Fayette, Colorado, Wharton and Matagorda counties. Rain falling below Mansfield Dam can cause severe flooding downstream, even with no releases from Lake Travis.
LCRA monitors river conditions throughout the lower Colorado River basin through the Hydromet. The Hydromet is a system of more than 275 automated river and weather gauges.