A procedure to estimate the peak discharge associated with large floods is needed for tributaries of the Colorado River downstream of Austin, Texas, so that appropriate peak discharges can be used to estimate floodplain boundaries and used for the design of bridges and other structures. The U.S. Geological Survey, in cooperation with the Lower Colorado River Authority, studied flood peaks for streams in all or parts of 22 counties in that part of the Colorado River Basin extending downstream of Town Lake in Austin to the Gulf of Mexico. The study area was selected because the streams in this area either are tributaries to the Colorado River or have flood characteristics similar to those tributaries.

Peak-flow frequency for 38 stations with at least 8 years of data in natural (unregulated and nonurbanized) basins was estimated on the basis of annual peak-streamflow data through water year 1995. Peak-flow frequency represents the peak discharges for recurrence intervals of 2, 5, 10, 25, 50, 100, 250, and 500 years. The peak-flow frequency and drainage basin characteristics for the stations were used to develop two sets of regression equations to estimate peak-flow frequency for tributaries of the Colorado River in the study area. One set of equations was developed for contributing drainage areas less than 32 square miles, and another set was developed for contributing drainage areas greater than 32 square miles. A procedure is presented to estimate the peak discharge at sites where both sets of equations are considered applicable. Additionally, procedures are presented to compute the 50-, 67-, and 90-percent prediction interval for any estimation from the equations.