Since the 1940s, the U.S. Department of Agriculture (USDA) has built over 11,000 flood-control reservoirs (FCR) throughout the United States. Most were designed to hold 50 years' worth of sedimentation. Many have reached that age, but the amount of sediment they contain is unknown. Conventionally, reservoir sedimentation is monitored by measuring the change in reservoir capacity over time relative to an initial survey. However, the vast majority of FCR in the United States have never been surveyed. We evaluate a new method for surveying FCR in which an acoustic sub-bottom profiler is used to measure both water depth and sediment thickness in one survey. From these measurements both the current reservoir capacity and post-impoundment sediment volume are determined. We evaluate the method by conducting surveys of 21 FCR to find the frequency with which the base of post-impoundment sediment can be mapped throughout the reservoir. In 18 of the 21 FCR surveyed, both the water bottom and the base of post-impoundment sediment could be mapped throughout, allowing both the current and initial reservoir capacity to be determined. Comparing our estimates of initial reservoir capacities with as-built capacities estimated prior to construction we find that only seven of the 18 agree to within 10 percent. Some disagree by more than a factor of 2. We conclude that as-built capacities of USDA FCR should be used with caution for estimating post-impoundment sediment volumes and that in the majority of cases more accurate estimates can be made by direct measurement using sub-bottom acoustic profiling.