Volume loss in water reservoirs due to sedimentation is conventionally estimated from the change in bathymetric surveys repeated five or more years apart. Despite many recent improvements in bathymetric surveying technology, such estimates of volume loss still depend on older and less accurate bathymetric surveys. We describe an alternative approach to reservoir surveying using a new multifrequency acoustic profiling system. The new system includes a 200 kHz precision fathometer and four subbottom profiling frequencies of 48 kHz, 24 kHZ, 12 kHZ, and 3.5 kHz, together with DGPS navigation. All data-recording components are contained in one suitcase-size instrument package. Specially designed software combines three or more signal frequencies into composite false-color displays for interpretation of sediment thickness and type. When calibrated by sparse coring or spud bar determinations of sediment thickness, the method can produce accurate long-term estimates of reservoir volume in one survey. As an initial test of the new system, we conducted a combined survey of water storage capacity and sediment volume of Waco Lake, Texas, in November, 1997. The water storage capacity of Waco Lake was previously determined by land surveying methods prior to the initial impoundment in 1965 and by bathymetric surveys in 1970 and 1995. Our 1997 estimate of water storage capacity agrees with the 1995 bathymetric results to within 3%. However, we find 62% more sediment in the reservoir by direct acoustic measurement of the sediment thickness than indicated from the difference between the 1970 and 1995 bathymetric surveys.

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