English: Researchers map the bottom of Castle Lake, near Mount St. Helens. For three days, researchers ran a small boat over the lake surface equipped with an acoustic Doppler current profiler (ADCP). The ADCP bounces an acoustic wave off the bottom that is reflected back to a mobile receiver in the boat. The frequencies of the send-receive pairs are used to calculate depth. More than 191,000 water-depth measurements were integrated with high-precision GPS and airborne lidar data. A GIS was then used to compute lake area and capacity for any given surface elevation and to generate a bathymetric map.