The PVC sleeve contains a SonTek M9, used to survey the lake bottom; the orange SonTek CastAway provided quick conductivity, temperature and depth data for correcting sound-speed variations. 11 HEADLINE SURFACE WATER Who’s Minding the Planet? Counting Strokes Counting his paddle strokes and navigating by compass and transect plan printouts, Webb was still able to follow his survey plan. And because the water level in the lake was constant, vertical data from absolute RTK (Real Time Kinematic satellite navigation) were not necessary, he notes. He just surveyed the water level into NAVD88 and corrected the measured depths to accurate labeled elevations. The M9 uses five of its nine acoustic beams at a time— selecting the optimum frequency for sampling conditions at the moment—to measure five discrete depths in a 50-degree swath. The ADP is equipped with an integrated GPS, so each data point is geolocated. In all, between wading measurements and data from the JagYak, Webb’s bathymetric survey included position and elevation measurements for more than 12,000 discrete locations. Webb supported his M9 data with readings from a CastAway- CTD, a palm-sized instrument from SonTek that provides vital data for correcting sound-speed variations in depth and velocity readings—a vital step in ensuring accurate depth measurements. Lowering the CastAway and reeling it back up at a steady rate provides geolocated conductivity, temperature and depth readings for the water column. “Even though there were no salinity changes in the reservoir, there were certainly temperature differences, and even some conductivity differences, that could affect the speed value,” Webb points out. “There were a couple of deep spots right in front of the dam where there was a fairly substantial change in temperature [with depth] in those holes." “I would never collect data without using a CTD profiler,” he adds. “And since I was on a kayak, the CastAway CTD was ideal due to its size and built-in GPS.” Stretching the Boundaries Bret Webb built the ideal sampling platform to map shoals and pools in Daphne, Alabama’s Lake Forest—mounting a 9-beam HydroSurveyor/RiverSurveyor-M9 Acoustic Doppler Profiler on an ocean kayak. But before his JagYak touched the water, he had to tackle another challenge with a computer rather than pipe adhesive. “We needed to determine the volume of sediment that had accumulated since the time of construction,” he explains. “To do that required a comparison of two surveys: pre-construction and present-day." “The only pre-construction survey we had was from 1958, and while of very good quality, it did not have any coordinate reference or stated vertical datum,” Webb notes. “Furthermore, the survey contained no recognizable features or landmarks that could be used for geo-referencing, because the entire area has changed substantially due to development since 1958!" “We had to get very creative,” he says. Ultimately, Webb and his team found a U.S. Geological Survey (USGS) quadrangle map from about the same year as the pre-construction survey, and stretched and scaled the 1958 survey over the map until the 20-foot contours of both charts were in perfect alignment. Webb could then geo-reference the 1958 survey using known coordinates from the USGS quadrangle. In all, Webb and his team manually digitized more than 800 elevations from the 1958 survey. That set the stage for a 12,000-point survey of Lake Forest and a new understanding of sedimentation in the system.