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Waterline Surveys One hindrance to using sonar to map the channel bottom involved the equipment's minimum depth limitation. In order for the sonar to get a reading off the bottom of the channel, the transducer had to have at least half a meter of water underneath it. The transducer was located approximately 15cm underwater to allow for the pitch and roll of the boat and to minimize air entrainment under the transducer head. Thus, the minimum depth to gather sonar data was 75 cm. In areas that were too shallow for data collection by the boat-mounted GPS/sonar method, additional topographic data was collected using RTK GPS in what we referred to as the "walking method". In this method, the Javad RTK GPS was mounted on a range pole and individual RTK GPS positions were recorded with a Psion data collector running Field Face software. After 2001, a TDS Ranger was used for collecting individual survey points. Reference Sites Data collected in the 15-mile reach of the Colorado River were geographically referenced to the Mesa County GPS Survey System (www.co.mesa.co.us, GIS section). Aerial photography of the 15-Mile Reach was purchased from Mesa County to aid in identification of landmarks and waterline boundaries. The web page of the Mesa County Dept. of Public Works Engineering Division/Survey Section showed the locations of the county markers. Using the latitude and longitude of the brass marker at the intersection of 31 and C Road as a known reference point, we placed secondary rebar survey pins at two. places along the 15-Mile Reach. One pin was placed near river mile 175. This pin was used as the reference point for the entire Corn Lake survey. The other pin, located on a bluff just upstream of the 32 Road Bridge on property owned by the Meso County Highway Department, was used for the Clifton surveys. On the Yampa and Dolores, initial surveys markers were placed using only uncorrected GPS coordinates. Base pins located by the GPS were then used for all subsequent surveys at each of the sites. Corn Lake, Colorado River The Corn Lake site was mapped during aseven-day period beginning June 27th and ending July 7`h. Flows were generally between 8,000 and 10,000 cfs. About 38,880 usable bathymetric survey points were collected along a 4.0-km reach. Water edge shots were obtained in October 1999 and July and August, 2000 using the walking method. 27