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14 I I I I I I I I I I I I I I I I I I I One of the greatest hindrances to using sonar to map the channel bottom is that there is a minimum depth requirement. In order for the sonar to get a reading off the bottom of the channel, the transducer must have at least half a meter of water underneath it. The transducer was located approximately 15cm underwater as to give room to roll and minimize air entrainment under the transducer head making it difficult to gather bathymetric data in areas shallower than 75cm. 2000 Yampa River, Sevens On July 12 1999, bathymetric data was collected along a 1.3km section of the Yampa River at the Sevens study section using the GPS/sonar technique. The length of this site was felt to be fairly short given the nature of the associated fish data and the habitats represented in this reach. The Sevens site was enlarged on June 23,2000 by surveying another 1.3km immediately upstream and overlapping the site mapped the previous year. The survey in 2000 used the sample boat and GPS/sonar equipment in both years. In order to compare bed and water surface elevations between years, three longitudinal profiles were made in the 1999 site and water lines were recorded for the entire 2.6 km station. Collection of bathymetric data was hampered in 2000 by the low and unusually short runoff period. The base pin established in 1998 was used as the reference position for both the 1999 and 2000 surveys. Bathymetric data was collected using the same boat, GPS and sonar equipment in both years. Shoreline and water surface shots were made using the Psion data collector running Field Face software. The waterline was surveyed on October 30 and 31, 2000