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I I I I I I I I I I I I I I I I I I I system has a published vertical accuracy of 15mm +/- 1.5 mm. The sonar unit used was an ODOM Hyrographic Systems, Hydrotrac - Single Frequency, Portable Survey Sounder. This unit used a 200kHZ frequency with a published accuracy of lcm +/- 1% of depth and an output resolution of lcm. The sonar unit pings and logs 20 depth readings per second and the GPS logs one position per second. The GPS system output a NMEA GGA string at a rate of 1HZ while the sonar output text strings indicating depth at a rate of 10HZ. Data from these instruments was sent to a laptop computer and recorded using the COMLOG software from ODOM Hydrographic. Because the GPS and Sonar data were received at different rates, all data entries collected by the COMLOG software were time-tagged to the millisecond using the computer's clock. The depth readings immediately before and after the GPS reading were interpolated by the computer clock time (nearest millisecond) to produce the XYZ coordinates used to map bed topography of the river channel. 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. 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 same 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 and 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. Shoreline and water surface shots were made using the Psion data collector running Field Face software. Waterline shots for the entire reach were surveyed on October 30 and 31, 2000. Another series of water edge/surface shots were made in June 26 & 27 and July 5 & 6 2001. Yampa River, Lily Park A semi-permanent base pin was established at Lily Park so that additional data can be collected using the same reference location. Bathymetric data was collected along a 2.8 km section of the Yampa River at Lily Park on June 12, 13 and 14,2000, using the GPS/sonar technique. Collection of bathymetric data was hampered in 2000 by the low and unusually short runoff period. There was a large, wide and shallow riffle near mid station that could not be surveyed by boat and was surveyed at a later time by logging points while walking. The shallow 13