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18 l 1 I I I I I I I I I I I I I I I I I Big Gypsum. Dolores River Over a three-day period, May 16, 17, and 18. 2000, bathymetric data was collected using the GPS/sonar methodalong a 3-km section of the Dolores River in the Big Gypsum Valley. As was the case with the Yampa River, the lower than normal runoff flows made data collection using sonar more difficult. Also, certain parts of the river were too shallow for the sonar. Several days were spent in June and July 2000 logging additional points by the walking method. Waterline/water surface shots were made on July 6 and 7,2000, and on June 13 and 14,2001. Velocity Measurements To calibrate modeled data, it is necessary to have obtained field measurements of depth and velocity at known flows. While depth can be gathered using the same technologies that are used in determining bathymetry, velocity measurements require another set of instruments. Two different technologies were used for measuring velocities in this project. In 1998, a Marsh McBirney Flo-Mate Portable Flowmeter was used in conjunction with the total station to determine point velocities. The Marsh McBernie had a published accuracy of 1.5cm/s +/- 2 percent. This flowmeter was based on the electrical principle known as Faraday's Law. Here the flow rates were determined by passing a conductive fluid through a magnetic field. A wading rod that held the meter head was placed at a depth chosen to represent average velocity (usually 0.6 oftota1 depth). Locations were recorded by shooting the point with the total station and then recording the average of three 10-second averages. After 1999, locations were recorded by locating the position with the RTK GPS and then recording the velocity of that point. At high flows it is often not practical to use a wading rod to measure velocities. In May of 2000, a 3MHZ Sontek River Surveyor Acoustic Doppler Profiler (ADP) was used to gather calibration data at the IS-mile reach. The ADP measured the velocity of water using a physical principle called the Doppler shift. This principle stated that if a source of sound was moving relative to the receiver, the frequency of the sound at the receiver was shifted from the transmit frequency. By determining the Doppler shift using three beams, it was possible to determine the relative speed and direction of the flow. Using the Doppler shift from the river bottom to determine the boat speed and direction, we computed absolute velocity and direction. The ADP measured velocities in 15 cm vertical increments down to the river bottom. These velocities were averaged over a specified time interval. Data Reduction and Preparation The GPS/sonar technique produced a large number of data in a short amount of time. It generally was not possible to perform any quality control as the data was being collected. Data reduction and quality control were accomplished using an Excel- macro that stripped out non-sensible or incomplete points so only points that met a set of