MISSION: WATER 28 FEATURE STORY HUGE SWINGS One of the most notable aspects of the diurnal cycle in meltwater flow was the sheer size of the fluctuations. Flow in Rio Behar could range from 4.61 to 26.73 cubic meters per second depending on the time of day. Channel width and depth varied widely to accommodate changes in discharge. That presented a significant challenge to the researchers. They had proven that acoustic Doppler profiling works even in the almost pristine water of supraglacial rivers, bouncing signals off of tiny bubbles and ice crystals in the water. However, the frequencies that work best in shallow, slower moving water are not necessarily most effective in deeper, fast-flowing channels, notes Brittany Jenner, application engineer for SonTek, a Xylem brand. In deep water, a lower-frequency, longer-wavelength acoustic signal is effective at providing a clear measurement of the bank and bottom, as well as the velocity and direction of flow, all of which are used to calculate volume. A higher-frequency, shorter- wavelength acoustic signal delivers a clearer profile of the velocity and flow in shallow water. The Greenland team used a SonTek RiverSurveyor-M9, which automatically uses its depth and velocity readings to determine whether to use its 1.0 MHz or its 3.0 MHz beams for the highest quality measurements, Jenner says. The RiverSurveyor's SmartPulse HD technology also selects its sampling cell size to optimize measurements in whatever conditions it is placed in. "They didn't have to know which configuration to set up in the beginning. It figured that out on the fly," she notes. Safety harnesses and robust instruments are vital when working on a fast river that drops deep into the ice. Photo Courtesy: Lincoln Pitcher