Figure 1. A diver deploying a bottom-mounted acoustic doppler current profiler (ADCP). These devices use sound waves to measure flow speed throughout the water column. As the sound hits particles in the water moving with the current, the particles shift the frequency of the return echoes (the Doppler effect). The shift in frequency increases with flow speed, so we can measure the water movements over the reefs of Bonaire at a multitude of heights. This is necessary to differentiate between the effects of waves passing overhead, and tidal currents running along the bottom. The hydrodynamics of reefs is not well known. Image courtesy of Bonaire 2008: Exploring Coral Reef Sustainability with New Technologies expedition.