UHF riversonde observations of cowlitz river flow velocity at Castle Rock, Washington

A UHF RiverSonde system, operating at a frequency of approximately 350 MHz, has been in operation on the Cowlitz River at Castle Rock, Washington since October 2003. The system uses SeaSonde electronics developed over many years for measuring ocean surface currents in salt water at HF. A three-Yagi antenna system designed using a genetic algorithm optimization procedure replaces the conventional crossed-loop HF array. The antennas, located on one bank of the river, are directed across the river at three different azimuths and are separated by approximately one-half wavelength, allowing both amplitude and phase differences to be used for MUSIC direction finding. Scattering from the fresh water surface is dominated by Bragg scattering as it is at HF, but the data processing algorithms are modified to accommodate flow velocities which can be several times the deep-water phase velocity of the Bragg waves. Data are processed in real time on a portable laptop computer and are available through a dial-up modem. The radar data provide hourly estimates of mean flow and cross-channel variations in the flow. Mean values of the radar flow profile track very closely continuous in situ stage height measurements. River flow velocities of 0.8-3.5 m s^-1 were observed in the first five months of the experiment, with a nearly linear relationship between radar-inferred flow velocity and stage height of 9-14 m. The radar velocity also appears to have a weak correlation with the local wind and several tidal frequencies. The strong correlation between surface velocity and stage height suggests that - with refinement - surface velocity could replace stage height in river gaging, as well as offering additional flow information