Bathymetry Differencing to Quantify Volumetric Change within the Snake River in Hells Canyon

A nearly complete baseline multibeam echosounder (MBES) survey of the 90 km of the Hells Canyon Reach of the Snake River that runs along the border of Idaho and Oregon, US was collected to monitor volumetric change in the sediment resources of this reach (e.g. fall Chinook salmon spawning gravel and beach-building sand). This baseline will be compared to future MBES surveys to determine the impact of the Hells Canyon Complex (HCC) that cuts off the supply of coarse sediment from the relatively small, unimpounded upstream area. MBES surveying is unique from other survey methods (terrestrial LiDAR scanning (TLS)), aerial LiDAR, RTK-GPS, or photogrammetry) in ways that lead to unique errors in the point measurements. For example, unlike static TLS acquisition, MBES surveys are performed from a moving platform that relies on GPS positioning, which introduces one of the largest sources of error into the point cloud. Because the GPS antenna is on the Earth's surface, this error is more extreme and more variable than aerial surveys where the sky view is unobstructed. Beyond the GPS positional accuracy, the errors of each MBES survey point are impacted by the geometry of the beam angle and range, which determine the beam footprint. The extremely rugged river bottom in the Hells Canyon Reach magnifies the error of the points when they are interpolated into a surface for differencing. The methods presented here account for both error sources in the surface (point and interpolation) in order to accurately determine the volumetric change between surveys.