Mapping gravel bed river bathymetry from space

Understanding river form and behavior requires an efficient means of measuring channel morphology. This study evaluated the potential to map the bathymetry of two clear-flowing, shallow (<3 m deep) gravel bed rivers <60 m wide from 2 m-pixel WorldView2 satellite images. Direct measurements of water column optical properties were used to quantify constraints on depth retrieval. The smallest detectable change in depth was 0.01-0.04 m and the maximum detectable depth was 5 m in green bands but <2 m in the near-infrared; lower sensor radiometric resolution yields less precise estimates over a smaller range. An algorithm for calibrating a band ratio X to field measurements of depth d proved effective when applied to spectra extracted from images (R-2 = 0.822 and 0.594 for the larger and smaller stream, respectively) or measured in the field (R-2 = 0.769 and 0.452). This procedure also identified optimal wavelength combinations, but different bands were selected for each site. Accuracy assessment of bathymetric maps produced using various calibration approaches and image types indicated that: 1) a linear d vs. X relation provided depth estimates nearly as accurate as a quadratic formulation; 2) panchromatic and pan-sharpened multispectral images with smaller 0.5 m pixels did not yield more reliable depth estimates than the original images; and 3) depth retrieval was less reliable in pools due to saturation of the radiance signal. This investigation thus demonstrated the feasibility, as well as the limitations, of measuring the bathymetry of clear, shallow gravel bed rivers from space.