LAUSR

ABSTRACT Quantifying bathymetry is essential in understanding the physical processes that control channel form and function. New applications in a wide range of disciplines have an urgent need for more continuous mapping of fluvial topography. Recent advances in through-water photogrammetry and optical imagery indicate that accurate, continuous bathymetric mapping may be possible in shallow, clear streams. This research directly compares the ability of through-water photogrammetry and spectral depth approaches to extract water depth for environmental applications. Imagery and cross sections were collected on a 140 m reach of the Salmon River in Clackamas County, Oregon, using an unmanned aerial vehicle (UAV) and real time kinematic (rtk)-GPS. Structure-from-Motion (SfM) software produced a digital elevation model (DEM) (1.5 cm) and orthophotograph (0.37 cm) derived from these data. The photogrammetric approach of applying a site-specific refractive index provided the most accurate (mean error, ME, 0.009 m) and precise (standard deviation of error, SD, 0.17 m) bathymetric data (R2 = 0.67) over the spectral depth and the 1.34 refractive index approaches. This research provides a quantitative comparison between and within bathymetric mapping methods, and suggests that a site-specific refractive index may be appropriate for similar gravel-bed, relatively shallow, clear streams.