Spatial Variability of Bedforms in a Large Lowland River

Our current understanding of bedforms in sand-bedded rivers is based on flume experiments and localized bedform fields found in natural channels. In controlled laboratory settings, bedforms are typically spatially uniform. Field observations are often made selectively, specifically focusing on bedform fields that contain the largest and most uniform features. While it is widely recognized that bedform fields are comprised of a spectrum of different sized bedforms and are hierarchical in nature, investigations of bedform spatial variability in large lowland rivers are lacking. Here, we examine bathymetric surveys collected in 2015, 2019 and 2020/21 for a 50 km reach of the sand-bedded Fraser River in British Columbia. To highlight the spatial variability, we categorize bed features as 1) continuous (2D, 3D and superimposed dunes), 2) discontinuous (dune with gaps in the troughs, isolated and barchan dunes), 3) flat bed, 4) unclassifiable (features smaller than the resolution of the surveys), and 5) non-bedforms (obstructions, wake and near-bank deposits). We find that ~40% of the classifiable river bed is composed of continuous 2D and 3D dunes. Discontinuous dunes make up ~20% of the classifiable bed and mixed morphology bedforms make up ~15%. A substantial proportion of the classifiable river bed is flat and composed of non-bedforms (~25%). The spatial variability is partially coherent as the largest dunes are often found on the edge slope of bars while smaller bedforms, or even flat beds, are generally found on bar tops and in deeper adjacent pools. Yet the river thalweg is made up of a wide variety of bedform types and sizes. Discontinuous dunes, characteristic of low sand supply, appear along the margins of many continuous dune fields, but are also present in the thalweg. Bedforms fields in large lowland sand-bedded rivers are not ubiquitous, or spatially uniform as in small scale laboratory channels. The results pose serious impediments to identifying a characteristic bedform size or scale in a large lowland river. Some caution is necessary in interpreting reported bedform sizes for large rivers since the size, scale, and type of bedform appears to depend on the location in the channel. Greater attention to the spatial variability of bedforms in sand-bedded rivers is required to enhance our current understanding.