Characterization of a gravel to sand transition on the Lower Yellowstone River, Montana

The gravel-sand transition in large rivers has been the subject of much discussion in the sedimentology literature. On the Yellowstone River in Montana, USA, the gravel-sand transition zone is also ecologically important because it is consistently selected for spawning by the endangered pallid sturgeon (Scaphirhynchus albus). The objective of this study was to characterize the gravel-sand transition to improve understanding of sediment transport processes and spawning habitat, and to explore explanations for its origin on this river. A transition from a gravel-bed to a sand-bed channel occurs approximately 50 kilometers upstream from the confluence of the Yellowstone with the Missouri River. We mapped water surface slope and bed topography in a 115 km-long, 1-pass swath map to quantify bedforms, bed sediment, and hydraulic conditions using a multibeam echosounder and post-processed GPS precise point positioning. Bathymetry and sidescan imagery from the multibeam echosounder quantify a range of bed conditions and types of bedforms over the transition from a gravel-dominant to sand-dominant bed. This transition is gradual; sand dunes occur on the bed in the gravel-bedded reach and gravel patches were mapped in the sand-bedded reach tens of kilometers downstream from the gravel to sand transition zone. Several hypotheses may explain the origin and location of the gravel-sand transition on the Yellowstone, including geologic controls on bed slope, changes in sediment supply, changes in channel confinement, and, possibly, backwater from the confluence with the Missouri River.