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Mueller, Erich R ¹ ; Pitlick, John ²

¹ Department of Geography, University of ÃÛÌÇÖ±²¥
² Department of Geography, University of ÃÛÌÇÖ±²¥

Channel morphology in streams and rivers is a product of the watershed scale flux of sediment and water to the channel network. As a result the scaling of channel geometry downstream through stream networks should reflect the variation in site specific sediment and water supply. Typically hydraulic geometry relations are formulated solely from measurements of water discharge, thus not explicitly addressing the role of sediment transport in dictating channel form. Here we use measurements of sediment transport for 35 streams and rivers in the northern Rocky Mountains of Idaho to explore the downstream scaling of sediment discharge with water discharge and channel width.

Bed load sediment transport data for individual sites suggest that, on average, streams and rivers in Idaho convey roughly 0.01 kg/m/s of sediment at bankfull discharge (Qb) (Figure 1). If this held uniformly, instantaneous sediment discharge at bankfull (Qs) should increase in exact proportion to width. Recent work by Parker et al. (2007) implies that Qs should be roughly proportional to Qb^0.5. Considering that width typically scales as Qb^0.5, the theoretical analysis of Parker et al. (2007) suggests that indeed the sediment discharge should approximately scale with channel width. While a clear generalization, this would imply that adjustments of channel geometry and grain size act to regulate sediment transport so as to convey, on average, the same unit transport rate (qb) everywhere.

Results from Idaho indicate that in fact unit transport rates are increasing downstream such that Qs is proportional to Qb^0.76 (Figure 2). Yet this deviation may reflect the downstream increase in the proportion of sand comprising the bed load. When considering only the non-sand fraction (>2 mm), Qs increases as Qb^0.59. This may suggest that channel width increases to convey the coarse fraction of the bed load at a relatively uniform unit transport rate, whereas sand transport may be controlled more by local supply. As a result, many streams and rivers may be resilient to morphologic change from episodic influxes of fine sediment.

Parker, G., P.R. Wilcock, C. Paola, W.E. Dietich, and J. Pitlick (2007) Physical basis for quasi-universal relations describing bankfull hydraulic geometry of single-thread gravel bed rivers, Journal of Geophysical Research, 112, F04005.