Geomorphic and sediment supply controls on transitions between single-thread and braided channel planforms, Sunlight Creek, Wyoming

Erich R. Mueller and John Pitlick
Department of Geography, University of Colorado, Boulder, CO, USA

	The response of stream channels to variations in sediment supply involves complex interactions between channel geometry, stream bed armoring, and planform adjustments.  Previous research in the Rocky Mountains of the Western U.S. has shown that single-thread channels can absorb wide variations in bed load sediment supply primarily through changes in sediment textures which modulate the stress available for sediment transport.  Nevertheless, there likely exists some sediment supply threshold beyond which a transition to braided channel morphology is likely.  In this study we attempt to address this directly through a combination of bed load transport sampling, morphologic measurements, and sediment transport modeling to explore the controls on planform adjustments of an emergent braided stream system.  The 354 km2 Sunlight Creek basin in northwest Wyoming drains erodible volcaniclastic rocks of the Absaroka volcanic province.  The resulting high sediment delivery likely drives Sunlight Creek to become braided, relatively rare for mountain streams in the interior western U.S.  Especially pertinent to this study, Sunlight Creek periodically alternates between a single-thread and braided planform, thereby allowing for the isolation of the geomorphic controls which drive planform transitions along adjacent reaches.
	Measurements of bed load transport were made in single thread reaches upstream and downstream of an approximately 1 km long braid plain and are consistent with high bankfull bed load transport rates on the order of 1 kg/m/s.  Additionally, pre- and post-peak surveys within this braided section suggest that, while channel change is significant in some cross-sections, little net aggradation or degradation occurred across the reach as a whole.  Despite being laterally dynamic the braided sections of Sunlight Creek thus appear to behave as pseudo-equilibrium conveyors of sediment to and from adjacent single thread reaches.  Further analysis of longitudinal trends in channel morphology and planform suggest that high gradient (>1%), coarse grained single-thread reaches upstream give way to braided, finer grained reaches downstream as gradient decreases.  Perhaps counter-intuitively, as gradient decreases further (<0.8%) the channel again becomes single-thread and relatively coarse grained, with downstream planform changes related to the local slope and grain size.   Modeled sediment transport rates between single-thread and braided reaches are consistent with near equilibrium transport capacity, despite markedly different cross-stream stress distributions and sediment textures.  The transitions between single-thread and braided channels in the Sunlight Creek basin result in a unique downstream hydraulic geometry that appears to be dynamically stable in the face of high sediment supply.