¾«Æ·SMÔÚÏßӰƬ

Mueller, Erich R ¹ ; Pitlick, John ²

¹ Dept. of Geography, University of Colorado
² Dept. of Geography, University of Colorado

Braided streams and rivers are relatively rare in the Rocky Mountains of the Western U.S., except in the Yellowstone region of northwest Wyoming. This raises the question: why do we find these morphologically complex channel types clustered there? While high rates of sediment transport may seem inherent to braided streams, limited sediment transport data exist to quantify this relation. In some cases braided streams may alternate longitudinally with single-thread reaches, necessitating a better understanding of the hydraulics and geomorphology between adjacent reaches. Here we attempt to address the relation between sediment supply, flow hydraulics, and braided channel morphology through a combination of GIS and sediment yield analysis, field studies, and flow modeling.

A unique set of sediment transport data for over 80 streams and rivers in the northern Rocky Mountains shows that basin lithology is a primary control of sediment supply, and braided streams indeed have distinctively higher sediment transport rates. In the Yellowstone region braided streams are most common in watersheds underlain by thick deposits of quartzite conglomerate in the Washakie Range and volcaniclastic rocks in the Absaroka Range. Field studies on five braided streams (Fig. 1) show a wide range in slope and grain size, suggesting that while locally important these factors are subsidiary to sediment supply in forcing a transition to a braided morphology.

The influence of slope and grain size on longitudinal transitions between single-thread and braided reaches was investigated in detail along Sunlight Creek in the Absaroka Mountains. Field data show that these transitions occur irrespective of channel slope. Instead the grain size of local sediment inputs from tributary fans is strongly coupled to longitudinal patterns in channel planform. In general, coarse grained tributaries tend to restrict valley width and coarsen the streambed and banks, resulting in more laterally confined single-thread channels (Fig. 2).

Finally, detailed topographic surveys of a braided reach along Sunlight Creek were used as input into a 2-dimensional hydrodynamic model (iRIC – USGS). Initial results suggest that spatially complex flow in braided channels produces shear stress fields best described by gamma or exponential distributions, where small zones of high shear stress greatly enhance sediment transport intensity. Despite being laterally dynamic, braided streams in the Yellowstone region likely represent a pseudo-equilibrium channel form in the face of high sediment supply.