The Role of Talus Pile Mobility in Valley Widening Processes and the Development of Wide Bedrock Valleys, Buffalo River, AR.

Valley width is largely controlled by lithology and upstream drainage area, but little work has focused on identifying the processes through which valleys widen. Bedrock valleys widen by first laterally eroding bedrock valley walls, followed by the collapse of overlying bedrock material that must then be transported away from the valley wall before the valley can continue widening. We hypothesize that talus piles that cannot be transported by the river protect the valley wall and slow valley widening, while talus piles that are rapidly transported allow for uninterrupted valley widening. We used field measurements from 40 locations in both wide and narrow valleys along the Buffalo River, AR to test this hypothesis. Our data show that wide valleys tend to have fewer talus piles and smaller talus grain sizes, whereas talus in narrow valleys is larger in size and more continuous along valley walls. We calculated potential talus block entrainment at each site location and found that talus blocks in wide valleys are potentially entrained and moved away from valley walls during moderate and large flood events, whereas talus blocks in narrow valleys are very rarely moved. Our results show that the potential transport of talus piles protecting bedrock valley walls from widening is controlled by the block size of collapsed bedrock wall material relative to stream competency. Our results also suggest that persistence versus mobility of collapsed talus piles is an important process in the development of wide bedrock valleys. Plain Language Summary: The width of a river valley largely depends on the rock type of the valley walls and the size of the upstream watershed, but we know very little about how a wide bedrock valley is formed. A river widens a bedrock valley by first undercutting the valley wall, causing the overlying material to collapse into the valley bottom. Then, the river must transport the collapsed material away from the valley wall so that the widening can continue. We use field data to test a conceptual model that links the width of bedrock valleys with how easily rivers can transport collapsed wall material. We measured the size of talus blocks and river channel geometry at 40 locations along the wide and narrow valleys of the Buffalo National River in northwest Arkansas. Next, we calculated whether moderate to very large floods can transport talus blocks away from valley walls. We found that in wide valleys the river can sometimes transport talus, while in narrow valleys, no talus blocks could be transported even in the 1000‐year flood. Our results suggest that the widening of bedrock valleys could be stalled by large, immobile blocks of talus that shield valley walls from undercutting and eventual widening. Key Points: Our data tests a process‐based model linking bedrock valley width with how quickly river export collapsed talus away from valley wallsIn this field site, talus blocks are larger and difficult to transport in narrow valleys, while wide valleys have smaller talus blocksWide bedrock valleys can develop where rivers efficiently transport talus away from valley walls; rarely transported talus can inhibit valley widening [ABSTRACT FROM AUTHOR]