Talus Block Characterization of Wide and Narrow Bedrock Valleys and the Implications of the Frequency of Talus Block Transport on Bedrock Valley Widening, Buffalo River, AR

We find bedrock river valleys in two end-member states: narrow, canyon like valleys, and wide, bedrock valleys many times wider than the river channel. Past research has shown that valley width is controlled by lithology and upstream drainage area, but there has been little exploration into the processes that control bedrock valley widening. The process of bedrock valley widening occurs in a two-step process. First, there is lateral erosion and undercutting from the river and eventual collapse of the undercut valley walls. Secondly, transport of the collapsed talus blocks moves them from the wall to allow for more undercutting to occur. Previous research has not investigated the role of the varying sized talus blocks that fall from bluff face walls, shielding valley walls from further widening. We gathered field data from Buffalo National River in north-central Arkansas to determine if, indeed, there is a difference in talus block grain sizes within wide and narrow valleys and how frequently talus blocks can potentially move, based on calculations of shear stress from bedload grain sizes. The Buffalo River is an ideal bedrock river to address this question as it has both wide and narrow bedrock valleys in similar lithologies. At about 30 sites in the wide and narrow valleys, we measured the grain sizes of a representative number of talus blocks at the base of bedrock valley walls, grain size distribution in the bedload, bankfull width, and valley wall height. If talus block size contributes to the development of wide bedrock valleys, we could expect that we will find large talus in the narrow valley and smaller talus grains in the wide valley. Preliminary analysis of field data shows that the locations in the narrow valleys, generally have larger talus grain sizes and smaller bedload sizes, than locations in the wide bedrock valleys. With knowledge of the movement of talus blocks and how often various sized blocks are moved, we can better determine the effect that the talus grains have on shielding the river valley walls, thus preventing bedrock river valley widening.