A long-standing paradigm in fluvial sedimentology is that logs and branches (often referred to as “woody debris”) can be used to measure paleocurrent orientations, assuming these debris typically align themselves parallel to flow direction. However, beginning with an observation made in 1986 while a Ph.D. candidate at the University of Cincinnati on a point bar along the Mississippi River, I began to question this assumption. As I looked at tree trunks and branches strewn across the bar surface, I noticed they adopted two dominant orientations, with little between. Trunks that contained root masses appeared to orient predominantly parallel to flow; whereas, those witch lacked roots or branches tended to come to rest perpendicular to flow. My thoughts were that those with roots began to drag as discharge waned and they came into contact with the bar surface, allowing them to maintain their original orientation. However, without roots or branches, the wood would settle on the bar top and become reoriented as it rolled across the surface during decreasing flow. This has important implications for using woody debris as a paleocurrent indicator.
A preliminary study was done in 2009 along a several-mile reach of the meandering Henrys Fork of the Snake River in Madison County, Idaho as an undergraduate senior thesis. Part of this study involved creating a template for data collection that considers both position of woody debris on and around a point bar and the degree of branching/rooting complexity. A similar study is now underway along a reach of the braided South Fork of the Snake River, also in Madison County, Idaho.
Forbush, T.F. & Little, W.W., 2010, Controls of Woody Debris Orientation in Meandering Fluvial Systems: Henrys Fork of the Snake River, Madison County, Idaho: Abstracts and Program for the Annual Meeting of the Geological Society of America, Denver, CO. (Abstract) (Poster)