When do logs move in rivers?

Large woody debris is an integral component of forested, fluvial systems th roughout the world, yet we know little about hydraulic thresholds for movem ent and transport of logs, We developed theoretical models of entrainment a nd performed flume experiments to examine thresholds for wood movement in s treams. Both the model and the experiments indicate that log entrainment is primarily a function of the piece angle relative to flow direction, whethe r or not the log had a rootwad, the density of the log, and the piece diame ter. Stability increased if the pieces had rootwads or were rotated paralle l to flow. Although previously reported as the most important factor in pie ce stability, piece length did not significantly affect the threshold of mo vement in our experiments or our physically based model, for logs shorter t han channel width. These physically based models offer a first-order approa ch to evaluating the stability of either naturally derived woody debris or material deliberately introduced to streams for various management objectiv es.