Dynamics of wood transport in streams: a flume experiment

The influence of woody debris on channel morphology and aquatic habitat has been recognized for many years. Unlikesediment, however, little is known about how wood moves through river systems. We examined some dynamics of woodtransport in streams through a series of flume experiments and observed three distinct wood transport regimes:uncongested, congested and semi-congested. During uncongested transport, logs move without piece-to-piece interactionsand generally occupy less than 10 per cent of the channel area. In congested transport, the logs move together as a singlemass and occupy more than 33 per cent of the channel area. Semi-congested transport is intermediate between these twotransport regimes. The type of transport regime was most sensitive to changes in a dimensionless input rate, defined as theratio of log volume delivered to the channel per second (Qiog) to discharge (Qw); this ratio varied between 0.015 foruncongested transport and 0.20 for congested transport. Depositional fabrics within stable log jams varied by transporttype, with deposits derived from uncongested and semi-congested transport regimes having a higher proportion of piecesoriented normal to flow than those derived from congested transport. Because wood input rates are higher and channeldimensions decrease relative to piece size in low-order channels, we expect congested transport will be more common inlow-order streams while uncongested transport will dominate higher-order streams. Single flotation models can be used tomodel the stability of individual pieces, especially in higher-order channels, but are insufficient for modelling the morecomplex interactions that occur in lower-order streams.

KEY WORDS: large woody debris; wood transport; jam architecture; flume experiment