Sources and dynamics of large logs in a temperate floodplain river.

Large logs, important agents of biophysical heterogeneity in temperate floodplain rivers, have been virtually eliminated from modified systems. Our purpose was to quantify the sources and dynamics of large logs (> or = 1 m diameter) in the mainstem of a nearly pristine system: the Queets River, Washington, USA. Erosion of forests by the river supplies 0.40 logs x (100 m)(-1) x yr(-1) to the channel. Most (72%) are new logs entering the river for the first time as the river undercuts mature fluvial terraces dominated by large conifers. Retrospective airphoto analyses demonstrate that, over 63 years, the Queets River recruits 95% of new logs from a riparian corridor extending 265 m laterally on both banks, mostly through channel meandering. However, input rates are patchy, with 10% of the valley length supplying 38% of the new logs. As the river moves laterally, remnant logs are left on channel surfaces that later develop riparian forests and reenter the river when those forests erode. Remnant logs lying on the floodplain forest floor surface or buried in alluvium constitute 21% and 7% of the annual inputs from bank erosion, respectively. We estimate that 50% of logs deposited in the channel in a given year, including those underpinning logjams, are transported downriver within five years. Over the next 55 years, bank erosion reclaims an additional 23%, leaving 27% of the logs stable for > 60 years. Simulations indicate that recurrent transport is common, with half of the large conifers being deposited in > or = 3 locations and transported > or = 1.5 km prior to disintegrating. One in ten logs links distant reaches by occupying > or = 7 locations spanning > or = 12.0 km. Instream supplies are therefore a mixture of new and old logs from nearby and upstream forests, sustained by the recapture and transport of stockpiled remnant logs during periods when new inputs are low. We propose that patchy input rates and the periodic rearrangement of large logs are important drivers of temporal variation in river valley habitats, adding to the spatial complexity created by stable logs. These findings underscore the importance of extensive mature forests and connectivity in temperate floodplain rivers.