[1] We reviewed 14 studies documenting the effects of tributaries on river morphology at 167 confluences along 730 km of river spanning seven orders of magnitude in drainage area in western United States and Canada. In both humid and semiarid environments the probability of observing significant confluence-related changes in channel and valley morphology due to tributary influxes of sediment (e.g., changes in gradient, particle size, and terraces, etc.) increased with the size of the tributary relative to the main stem. Effects of confluences on river morphology are conditioned by basin shape and channel network patterns, and they include the nonlinear separation of geomorphically significant confluences in river networks. Other modifying factors include local network geometry and drainage density. Confluence-related landforms (i.e., fans, bars, terraces, etc.) are predicted to be dominated by older features in headwaters and younger features downstream, a pattern driven by the frequency and magnitude of floods and punctuated sediment supply that scale with watershed size.

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From an ecological perspective, one aim of forest management is to supply wood to streams to protect and enhance aquatic habitats. An analysis was made of the mass balance of in-stream wood along 9 km of channels in old-growth and 50-year-old second-growth redwood (Sequoia sempervirens (D. Don) Endl.) forests in northern California, U.S.A. High volumes of wood storage in streams in old-growth forests were due primarily to streamside landsliding and bank erosion. Logging-related debris and high forest mortality rates in conifer and deciduous forests contributed to high wood storage in second-growth forests. Volumes of in-stream wood in second-growth forests were similar to volumes in one old-growth system and less than another. Diameters of wood were significantly greater in older forests. Wood recruitment from forest mortality in old-growth forests was low compared with second-growth sites, driven by differences in conifer mortality rates of approximately 0.04 and 0.9%·year–1, respectively. Contrasting old-growth redwood mortality with values reported for unmanaged Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests in Washington State (0.5%·year–1) and unmanaged Sitka spruce (Picea stichensis (Bong.) Carrière) forests in southeastern Alaska (1.2%·year–1) point to a strong latitudinal gradient of forest mortality reflected in tree size. The mass balance analysis of in-stream wood also allowed an estimation of bank erosion along large channels and soil creep along small, steep streams.

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Seasonal scour and fill from bankfull flows were measured in Freshwater Creek, a gravel-bed coastal stream of northern California, to test a previously developed approach predicting the reach-average and distribution of scour or fill depths based on Shields stress and the exponential function. Predictions of reach-average scour and fill depths were within 4-60% of measured depths. Three of the four predicted distributions of scour and fill depths were statistically different (p < 0.05) from measured distributions. Differences between predicted and measured values were likely due to scour and fill patterns in Freshwater Creek that were influenced by sediment supply and location within the channel network, channel form roughness, and possibly multiple peak flows. Consequently, the predictive approach may be better suited for individual peak flows on straight reaches that are in equilibrium between sediment supply and transport, and with form roughness similar to the creeks where the approach was developed. Improved predictions of scour and fill are possible with adjustments for aggrading reaches and form roughness.