[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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Research Impact Statement: A new model evaluates road network erosion and sediment delivery to streams and then prioritizes new drain and surfacing locations to optimize road-stream disconnection and reduce sediment delivery.ABSTRACT: The Road Erosion and Delivery Index (READI) is a new geographic information system-based model to assess erosion and delivery of water and sediment from unpaved road networks to streams. READI quantifies the effectiveness of existing road surfacing and drain placements in reducing road sediment delivery and guides upgrades to optimize future reductions. Roads are draped on a digital elevation model and parsed into hydrologically distinct segments. Segments are further divided by engineered drainage structures. For each segment, a kinematic wave approximation generates runoff hydrographs for specified storms, with discharge directly to streams at road-stream crossings and onto overland-flow plumes at other discharge points. Plumes are attenuated by soil infiltration, which limits their length, with delivery occurring if plumes intersect streams. Sediment production and sediment delivery can be calculated as a relative dimensionless index. READI predicts only a small proportion of new drains and new surfacing results in the majority of sediment delivery reductions. The model illustrates how the spatial relationships between road and stream networks, controlled by topography and network geometries, influence patterns of road-stream connectivity. READI was applied in seven northern California basins. The model was also applied in a recent burn area to examine how reduced hillslope infiltration can result in increased hydrologic connectivity and sediment delivery. (

An integrated suite of numerical models and analysis tools (NetMap) is created for three purposes: (1) Develop regional scale terrain databases in support of watershed science and resource management, (2) Automate numerous kinds of watershed analyses keying on environmental variability for diversifying resource management options, and (3) Improve tools and skills for interpreting watershed-level controls on aquatic systems, including natural disturbance. Hillslope attributes, such as erosion potential, sediment supply, road density, forest age, and fire risk are aggregated down to the channel habitat scale (20–200 m) allowing unique overlap analyses, and they are accumulated downstream in networks revealing patterns across multiple scales. Watershed attributes are aggregated up to subbasin scales (10,000 ha), allowing comparative analyses across large watersheds and landscapes. Approximately 25 automated tools address erosion risk, habitat indices, channel classification, habitat core areas, habitat diversity, and sediment and wood supply, among others. Search functions target overlaps between specific hillslope and channel conditions and between roads and landslide or debris flow potential. To facilitate its use, NetMap contains hyperlinked users' manuals and reference materials, including a library of 50 watershed parameters. NetMap provides decision support for forestry, restoration, monitoring, conservation, and regulation.