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Introduction to Sediment & River Stability

Problem Statement. Recent National Water Quality Inventory reports consistently implicate sediments as a leading reported cause of water quality impairment in assessed rivers, streams, and lakes. Accelerated erosion and sediment deposition/inputs from hillslope and channel processes have impaired designated uses in rivers in many ways. Stream channel instability caused by excess deposition of sediment can severely impact aquatic life including the food chain, spawning and rearing habitat, in-stream cover, water temperature extremes, and other structural and functional components. Adverse sediment impacts also include harmful effects on recreation, navigation, conveyance, water treatment systems, and water storage. Economic consequences of excessive sediment, deposition, and transport problems in surface waters have resulted in annual costs for damages of approximately $16 billion (Osterkamp, et al.,1992). More on suspended and bedded sediment impacts.

Numerical water quality standards for clean sediment have been difficult to establish due to the complexity and uncertainty of prediction within the context of significant natural variability across temporal and spatial scales. Nonetheless, widespread sediment impacts on beneficial water uses requires an organized, consistent, and quantitative assessment approach to address the problem. The ideal assessment approach would be informative at several points along the analytical continuum, i.e., from sediment sources to stressors to effects. This would require not only addressing sediment effects and the desired stable condition for the impaired waterbody, but also quantification of sources and their locations in so that management actions can allocate reduced sediment loads among different sources and resolve sediment impairments. The complexity of this challenge points to the need for a broad assessment framework composed of useful methodologies that are linked to models and technical tools and are adaptable to the highly varied settings encountered in sediment-impaired watersheds.

No single model includes all of the assessment protocols incorporated into the WARSSS framework. There are, however, some components such as RUSLE for surface erosion that have been modeled and other computer assisted iterations that speed up the mapping and analytical process. WARSSS addresses landscape and channel sensitivity and response, and focuses on understanding cumulative effects from past land use activities. This is not a "desktop analysis" procedure, but it does rely heavily on documentation of field observations and corresponding analysis. Due to the nature of the assessment methodology, it is essential that predictions and other analyses be conducted by individuals with experience in geomorphology, hydrology, engineering and other scientific disciplines specifically trained in hillslope (e.g. land use/cover and soil properties), hydrologic processes, and channel processes specifically related to sediment erosion and deposition. More on assessment approach.

Factors affecting sediment and stability. The assessment of the erosional impacts of land use on sediment yields and stream ecosystems is often complicated by the episodic nature of major flood events. Such events generate large sources of sediment to river systems. More on flow-related factors.

How well landscapes and streams accommodate these natural events is influenced by modifications to surface and internal drainage by vegetative changes and road systems, conversion from woody riparian to grass/forb vegetative communities, direct alterations to stream channels (e.g., channelization, abandonment of floodplains, confinement of river systems), and a variety of land use changes. For more information on sediment, natural processes and river stability, use the links in the navigation box to the right above.

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