Abstract The removal of obsolete and unsafe dams for safety, environmental, or economic purposes frequently involves the exploration of sediments trapped within the impoundment and the subsequent assessment of sediment management needs and techniques. Sediment management planning requires a thorough understanding of the watershed’s surficial geology, topography, land cover, land use, and hydrology. The behavior of sediments is influenced by their age, consolidation, and stratigraphy. All watersheds have a history that helps forecast sediment loads, quality, gradation, and stratigraphy. Impounded sediment deposits may include coarse deltas and foreset slopes, fine or coarse bottom deposits, cohesive or organic matter, and wedge deposits immediately behind the dam. Some watersheds have anthropogenic pollutants from agricultural activities, mining, industries, or urban runoff. The volume and rate of sediment release during and after small dam removal can be limited by active management plans to reduce potential downstream impacts. Management strategies include natural erosion, phased breaches and drawdowns, natural revegetation of sediment surfaces, pre-excavation of an upstream channel, hazardous waste removal or containment, flow bypass plans, and sediment dredging.

More than 30 million dollars are expended annually to assess environmental quality of the Southern California Bight, yet only 5% of the Bight area is surveyed on an ongoing basis. Because decision makers lacked the data to make regional assessments of ecosystem condition, multiple stakeholders collaborated to create a Southern California Bight Regional Monitoring Program. The third survey in this program was conducted in 2003. A primary goal of this regional monitoring program was to determine the extent and magnitude of sediment contamination in the Southern California Bight, and to compare these assessments among several different habitats. A stratified random design was selected to provide unbiased areal assessments of environmental condition; 359 surficial sediments were collected, representing 12 different habitats that extend from shallow embayments and estuaries to deep offshore basins. Each sample was analyzed for grain size, total organic carbon and nitrogen, 15 trace metals, and a suite of persistent organic constituents (total dichloro-diphenyl-trichloroethane [DDT], total polychlorinated biphenyl [PCB], and total polynuclear aromatic hydrocarbon [PAH]). The greatest accumulated mass of these constituents (76% on average; range 70% to 87%) was located at depths >200 m, which was proportional to its relatively large area (67% of entire Southern California Bight). The greatest sediment concentrations of trace metals, total PAH, and total PCB were observed in embayments (e.g., marinas, estuaries draining urbanized watersheds, and industrialized port facilities). These shallow habitats also contained a disproportionately high mass of contaminants relative to their area. Despite the relatively widespread anthropogenic enrichment of Southern California Bight sediments, only 1% of the Southern California Bight was at a moderate to high risk of adverse biological effects based on empirically derived sediment quality guidelines. Risk, however, was not evenly distributed throughout the Southern California Bight. The greatest risk of adverse biological effects was found in sediments of marinas, Los Angeles estuaries, and large publicly owned treatment works (POTWs); these were the only habitats for which the mean effects range-median quotient exceeded 0.5. The least risk was observed in sediments associated with the Channel Islands and small POTWs, for which all sites were considered to be at low risk of adverse biological effects.

Abstract Human activities impact an estimated 98% of rivers in the United States. This chapter summarizes impacts associated with pioneer societies, commercial activities, and public works. In pioneer societies, individuals or small groups undertake activities such as timber harvest, agriculture, or navigation improvements. Nineteenth-century placer mining of gold along rivers of California's Sierra Nevada is used as a case study. Commercial activities are conducted by groups of people seeking profit through industry, commerce, or agriculture. Commercial activities impacted rivers much more extensively than most activities of pioneer societies. Impacts to water quality, and particularly the U.S. Geological Survey National Water Quality Assessment (NAWQA) program, provide a case study. Results from the 1991–1995 NAWQA program indicate that approximately half the sites sampled in urban areas have surface-water contamination exceeding levels at which adverse biological effects can occur in aquatic biota. Public works such as dam and levee construction undertaken by local and federal governmental agencies caused massive alteration of river systems. Channelization is used as a case study of the impacts of public works on rivers; more than 56,000 km of waterways were channelized by the Corps of Engineers and the Soil Conservation Service after 1940. The net effect of human activities in the United States has been to disconnect rivers from adjacent hillslopes, floodplains, and valley bottoms, underlying hyporheic and groundwater zones, and from headwaters and downstream processes. Because a connected river is a functioning ecosystem, rather than simply a canal for moving water and sediment, disconnection simplifies and impoverishes the ecosystem. This has resulted in widespread loss of biological diversity in rivers of the United States.