The spatial scale and location of land whose development has the strongest influence on aquatic ecosystems must be known to support land use decisions that protect water resources in urbanizing watersheds. We explored impacts of urbanization on streams in the West River watershed, New Haven, Connecticut, to identify the spatial scale of watershed imperviousness that was most strongly related to water chemistry, macroinvertebrates, and physical habitat. A multiparameter water quality index was used to characterize regional urban nonpoint source pollution levels. We identified a critical level of 5% impervious cover, above which stream health declined. Conditions declined with increasing imperviousness and leveled off in a constant state of impairment at 10%. Instream variables were most correlated (0.77 ≤ |r| ≤ 0.92, p < 0.0125) to total impervious area (TIA) in the 100‐m buffer of local contributing areas (∼5‐km2 drainage area immediately upstream of each study site). Water and habitat quality had a relatively consistent strong relationship with TIA across each of the spatial scales of investigation, whereas macroinvertebrate metrics produced noticeably weaker relationships at the larger scales. Our findings illustrate the need for multiscale watershed management of aquatic ecosystems in small streams flowing through the spatial hierarchies that comprise watersheds with forest‐urban land use gradients.
Stream rehabilitation and enhancement projects in the Norwalk River (urban-forest watershed) and Merrick Brook (agriculture-forest watershed) were evaluated. Instream structure installation, streambank stabilization and meander re-creation were performed 2-5 years before monitoring. Physical, chemical and biological variables were monitored at control, enhanced (treatment sites originally controls), impaired and rehabilitated (treatment sites originally impaired) sites for three field seasons to evaluate the projects and formulate monitoring strategies. Small improvements in local habitat and macroinvertebrate assemblages were observed at rehabilitated sites on the Norwalk River however control conditions were not attained. Changes to stream health were less evident at the reach scale suggesting that watershed processes that form and maintain habitat were too altered for more widespread recovery. A localized sediment source from a failing streambank was eliminated from Merrick Brook protecting the abundant nearby quality habitat, yet fining occurred at the rehabilitation site due to hydraulic changes leading to localized shifts in macroinvertebrate assemblages. Single-season sampling created a useful snapshot to compare enhanced and rehabilitated sites to control and impaired sites. We recommend a tiered sampling strategy where effectiveness monitoring may include a detailed effort at many sites over a short time (as performed here), a relatively low level of detail (e.g. a rapid assessment) at an intermediate number of sites over a short time, and a detailed long-term monitoring at few sites (e.g. before-after-control-impact, BACI). More research is needed to continue the trend of increased project evaluation to advance the science and application of stream restoration.
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.
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