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Technical Guidance document

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Hydromodification What Are the Nonpoint Source-Related Problems Associated with Hydromodification? Hydromodification activities have been separated into the categories of channelization and channel modification, dams, and streambank and shoreline erosion. A frequent result of channelization and channel modification activities is a diminished suitability of instream and streamside habitat for fish and wildlife. They can also alter instream patterns of water temperature and sediment type, as well as the rates and paths of sediment erosion, transport, and deposition. Hardening of banks along waterways has increased the movement of NPS pollutants from upper reaches of watersheds into coastal waters. Dams can adversely impact the hydraulic regime, the quality of the surface waters, and habitat in the stream or river where they are located. A variety of impacts can result from the siting, construction, and operation of these facilities. The erosion of shorelines and streambanks is a natural process that can have either beneficial or adverse impacts on the creation and maintenance of riparian habitat. Excessively high sediment loads can smother submerged aquatic vegetation, cover shellfish beds and tidal flats, fill in riffle pools, and contribute to increased levels of turbidity and nutrients. Management Measures Summary Management Measures for Channelization and Channel Modification Physical and Chemical Characteristics of Surface Waters This measure ensures that the planning process for new channelization projects includes an evaluation of the potential effects on the physical and chemical characteristics of surface waters that may occur as a result of the proposed work. The measure encourages planning and design of new projects to reduce undesirable impacts. The operation and maintenance programs for existing modified channels should identify and implement any available opportunities to improve the physical and chemical characteristics of surface waters in those channels. Instream and Riparian Habitat Restoration for Channelization and Channel Modification this measure ensures that the planning process for new channelization projects includes an evaluation of the potential effects on instream and riparian habitat that may occur as a result of the proposed work. The measure encourages planning and design of new projects to reduce undesirable impacts. The operation and maintenance programs for existing modified channels should identify opportuniti4s to restore instream and riparian habitat in those channels. The habitat characteristics that may be influenced by channelization and channel modification include: elimination of stream bank vegetation, reduce freshwater availability, and accelerated delivery of pollutants. Management Measures for Dams These management measures apply to dams 25 feet or more in height and greater than 15 acre-feet in capacity, or to dams six feet of more in height and greater than 50 acre-feet in capacity. The measures also apply only to those projects and activities that fall outside of existing jurisdiction of the National Pollutant Discharge Elimination System permit program. Erosion and Sediment Control this measure provides for reducing erosion and retaining sediment onside, to the extent practicable, during and after construction of dams. An approved erosion and sediment control plan, or similar administrative document that contains erosion and sediment control provisions, should be prepared and implemented prior to land disturbance. Chemical Pollutant Control This measure ensures the proper storage and disposal of certain chemical, substances, and other materials that are used in construction or maintenance activities at dams. These include construction chemical s such as concrete additives. Petrochemicals, solid wastes, cement washout, pesticides, and fertilizers. The measure limits the application, generation, and migration of toxic substances, and ensur

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