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 the 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.
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 -- The planning process for new channelization projects should include 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 opportunities 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, reduced freshwater availability, and accelerated delivery of pollutants.
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 or more in height and greater that 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 onsite, 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 AND POLLUTANT CONTROL -- This measure ensures the proper storage and disposal of certain chemicals, substances, and other materials that are used in construction or maintenance activities at dams. These include construction chemicals such as concrete additives, petrochemicals, solid wastes, cement washout, pesticides and fertilizers. The measure limits the application, generation, and migration of toxic substances, and ensures their proper storage and disposal. The measure also ensures that nutrients are applied at rates necessary to establish and maintain vegetation without
causing significant nutrient runoff to surface waters.
PROTECTION OF SURFACE WATER QUALITY AND INSTREAM AND RIPARIAN HABITAT -- This measure ensures that the operation of dams will be
assessed for impacts to surface water quality and instream and riparian habitat, and that the potential for improvement will be evaluated. Significant nonpoint source pollution problems that exist from excessive surface water withdrawals will also be assessed and evaluated.
STREAMBANK AND SHORELINE EROSION -- Eroding streambanks and
shorelines should be stabilized, where streambank and shoreline erosion is a nonpoint source problem. Vegetative methods such as marsh creation and vegetative bank stabilization ("bioengineering") are the preferred methods. The measure also ensures that streambank and shoreline features such as wetlands and riparian areas with the potential to reduce NPS pollution are protected. Streambanks and shorelines should also be protected from erosion due to uses of either the shorelands or adjacent surface waters.