5.0 – Hydromodification – Stream Channel Modification

Hydromodification can be any activity that increases the velocity and volume (flow rate), and often the timing, of runoff. Such activities include:

  • Construction and maintenance of channels, levees, dams, and other water conveyance structures and/or impoundments for purposes of flood control, water storage, water conveyance, and navigation;
  • Dredging and/or filling or other alterations to natural land contours for the purposes of new development (including transportation and other infrastructure) or navigation;
  • Development of impervious surfaces (asphalt, concrete, most buildings, etc.); and
  • Deforestation or removal of vegetation.

A self-sustaining stream and river within a healthy watershed is in equilibrium within its floodplain when channel forming processes (sediment erosion, transport and deposition) are not constrained and uncontrolled flooding occurs at frequent intervals (ranging from every 1-3 years to several per decade). Unconstrained streams adjust their channel dimensions (width and depth) in response to long-term changes in sediment supply and reoccurring streambank overflow events (i.e. flooding the floodplain). Hydromodification occurs in constrained rivers and streams and/or within watersheds that contain highly modified drainage patterns, extensive deforestation, and/or significant amount of impervious surfaces. Hydromodification effectively reduces base-flow (groundwater flow into streams) and increases overland or storm-flow which causes reduced groundwater recharge and increased peak discharge rates into streams and rivers. Hydromodification may result in stream channel instability and streambank or shoreline erosion which in turn modifies the streams natural water temperatures and flow.

The 2006 version of the Hydromodification Land Use category was reorganized using terms that the end-users of this encyclopedia are familiar with rather than water quality management goals and objectives. Additionally, some management measures (MM) were moved to the Wetlands/Riparian Areas/Vegetated Treatment Systems land-use category in order to reduce overlap and a new, draft MM was created 5.2C Low Impact Development because of the recent popularity of this term for addressing polluted runoff and hydromodification effects from watersheds with significant amount of impervious surfaces. The following describes specific changes made to the 2006 edition of the NPS Encyclopedia. Former MM 5.1A - Channelization and Channel Modification, Physical and Chemical Characteristics of Surface Waters has been reorganized into MM 5.1A - Stream Channel Modification, Channelization/Channel Modification and 5.2B - Managing Hydromodification Impacts, Flow and Temperature Maintenance. Former MM 5.2A - Dams, Erosion and Sediment Control, 5.2B - Dams, Chemical and Pollutant Control, and MM 5.2C - Dams, Protection of Surface Water Quality were reorganized into MM 5.1B - Dams and Levees, Construction, 5.1C - Dams and Levees, Operation and Maintenance, and 5.1D - Dams and Levees, Removal. Due to overlap, former MM 5.1B - Channelization/Channel Modification, Instream and Riparian Habitat Restoration and former MM 5.2C – Dams, Protection of Instream and Riparian Habitat have been moved and are now included in the Wetlands land-use category in the following MM: 6B - Restoration, Rehabilitation, and Mitigation of Wetlands and Riparian Areas and 6C - Protection and Conservation of Wetlands and Riparian Areas. Former MM 5.3A - Streambank and Shoreline Erosion, Eroding Streambanks and Shorelines is now included in MM 5.3A - Managing Hydromodification Impacts, Streambank and Shoreline Erosion. Former MM 5.4A - Education/Outreach, Educational Programs was renamed to MM 5.3 - Education and Outreach.

Please click on the links below to find more specific information for each of the following management measures.

Programs, Policy and Government Agencies

  • The California Coastal Commission's primary mission is to plan for and regulate land and water uses in the coastal zone consistent with the policies of the coastal act. The California Coastal Act - Public Resources Code includes permitting, planning, enforcement, and resource protection. The Coastal NPS program is involved in programs pertaining to marinas and recreational boating, as well as wetland/riparian areas, urban and hydromodification land-uses.
  • California Environmental Resources Evaluation System (CERES) is an information system developed by the California Resources Agency to facilitate access to a variety of electronic data describing California’s rich and diverse environments. The goal of CERES is to improve environmental analysis and planning by integrating natural and cultural resource information from multiple contributors and by making it available and useful to a wide variety of users.
  • CALFED Bay-Delta Program aims to improve the quality and reliability of California’s water supplies and revive the San Francisco Bay-Delta ecosystem. Its Web site contains information about water supply, water quality, and ecosystem restoration.
  • Clean Water Act Section 401 Certification Program administered by Regional Water Quality Control Boards (RWQCBs), review projects that require a federal permit under CWA section 404 or that involve dredge or fill activities that may result in a discharge to waters of the United States. This is to ensure that the State’s interests are protected on any federally permitted activity occurring in or adjacent to waters of the State. Detailed information about CWA section 401 in California, including a description of the program, resources, legal background information, proposed projects, and links, are described on the SWRCB Web site.
  • Department of Fish and Wildlife, Habitat Conservation Branch consists of multiple programs dedicated towards the conservation and preservation of habitats and species in California and include the following: California Endangered Species Act (CESA), California Environmental Quality Act (CEQA), Lake and Streambed Alteration Program (1600), Timber Harvest Plan Review, and Natural Community Conservation Program (NCCP).
  • NRCS, Watershed Technology Electronic Catalog is a source of technical guidance on a variety of restoration techniques and management practices, to provide direction for watershed managers and restoration practitioners. The site is focused on providing images and conceptual diagrams.
  • NRCS, Conservation of Highly Erodible Lands provides dis-incentives to farmers and ranchers who produce annually tilled agricultural commodity crops on highly erodible cropland without adequate erosion protection. In addition, these dis-incentives apply to farmers and ranchers who produce annually tilled agricultural commodities or make possible the production of agricultural commodities on land classified as wetlands.
  • NRCS, Wildlife Habitat Incentives Program (WHIP) is a voluntary program for people who want to develop and improve wildlife habitat primarily on private land. Through WHIP USDA's Natural Resources Conservation Service provides both technical assistance and up to 75 percent cost-share assistance to establish and improve fish and wildlife habitat.
  • Sacramento Area Flood Control Agency’s mission is to provide the region with at least a 100-year level of flood protection as quickly as possible while seeking a 200-year or greater level of protection over time.
  • U.S. Army Corps of Engineers (USACE) mission is to provide quality, responsive engineering services to the nation including: planning, designing, building, and operating water resources and other civil works projects; designing and managing the construction of military facilities for the Army and Air Force; and providing design and construction management support for other defense and federal agencies.
  • U.S. Fish and Wildlife Service South Pacific Division mission is to conserve, protect, and enhance the nation’ s fish and wildlife and their habitats for the continuing benefit of people.

General Modeling Tools

Continuous Simulation Hydrologic Models

  • Bay Area Hydrology Model (BAHM) is a tool for analyzing the potential hydrograph modification effects of land development projects and sizing structural solutions to mitigate the increased stormwater runoff from these projects. This software was developed for use in three counties in the San Francisco Bay Area: Alameda, San Mateo and Santa Clara.
  • Contra Costa County, IMP Sizing Calculator, may be used to size Low Impact Development Integrated Management Practices (IMPs) to meet treatment-only requirements or treatment-plus-flow-control requirements in Contra Costa municipalities. Appendix I also includes detailed instructions for sizing IMPs using a manual calculation method. (updated December 5, 2006)
  • US Army Corps of Engineers, Hydrologic Modeling System (HEC-HMS) is designed to simulate the precipitation-runoff processes of dendritic watershed systems. It is designed to be applicable in a wide range of geographic areas for solving the widest possible range of problems. This includes large river basin water supply and flood hydrology, and small urban or natural watershed runoff. Hydrographs produced by the program are used directly or in conjunction with other software for studies of water availability, urban drainage, flow forecasting, future urbanization impact, reservoir spillway design, flood damage reduction, floodplain regulation, and systems operation.
  • USEPA's Storm Water Management Model (SWMM) is a dynamic rainfall-runoff simulation model used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. The runoff component of SWMM operates on a collection of subcatchment areas that receive precipitation and generate runoff and pollutant loads. The routing portion of SWMM transports this runoff through a system of pipes, channels, storage/treatment devices, pumps, and regulators. SWMM tracks the quantity and quality of runoff generated within each subcatchment, and the flow rate, flow depth, and quality of water in each pipe and channel during a simulation period comprised of multiple time steps.
  • US Geologic Survey, Hydrological Simulation Program-Fortran (HSPF) simulates for extended periods of time the hydrologic, and associated water quality, processes on pervious and impervious land surfaces and in streams and well-mixed impoundments.

General Resources

  • American Rivers is the only national organization standing up for healthy rivers so our communities can thrive. Through national advocacy, innovative solutions and our growing network of strategic partners, we protect and promote our rivers as valuable assets that are vital to our health, safety and quality of life.
  • Central Valley Joint Venture (CVJV) was established in 1988 to "protect, maintain, and restore habitat to increase waterfowl populations to desired levels in the Central Valley of California consistent with other objectives of the North American Waterfowl Management Plan." An Implementation Board of representatives from the California Waterfowl Association, Defenders of Wildlife, Ducks Unlimited, National Audubon Society, Waterfowl Habitat Owners Alliance, and The Nature Conservancy guides the CVJV. The U.S. Fish and Wildlife Service, California Department of Fish and Game, California Department of Food and Agriculture, and other organizations and agencies provide technical assistance and advice to the Board.
  • The River Basin Center in the Eugene P. Odum School of Ecology, aim is to increase the capacity of communities and other individual stakeholders to manage and protect their water and related land resources in a sustainable manner. Many publications about various river topics, including riparian buffers can be found in the publications page.
  • U.S. Geological Survey's (USGS) National Water Information System (NWIS) is a comprehensive and distributed application that supports the acquisition, processing, and long-term storage of water data. NWISWeb serves as the publicly available portal to a geographically seamless set of much of the water data maintained within NWIS (additional background).


SWRCB and CCC. 2000. Plan for California’s Nonpoint Source Pollution Control Program Volume II: California Management Measures for Polluted Runoff. State Water Resources Control Board and the California Coastal Commission, Sacramento, CA.

USEPA. 2007. National Management Measures Guidance to Control Nonpoint Source Pollution from Hydromodification. EPA 841-B-07-002. U.S. Environmental Protection Agency, Washington, DC.

NPS Encyclopedia Site Map