Why do we delineate watersheds?
Reasons relate both to traditional stewardship of lands and waters, and to developing an ecosystem perspective for a project and its resources. An interest exists in knowing the character of streams that are flowing into or through an area, such as a Corps project. Associated with those streams may be issues and needs with water quality, water yield, erosion and sedimentation, recreation and navigation, riparian habitat, fishery habitat, invasive species, and esthetics. Because water often acts as an integrator of events at other locations in the watershed, and because the Corps routinely affects aquatic systems, knowledge of the drainage system within a watershed is important.
Encouragement for working in watersheds came in October 2000 when the Clean Water Action Plan developed and published a "Unified Federal Policy for a Watershed Approach to Federal Land and Resource Management." The policy pertains to Federal lands of the Corps and the Departments of Agriculture, Commerce (National Oceanic and Atmospheric Administration), Defense, Energy, Interior, the Environmental Protection Agency, and the Tennessee Valley Authority. The home page for this initiative is http://cleanwater.gov/.
Why are watersheds a useful piece of the landscape to work with?
Watersheds are a physical boundary that can be mapped and analyzed, with the results communicated to others such as the public. People can identify with activities that occur in a watershed and focus on a central point, the outlet, where human activities are often centered. Multiple government agencies typically have jurisdictions and interests in a watershed, so it can also form the basis for partnering on resources and projects.
Delineating a watershed provides a bounded area wherein the physical processes are similar. Aquatic and hydrologic processes can be described and to some degree controlled or managed within a watershed. Because of the integrating nature of water in a drainage, analysis of the water can track activities elsewhere in the watershed.
Watersheds are scalable, in that one can define a watershed large enough to exhibit or accommodate the ecological processes of interest. Multiple sizes of watersheds can be delineated, and they can be nested for hierarchical analysis.
When may watersheds not be useful or appropriate as a physical unit?
In those parts of the country where there is no organized surface water drainage, watersheds may not be mappable or usable. Examples are large floodplains or other large flat areas, areas of karst topography with subterranean drainage, and arid regions with little channeled drainage.
What is the process of delineating watersheds?
This can be done manually or automatically. Manual delineation involves drawing lines on a topographic map, and connecting the slope or ridge tops. Assuming the water will drain away from those points, the watershed is delineated by enclosing a polygon.
The automated process involves some technical skills with GIS databases. Automated delineation involves obtaining a Digital Elevation Model (DEM), obtaining a stream network, and identifying stream outlets. The DEM and stream outlets can either be downloaded or constructed. Consult the Environmental Protection Agency (EPA) Watershed page http://www.epa.gov/win/ for available data and further information on watersheds. If a DEM must be constructed, consider cost-sharing with the U.S. Geological Survey to do so.
The user of the end result of a watershed delineation can influence the process at two points. One is in the beginning and related to objectives, when the desired scale of mapping and classification is determined. The second is during the process itself, when choices on obtaining data are associated with different costs.
Watershed Tools and Information
An EMRRP Tech Note titled "Preliminary Watershed Assessment" contains a suggested strategy for characterizing watersheds for stream restoration planning. Access that document at http://el.erdc.usace.army.mil/emrrp/pdf/sr03.pdf.
The Environmental Protection Agency supplies a large variety of watershed tools of use in planing restoration projects and working with others for common goals. Below is a listing of just the A-E tools for Data Collection/Measurement/Assessment, one of 12 categories of tools. The Web page for the Watershed Tools Directory is http://www.epa.gov/OWOW/watershed/tools/.
Advanced Identification
Aerial Photography
Analytical Methods for the Determination of Pollutants in Waste Water
Analytical Procedures and Quality Assurance Plan for the Determination of Xenobiotic Contaminants in Fish
Assessment of Uncertainties and Variability in Wildlife Toxicity Data
ASTM 1993 Annual Book of ASTM Standards Water and Environmental.
Barny
Bibliography of Methods for Marine and Estuarine Monitoring
Construction of Aquatic Based Webs
Culture and Test Methods
CWA Section 106 Guidance for Water Quality Monitoring
CWA Section 403: Procedural and Monitoring Guidance
Ecological Risk Assessment
Electric Log
EMAPS Ecological Indicators for Aquatic Systems
Environmental Indicators
Equilibrium Partition Approach for the Development of Sediment Quality Criteria for Non Ionic Organic Contaminants for the Protection of Benthic Organisms (Technical Basis Document)
Evaluation of Dredge Material Proposed for Ocean Disposal Testing Manual