Watershed Definition

"Watershed" is the term used to describe the geographic area of land that drains water to a shared destination. In San Mateo and Santa Cruz Counties, this drainage pattern includes small streams high in the Santa Cruz Mountains that flow into larger tributaries, then into mainstem rivers before reaching the Pacific Ocean or San Francisco Bay. The drainage system (and the watershed) also includes the geographic area surrounding the stream system that captures precipitation, filters and stores water, and determines water release into stream systems. The stream system is the visible, aboveground portion of a larger drainage system. A watershed, therefore, is "an area of land that drains water, sediment, and dissolved materials to a common outlet" (FISWRG 1998).

Any activity that changes soil permeability, vegetation type or cover, water quality, quantity, or rate of flow at a location can change the characteristics of a stream or even the watershed at downstream locations. Land use practices such as clearing land for timber or agriculture, developing and maintaining roads, housing developments, and water diversions may have environmental consequences that greatly affect stream conditions even when the land use is not directly associated with a stream. Proper planning and adequate care in implementing projects can help ensure that one activity within a watershed does not detrimentally impact the downstream environment.

In recent years, watersheds across the United States have become a focal point for community-based environmental conservation. Through the collaborative efforts of watershed groups – consisting of the people living and working within each watershed – major improvements in water quality, fisheries enhancement, wildlife habitat, and overall quality of life have been accomplished. A watershed provides water for drinking, recreation, and agriculture, and is a rich source of biological diversity that includes habitat for many threatened and endangered species, including salmon and trout. Coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss) return from the ocean and travel upstream to reach spawning grounds deep within many watersheds in San Mateo and Santa Cruz Counties. Spawning success depends upon good environmental conditions to ensure the survival of the next generation of salmon.

The San Lorenzo River watershed begins high in the mountains north of the City of Santa Cruz. The land uses within this watershed are varied and include farming, mining, timber harvest, and residential, commercial, and industrial uses. The people living in the watershed – including the residents of the City of Santa Cruz – rely on this watershed to provide drinking and agricultural water as well as scenic beauty and recreation activities. Through collaboration, public agencies and residents of Santa Cruz can work together to ensure a healthy watershed.

Delimiting and Mapping Watersheds

Every waterway lies within a watershed, and smaller watersheds join together to become larger watersheds. Watershed boundaries always follow the highest ridgeline around the stream channels and meet at the bottom or lowest point of the land where water flows out of the watershed. The boundary between watersheds is defined as the topographic dividing line from which water flows in two different directions. However, the scale at which the landscape is examined is relevant for identifying and defining watersheds.

A watershed may be small and represent a single tributary within a larger system, or be quite large and cover thousands of miles. Because using a common language will help with coordination and management, watersheds have been defined and named using standardized protocols. The naming conventions used by federal and state agencies are defined at a regional scale, and then these large hydrologic units are broken down into smaller watershed units for management purposes. The federal system divides the United States into a four-tiered hierarchical system, which is defined by the United States Geological Survey's (USGS) hydrologic unit codes (HUC). Based on hydrologic features, this system divides the country into:

• 21 Regions
• 222 Subregions
• 352 Accounting Units, and
• 2,149 Cataloging Units.

The smallest unit, the Cataloging Unit, which has an eight digit HUC is commonly referred to as a 4th field HUC or a “Sub Basin.” The 4th field HUC is generally used for local planning efforts but the scale is still quite large (mapping scale is 1:500,000, or approximately 448,000 acres for the smallest watersheds). Knowing a watershed's HUC designation may be important for understanding management issues within the region, as well as necessary for applying for watershed assistance granting opportunities (visit the United States Environmental Protection Agency's Surf Your Watershed Web site to learn more about your HUC watershed). The Natural Resources Conservation Service (NRCS) is presently developing a national system that will map watersheds on a smaller scale – 1:24,000 (see NRCS' Watershed Boundary Dataset Web site for more information).

In California, the most widely used watershed mapping delimitation system is the digital California Watershed Map program (also known as CALWATER 2.2). This is the system used throughout the Salmonid Habitat Restoration Planning Resource for San Mateo and Santa Cruz Counties Web site when referring to “watersheds.” CALWATER maps are available for use with Geographic Information System (GIS) applications, and delineate watersheds down to 3,000–10,000 acres. The maps are presently being revised to remove old political boundaries so that they are based on hydrologic features and are consistent with national datasets. Similar to the federal system, CALWATER is hierarchically structured so that large watersheds are broken down into smaller planning watersheds. CALWATER provides a standard nested watershed delineation scheme that uses the State Water Resources Control Board numbering scheme. The hierarchy of watershed designations consists of six levels of increasing specificity:

• Hydrologic Region (HR),
• Hydrologic Unit (HU),
• Hydrologic Area (HA),
• Hydrologic Sub-Area (HSA),
• Super Planning Watershed (SPWS), and
• Planning Watershed (PWS).

Reference

Federal Interagency Stream Restoration Working Group (FISWRG). 1998. "Stream Corridor Restoration: Principles, Processes, and Practices." GPO Item No. 0120-A; SuDocs No. A 57.6/2:EN3/PT.653. View on-line document.

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