Making decisions about a watershed is an important responsibility; decisions must be based on a solid understanding of the characteristics of the watershed and how physical processes shape watershed conditions. This section provides basic background information on watershed functions and processes to help users understand the assessment procedure and the results of the assessment process. Watershed "processes" refer to those natural physical, chemical, and biological mechanisms that interact to form aquatic ecosystems. For example, the input and routing of water, sediments, and large wood through stream channels involve many inter-related processes occurring both in channel and upslope.
What is a Watershed?
The term "watershed" describes an area of land that drains downslope to the lowest point (Figure 1). The water moves by means of a network of drainage pathways that may be underground or on the surface. Generally, these pathways converge into a stream and river system that becomes progressively larger as the water moves downstream. However, in some arid regions, the water drains to a central depression such as a lake or marsh with no surface-water exit.
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Figure 1
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Watersheds can be large or small. Every stream, tributary, or river has an associated watershed, and small watersheds aggregate together to become larger watersheds. It is a relatively easy task to delineate watershed boundaries using a topographical map that shows stream channels. The watershed boundaries will follow the major ridge-line around the channels and meet at the bottom where the water flows out of the watershed, commonly referred to as the mouth of the stream or river.
The connectivity of the stream system is the primary reason why aquatic assessments need to be done at the watershed level. Connectivity refers to the physical connection between tributaries and the river, between surface water and groundwater, and between wetlands and these water sources. Because the water moves downstream in a watershed, any activity that affects the water quality, quantity, or rate of movement at one location can change the characteristics of the watershed at locations downstream. For this reason, everyone living or working within a watershed needs to cooperate to ensure good watershed conditions.
Watershed Definition
“Watershed” can refer to drainage areas of a wide variety of sizes, and this may create confusion about what is being discussed. Words such as draw, drainage, basin, and sub-basin are also used to describe areas that drain water. The term basin usually refers to a larger river basin, and the terms drainage and draw are usually a component of a watershed or subwatershed. The Federal Guide to Watershed Analysis suggests using specific terms for the different levels. In addition, the hierarchical structure of the watershed system allows for the systematic assignment of identification numbers. The US Geological Survey (USGS) has developed maps that assign a Hydrologic Unit Code (HUC) number to the larger watersheds. This number is often referred to as the watershed HUC Number and consists of four pairs of numbers or “fields.” Thus, HUC Number 17090001 denotes the watershed that is the Middle Fork of the Willamette River and is referred to as a 4th field HUC. The State of Oregon has 49 4th field HUC’s. These 4th field HUC’s are quite large and the State of Oregon has subdivided them further into 5th field HUC’s. There are 1,063 5th field watersheds, with an average size of 58,218 acres. Further subdivision is possible, and portions of the assessment process summarize information by subwatershed. The State of Oregon is currently working on delineating 6th field subwatersheds statewide.
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