A watershed is an area of land that catches rain and snow and drains it into a body of water, such as a river, pond, estuary, bay, lake, or ocean. A watershed is also called a drainage basin. Ridgetops of hills or mountains form the boundaries between watersheds.
A drop of water falling on the top of a ridge will flow by gravity into one watershed or another depending upon the topography or lay of the land. Watershed boundaries often cross private property, county, state, and even international borders.
The upstream areas of a watershed are called the headwaters. As you move downhill and downstream, tiny rivulets and streams combine to form larger rivers which eventually empty into a larger body of water such as a lake or bay.
The area of land in a watershed can be immense or it can be very small. Large watersheds often are composed of several smaller watersheds, called sub-watersheds or sub-drainages. For example, Sulphur Creek flows into the Napa River and has its own watershed, but it is also part of the much larger Napa River watershed, which is part of the even larger San Pablo Bay watershed.
It is important to recognize that activities and conditions anywhere within a watershed can influence the condition of creeks. What takes place in the upper watershed will influence the downstream area. Likewise, what happens downstream may also influence conditions in the upper watershed.
Napa County Watersheds
Napa County encompasses over half a million acres (507,437) and has three major watersheds: the Napa River, Putah Creek, and Suisun Creek Watersheds.
Napa River Watershed
The Napa River winds through varied landscapes of forested mountain slopes, vineyards, urban areas, open pasture, grasslands, industrial zones, and marshes. The Napa River Watershed is home to most of the residents and developed areas in the county. It is estimated that 95% or more of the entire population of Napa County live in the Napa River Watershed. Reservoirs in the watershed include Lake Hennessey, Lake Milliken, and Bell Canyon Reservoir. This watershed also provides many different habitats for fish and wildlife, including chaparral, brackish and salt water marsh, vernal pools, forests, oak woodlands, grasslands, and riparian and freshwater communities.
Putah Creek Watershed
The watershed is flanked by Howell Mountain and Atlas Peak to the west and the Blue Ridge and Vaca Mountains to the east. The dominant feature in the watershed is Lake Berryessa, formed when Monticello Dam was built on Putah Creek in 1957. Major land uses in the watershed are recreation and rangeland. The watershed also supports a unique assemblage of natural communities including serpentine chaparral, grasslands, oak savanna, oak and mixed oak/ coniferous woodlands, riparian, freshwater lake, and cliff habitats.
Suisun Creek Watershed