What is a Watershed?

A watershed is an area of land where all the surface water and underground water drain to the same location. Typically, watersheds are separated by geological features such as mountain ridges and drainage divides. Therefore, it is possible to have two watersheds right next to each other but the water in each watershed is independent of one another with regards to flow paths and endpoints. Watersheds can be comprised of natural and artificial waterbodies. Natural waterbodies include streams, lakes, ponds, and springs. Artificial waterbodies are man-made and include reservoirs, ditches, irrigation ponds. channelized streams, and harbors. These artificial waterbodies are the result of either altering a natural waterbody (e.g., damming a stream to form a reservoir) or creating an artificial waterbody from non-water-related features (e.g., digging a pond and importing water from an outside source). The graphic below is a simplified diagram of a single watershed. Notice how the runoff, streamflow, and groundwater end up in the same location?



Ground Water

Another side to watersheds that most people often overlook is the groundwater underneath the ground, roads, and floors that we walk on. When rain falls onto the ground, the water seeps into the soil and rock, saturating the substrate and raising the watertable. This groundwater may eventually make its way into what is known as an aquifer. An aquifer is a layer of porous rock and soils which conveys subsurface water movement. You may have heard of a percolation pond. These ponds are artificially built to allow water to seep into the water table and are typically found in cities which have extensively covered the ground with hard surfaces like roads and pavement. Many private and publicly funded water agencies operate wells that extract water from the aquifer to be treated and distributed to cities and towns.

Why Protect Watersheds?

As we discussed earlier, a watershed is not just a stream. It is all of the land that funnels surface water into the water way. Several wildlife, fish, and plant species live in and depend on the land and water of a functional healthy watershed. Watersheds provide a number of resources to all sorts of people. The watersheds of San Mateo County provide water for agriculture and municipal water treatment plants, scenic and aesthetic value for recreation and ecotourism, hunting and fishing opportunities and more. From everyday city dwellers to the avid outdoors person, one can find a little of everything to enjoy from a healthy thriving watershed. Click here for more information.