Anatomy of a Watershed
A cross section view of a watershed from the headwaters downstream to the bay.
Drawing by Alekz Pang
“A watershed is that area of land, a bounded hydrologic system, within which all living things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community."
– John Wesley Powell
The cities and communities of eastern Marin County tend to each be nestled in their own valley, surrounded by rolling hills and steep ridgelines, with the waters of San Pablo and Richardson Bays bordering their eastern shores. West Marin, with fewer communities and more agricultural and public lands, is bordered to the west by the Pacific Ocean or waters of Tomales Bay.
East Marin, and to a lesser extent West Marin, watersheds have three very distinct hydro-ecologic zones: the uplands, the valley floor, and the baylands. These zones are delineated by elevation, topographic slope, hydrology, vegetation and animal communities.
The uplands encompass the hilly, often steep, terrain from the top of the ridges down to where the valleys flatten out and are dominated by mixed evergreen forest and oak-bay woodlands, interspersed with open grasslands, chaparral, and coastal scrub. These diverse native communities support a broad range of wildlife, including federally threatened northern spotted owl. Much of the upland habitats in Marin County are protected as public and municipal open space.
These are the headwaters. Here the streams begin and the small, steep, ephemeral channels join to form the major waterways that denote a watershed. Steelhead trout spawn and rear in the upland creek reaches. Their survival is dependent upon cool, clear, spring-fed water shaded by deep forest cover.
The uplands, due to the steep terrain, shallow soils, and unstable geology, respond quickly to changes in land use and vegetative cover. If disturbed by development or fire, erosion quickly destabilizes slopes. Forest canopies and healthy grasslands stabilize the land and intercept rainfall, softening its erosive impact on the soil below, allowing it to infiltrate and recharge the groundwater aquifer.
Fire, a natural process in the coastal woodland and chaparral ecosystems of the uplands, has been suppressed for decades. Fuel loads are high. The spread of Sudden Oak Death is also adding to fuel loads. These changes to the natural fire regime have increased the risk of destructive wildfires.
At the base of the uplands the rich soils and easy terrain of the valley floors have enticed humans to settle here for millennia. Valley oak woodlands, with an understory of native perennial grasses and wildflowers, once covered these lands. Creek corridors crossed the valley floor. The creeks were lined with riparian plants like willows, boxelders, bays, alders, native berries, sedges and rushes; providing crucial shade and channel complexity for fish, and cover for wildlife.
These are the sinks, where productive lands are created by the accumulation and storage of sediment, nutrients, and water. The well-vegetated, wide floodplains on the valley floor helped slow the water coming out of the hills, trapping nutrient-rich sediments and promoting infiltration and groundwater recharge. Deep valley-floor sediments act as a sponge, soaking up winter precipitation and releasing it to the streams and vegetation during the dry summer months.
Today, human settlements dominate east Marin’s valley floors. The oak woodland and grassland that once carpeted these areas has been replaced by paved city centers, dense residential areas, and, in part, by an “urban forest” of street trees and other ornamental landscaping.
The streams are often incised (deeply cut into the valley), and are largely separated from their floodplains. Along the valley floor the creeks are confined by urban land uses, and are maintained as flood channels. Regular channel clearing, bank hardening, and concrete culverts have reduced or eliminated valuable habitat.
Vegetated riparian buffers – critical habitat for songbirds, frogs, fish, and other wildlife – are often lacking. Where they remain intact, streams and their forested floodplains act as wildlife corridors, linking the baylands to the uplands.
In the urban valley floor, stormwater runs off impervious surfaces, is collected, and is directed to the creeks. Pollutants from roads, parking lots, and managed landscapes are concentrated in the runoff. High percentages of paved and impervious area significantly reduce infiltration, affecting groundwater recharge rates and volumes, as well as summer streamflow volumes.
The baylands are the interface between the tidal waters and surrounding habitats and the valley floors. Freshwater and tidal wetlands support rich plant and animal life in these low-lying lands. Marsh vegetation provides foraging, breeding habitat, and cover for resident bird populations, including sensitive species like the California clapper rail, salt marsh common yellowthroat, and San Pablo song sparrow. The baylands are also important for migratory waterbirds stopping along the Pacific Flyway and other water fowl depend on wetland habitats for feeding and some for nesting. Several species of fish rely on the baylands and local streams at certain times of the year for spawning and acclimating as they transition from salt to freshwater. Marine mammals, harbor seals and sea lions, use the baylands for foraging and resting. The endangered salt marsh harvest mouse also utilizes levees and adjacent uplands.
These are the buffers, where water and pollutants are stored and filtered. Intact wetlands absorb floodwaters, reducing flood heights and associated impacts upstream. Salt marsh plants remove pollutants from upstream urban and agricultural sources. With the floodplains restricted and developed on the urbanized valley floor, the baylands’ function becomes more important.
Only a small fraction of the county’s salt and brackish marshes remain intact. Many of these historically-extensive wetlands have been drained and filled for development or converted to agricultural lands. Where they still occur, the marshes support an array of grasses and low-growing plants, including cordgrass, pickleweed and jaumea, that are adapted to different levels of inundation and salinity. Herons, egrets, ducks, and other water fowl depend on the remaining wetland habitats for feeding and some for nesting.
Many tidal sloughs at the mouths of the creeks were once large and deep enough for ships to travel up to the city centers. Tidal slough size is directly related to marsh area and the volume of water exchanged during the daily tide cycles. When early settlers built levees around the tidal wetlands, drained them, and planted fields, they reduced the marsh area. Today these sloughs require dredging to maintain navigable channels. Efforts to restore areas of tidal marsh are underway in much of Marin and the Bay Area.