The Watershed

A watershed is all the land drained by a particular creek or river. The watershed catches and stores rainfall, releasing it gradually to the creek. Climate, terrain, soil quality, and plant cover all affect the rate at which stored water makes its way to the creek. In a built-up watershed, impervious surfaces like roofs and pavement prevent rainfall from soaking into the ground so it runs off very quickly.

A watershed functions as an interconnected system, so that natural changes or human activities in one part of a watershed affect other areas downstream. As watersheds change due to natural processes and human activities, these changes can alter water quality, runoff rates, habitat values and erosion processes along the creek and, eventually, have impacts throughout the watershed.

What is a Watershed?

Illustration by Susan Riedley

San Pedro Creek is a perennial stream that drains a 5,114 acre basin (8 square miles) composed of 5 main tributaries that define seven subwatersheds. The upper reaches of the watershed are formed by the north, middle, and south forks of San Pedro Creek and stream flows are maintained by springs in the south and middle forks of the basin. After their convergence at the head of the valley floor, the main stem flows northwesterly toward the Pacific Ocean.


Land Use

Land use in a watershed affects stream dynamics, the behavior of the stream. Where urbanization extends impervious surfaces, less storm water soaks into the ground and more runs off, increasing erosion and delivering more sediment to the creek. Increased sediment load and more frequent peak flows can cause the stream channel to migrate, eroding banks, degrading water quality for fish and wildlife, and increasing the likelihood of flooding. San Pedro Creek is located in one of the most populous seaside communities in the City of Pacifica. Although its headwaters and most of its south slope remain relatively undisturbed, covered in native shrub and brush, urban land uses dominate the lower hillsides and the valley floor. Land uses in the lower reach include four shopping centers, extensive residential development (the Linda Mar, Sun Valley and Pack Pacifica neighborhoods), several schools and numerous commercial properties, as well as an extensive network of paved roads and parking lots.

San Pedro Creek - Land Use Map


Subwatershed Areas

Hydrographic analysis of a 10-m digital elevation model (DEM) allows us to study the watershed as a system of subwatersheds, each with somewhat contrasting characteristics land uses:

North Fork. 614 ha (1517 acres or 2.37 mi2)
The North Fork is most problematic of the subwatersheds in that its upland drainage areas are steep and drain rapidly into culverts. Coupled with stormdrains from impervious surfaces along developed areas, the North Fork subwatershed responds rapidly to rainfall events.

Middle Fork. 329 ha (813 acres or 1.27 mi2)
The Middle Fork is entirely within public lands, with the exception of a small inclusion of private land which cannot be developed, and thus has fewer problems than the North Fork. It drains primarily sandstone bedrock with minor inclusions of limestone.
South Fork. 284 ha (703 acres or 1.10 mi2)
The South Fork is entirely within public lands, primarily San Pedro Valley County Park, but also including North Coast County Water District watershed lands. Much of the drainage is from granitic rocks of Montara Mountain, and this subwatershed includes the steepest relief in the area. Thus while there are few impervious surfaces, the steep slopes and thin soils of the Montara Mountain granitics should produce relatively rapid runoff. Clearly however there is significant groundwater input, since even smaller tributaries such as Brooks Creek (in the western section of this subwatershed) is perennial.

Combined Middle & South Fork. 624 ha (1543 acres or 2.41 mi2) This combination is useful to provide a contrast, in terms of land use and water quality, with the North Fork.

Sanchez Fork. 237 ha (582 acres or 0.92 mi2)
The Sanchez Fork is an important subwatershed which could provide significant steelhead habitat but unfortunately has a barrier to fish migration near its confluence with the main stem. Upland drainage flows from Montara Mountain and San Pedro Mountain, and development is low in density, but new houses are going in as we speak; some of these houses are being built far too close to perennial streams and riparian corridors are being destroyed.
Shamrock. 146 ha (361 acres or 0.56 mi2)
This creek tributary is not mapped by the USGS, but is perennial. Much of the area is in the Shamrock Ranch, but also includes significant upland drainage from San Pedro Mountain.
Pedro Point watersheds. 51.4 ha (127 acres or 0.20 mi2)
These are actually two separate small watersheds that drain the major portion of the Pedro Point headlands. Not included are small culverted intermittent drainages within the residential areas, though these also feed to the San Pedro Creek outflow at Linda Mar Beach. The two watersheds drain areas of steep terrain significantly impacted by off-road motorcycles, and are being managed and restored by the Pacifica Land Trust.
Hinton subwatershed. 65 ha (161 acres or 0.25 mi2)
This is a small subwatershed that has one fairly large landholding, the Hinton Ranch, but is primarily residential except for the steeper slopes. It feeds into a ditch that extends to the main stem just below the Adobe bridge.

Smaller Subwatersheds
The entire watershed is 2127 ha (5257 acres or 8.21 mi2). The sum of the above is 2021 ha (4994 acres or 7.80 mi2). The difference, 106 ha (262 acres or 0.41 mi2) comes from smaller intermittent watersheds draining minor upland areas, but primarily small drainages immediately adjacent to the main stem. Much of this area is built over.


Geology

  • Sandstone and sandstone-dominated melange. This is the most common upland rock type. Slopes are typically steep, and soils are well drained.
  • Montara Mountain granitics, which have been classified as tonalite. Characteristic soil development is thin, well-drained soils with outcrops on slope convexities. Slopes are typically quite steep, since this is all on Montara Mountain, extending to the west to San Pedro Mountain.

  • Greenstone. Quite in contrast to the above, greenstone doesn’t tend to produce significant outcrops. This rock is common in the northeast part of the watershed, and is drained by the North Fork.
  • Alluvium. Flat valley bottoms are predominantly underlain by gravelly alluvial deposits. Most of these areas are covered by residential development.
  • "Conglomerate". Most of the areas mapped as "conglomerate" by the USGS appear to be unconsolidated colluvial deposits from debris flows and other slope movements.
  • In addition, there are scattered outcrops of serpentinite and limestone. Lower parts of San Pedro valley are mapped as fill.

 
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