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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.
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 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.
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:
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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. |
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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.
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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.
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This combination is useful to provide a contrast, in terms
of land use and water quality, with the North Fork. |
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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.
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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.
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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.
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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.
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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
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- 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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