«A thesis submitted to the faculty of San Francisco State University In partial fulfillment of The requirements for The degree Master of Arts In ...»
Humans have dramatically transformed the San Pedro Creek Watershed since the Spanish explorers first encountered the area in 1769. The Spanish explorers found a valley with a meandering creek, wetlands, a lake, and a large willow patch. However, soon after the explorers’ encounter, land use shifted from subsistence activities by indigenous Ohlone to agriculture and ranching by Spanish missionaries. A private landowner later obtained the land and continued ranching until the 1860s, after which the area transitioned to agriculture. Farmers transformed the valley floor and some surrounding hills of the watershed into agricultural fields, straightening streams and building irrigation canals. The latest shift in land use came after World War II when developers created a new suburban community in San Pedro Valley. The developers built up to and in some places on top of the creek and former lake and wetlands. All of the land use changes resulted in an increased flood risk by altering the stream drainage
Kelsey McDonald - Thesis 38network and storage capacity in the watershed, which decreased the lag time of runoff after rainfalls and in turn increased the peak amount of runoff.
The San Pedro Creek Watershed: c. 1769 Notes from a Spanish expedition led by Captain Gaspar de Portola that passed through San Pedro Valley in 1769 provide the first written accounts of the valley. They paint a picture of a valley surrounded by hills with a stream running through it into a large marsh extending to the Pacific Ocean (Teggart 1911). Portola’s chaplain, Juan Crespi, noted in his journal entry of October 31, 1769, that two creeks met and flowed into a freshwater “inlet” before emptying into the ocean (Stanger and Brown 1969, 96Crespi also noted the presence of willows around the creeks and the inlet (Stanger and Brown 1969, 96-97). The expedition’s discovery of the bay prompted further exploration of the area by Captain Fernando Rivera in 1774. Francisco Palou, the chaplain, suggested that a mission could locate in the valley given its many amenities, including pasture land and two creeks that ran into a lake (Bolton 1926, 285-286).
The indigenous people of the San Pedro Creek Watershed were the Ohlone.
On the first Portola expedition in 1769, Crespi described being greeted by Ohlone in San Pedro Valley (Stanger and Brown 1969). The Ohlone were hunters and gatherers, and routinely set fires to encourage the growth of particular grasses for their seeds (Brown 1973-74). Fish were also an important part of their diet and the Ohlone may have used small, temporary dams along the creek to help catch fish, perhaps aided by
Kelsey McDonald - Thesis 39fish poisons or nets (Margolin 1978). Ohlone interactions with the watershed probably caused minimal changes to the area, and any flooding likely had little impact on the Ohlone, who as hunters and gatherers relocated as necessary based on where resources were available.
The Mission’s Agricultural Outpost in San Pedro Valley: 1786-1792 The sighting of San Francisco Bay and the abundance of resources in the area described by Spanish explorers ushered in an era of change for San Pedro Valley.
Within a decade of the establishment of the Mission San Francisco de Asis (later called Mission Dolores) and the nearby presidio in San Francisco in 1776, the San Pedro Valley housed an agricultural outpost to supply food to the mission (Dietz 1979). The outpost was located on the valley floor southeast of the lake and its wetlands on the site of the former Ohlone village of Pruristac (Dietz 1979; Before the Sanchez Adobe 1957).
The mission farmers built ditches to drain excess water off of the fields, and also irrigation channels to water crops and willow hedges. The availability of water in the valley was clearly an asset for farmers, who grew a variety of crops including corn, beans, wheat, peas, barley, and grapes (Milliken and Sanchez 1979). The ditches likely increased the velocity of runoff in the area and thereby increased channel incision, which would also have increased the sediment in the stream (Collins, Amato, and Morton 2001). Likewise, the outpost presumably used some of the limited wood in the area for fuel or building, which may have increased erosion and thereby the amount of sediment
Kelsey McDonald - Thesis 40in the stream. Some of the increased sediment was likely deposited downstream in the lake and wetlands in the northwestern portion of the valley floor. Mission farmers decreased the storage capacity for runoff after rainfall, in part by creating drainage channels to move water out of the immediate area and also by adding sediment to the lake and wetlands. These actions would have increased the potential for flooding, although perhaps not significantly yet.
Ranching in San Pedro Creek Watershed: 1792-1860s Ranching in San Pedro Valley started with the mission outpost and continued into at least the 1860s. After flourishing for several years, the mission’s agricultural outpost experienced a rapid decline in population in 1792, likely due to an epidemic, and the it’s primary function changed from labor-intensive crops to livestock, primarily cattle and sheep (Dietz 1979).
After Mexico’s independence from Spain, land in the San Pedro Valley became available to settlers through grants from the Mexican government. Maps sketched by several people applying for land grants in San Pedro Valley provide early images of the valley (Dietz 1979). Guadalupe Barcena’s sketch from 1835 (Map 8) shows a valley surrounded by mountains and a creek (arroyo) running through a group of willows (sausal) and into a lake. Francisco de Haro’s 1838 map (Map 9) covers a larger area than Barcena’s sketch, and shows San Pedro Creek (arroyo de S Pedro) running on the
Kelsey McDonald - Thesis 41
MAP 9. De Haro’s 1838 Map for Rancho San Pedro Land Grant. Source: Dietz 1979.
valley floor, into a grove of willows (sausal) and a lake (laguna). The map also points out the mission outpost ruins in the valley.
In 1839 Francisco Sanchez succeeded in obtaining a land grant including San Pedro Valley, where he soon built his adobe. Sanchez raised crops (Dietz 1979) and livestock, including a large number of cattle, as well as horses and sheep (Brown 1961), which would have been allowed to roam freely over the area to feed on the vegetation (Hynding 1982).
In 1853, almost 15 years after Sanchez acquired the land, the U.S. Coast Survey mapped the San Pedro Creek Watershed, providing a more precise look around this time (Map 10). San Pedro Creek runs along the valley floor and disappears into a thicket of willows surrounded by wetlands and to the northwest by a lake, which empties into the ocean. Sanchez’ adobe is also shown with a road leading up to it.
Lines east of the adobe and leading into the creek may be drainage ditches. Another topographical map by the U.S. Coast Survey from 1866 shows much of the same detail;
a fork of the creek (now called the Sanchez Fork) enters the valley from the south side, and the road leading up to the adobe now continues across the valley floor.
Livestock ranchers altered the San Pedro Creek watershed by bringing livestock into the area and allowing them to roam freely on the hillsides and in the valleys.
Historically in California, grazing livestock initiated changes in the vegetation, from taller, perennial bunchgrasses to shorter, exotic annual grasses – the intensity of grazing would have influenced the extent of the shift (Burcham 1957). In California Rivers and Streams
Kelsey McDonald - Thesis 44Jeffrey Mount (1995) concurs that grazing results in a conversion to non-native vegetation, and he notes that the compaction of the soil by the livestock increases runoff and the erosion rate, thereby increasing the sediment yield from the area.
Likewise, adding unpaved roads impacts a stream system. New, unpaved roads function as drainage channels moving water downstream more quickly and increasing the sediment load in the stream (Mount 1995). The increase in the amount and velocity of runoff disrupted the equilibrium of the creek leading to channel incision, which increases the sediment supply until a new equilibrium is found (Collins, Amato, and Morton 2001).
In San Pedro Valley as the increased sediment moved downstream to areas with a more gradual slope it was likely deposited and helped to start filling in the wetlands and lake, reducing their capacity, and potentially contributing to flooding during high flows. Flooding was unlikely to have been a major concern for ranchers, as their livestock could graze up in the surrounding hills.
Commercial Agriculture: 1870s-1950s After Sanchez’s death in 1862, his land was sold off (Dietz 1979; Brown 1961), and crop farming returned to the valley. Commercial truck farming began developing with the arrival of Italian immigrants who took up parcels of Sanchez’ subdivided land (Richards 1974; Brown 1961).
By the 1890s the changes farmers had made to the watershed over the past several decades of agriculture were evident, as seen on the USGS topographical map
Kelsey McDonald - Thesis 46from 1896 (Map 11). Lake Mathilde in the northwestern part of the valley had become two smaller lakes. The creeks no longer flowed into the lakes; Crespi Creek flowed between the lakes and San Pedro Creek flowed into the ocean west of the lakes. San Pedro Creek appears to have been straightened to run more directly out to the ocean, especially in the lower reach, to drain the excess water out of the area and provide more arable land for farming. The valley now contained many more structures and roads, including the Half Moon Bay-Colma Road, built in 1879, which ran through the valley and south over the mountains (VanderWerf 1994). The new roads enhanced access to the valley and also increased the runoff and sediment entering the stream system.
Italians introduced artichokes in San Pedro Valley by the early 1900s (Parker 1915; Gervais c.1984), and they became the prevalent crop in the valley (Azevedo and Azevedo 2002). Globe artichokes were planted in rows six feet apart on the valley floor and even farther apart on higher ground, in part to allow for irrigation channels (Parker 1915). A photo in the Pacific Rural Press from 1915 (Photograph 3) looking down on the artichoke fields in San Pedro Valley shows the northwestern portion of the valley floor, covered with artichoke fields except for some wetlands near the ocean, where Lake Mathilde was once located. Irrigation and/or drainage ditches lead to San Pedro Creek, which is hidden by a narrow line of riparian vegetation. Large areas of exposed soil lead to increases in soil erosion and sediment in a creek (Mount 1995).
Plowing the land and creating irrigation and drainage ditches reduces the storage
Kelsey McDonald - Thesis 47PHOTOGRAPH 3. San Pedro Valley, 1915. Source: Parker 1915. Courtesy of San Mateo County Historical Assn.
capacity by eliminating depression storage and allows runoff to more quickly enter the larger stream (Mount 1995). These changes also decrease the lag time for the peak discharge, which increases as well, all of which create a greater flood risk downstream (Mount). The net effect of these changes to San Pedro Valley was likely that sediment continued to fill in the lake and wetlands.
Several innovations facilitated truck farming in San Pedro Valley, including the development of the Ocean Shore Railroad. Work began in 1905 on a railroad line intended to run from San Francisco down along the coast to Santa Cruz (Wagner 1974).
In San Pedro Valley the railroad built a berm along the coastline for the tracks, and a trestle over San Pedro Creek. The berm increased the flood hazard in San Pedro Valley by impeding floodwaters in the valley from easily draining into the ocean. By 1907 passengers could travel from San Francisco to Tobin Station at the westernmost end of San Pedro Valley (Wagner 1974). Passengers from San Francisco enjoyed the scenic beauty of the coast, while truck farmers in the valley benefited from the manure transported out of San Francisco for use on the artichoke fields (Parker 1915) and from the quicker transportation of their produce out of the area. A topographical map from 1915 shows the Ocean Shore Railroad line running along the coast and Tobin Station (U.S. Dept. of Interior 1915, reprinted 1947). Other noticeable changes since the 1896 map include the disappearance of the lakes and a few new structures along the railroad line.
Pedro Valley (Pinkson 1915), also aided the development of truck farming in San Pedro Valley. The new road allowed truck farmers to more easily ship their produce out of the area, and became even more important after the Ocean Shore Railroad stopped operating in 1920.
A photo of the northwestern part of the valley along the ocean, dating from around the 1910s-1920s, shows the railroad berm running along the shore, and passing over San Pedro Creek, which is flowing directly into the ocean (Photograph 4).
Artichoke fields are seen on the south side of the valley near the coast. The photo also shows standing water and wetlands in the area formerly covered by Lake Mathilde, suggesting that despite attempts to drain the water out of the valley over the previous forty years or more, farmers could not drain this area well enough to make it arable.
A topographical map from 1943 (Map 12) (War Dept. 1943) highlights additional changes since the 1915 map. These changes include a small development at San Pedro Terrace, near the Ocean Shore Railroad; California Highway One, built in 1937; and Coastside Boulevard, built in 1915. Bridge crossings are shown over San Pedro Creek for both of these roads. Instead of disappearing into the wetlands, Crespi Creek now runs directly into the ocean north of San Pedro Creek, suggesting that farmers altered its course, probably in an attempt to further drain water from the wetlands and lakes and increase the arable land.