This is an excerpt of my senior seminar group project on biowales for stormwater remediation in Claremont, California. I researched and wrote this section introducing the hydrological context of the Pomona Valley; the full report can be found here.

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The Pomona Valley is a broad alluvial fan, formed from the gradual deposition of sediments by the tributaries of the Santa Ana River as the flow from their source in the San Gabriel Mountains. One of these tributaries, San Antonio Creek, originates from snow-melt on Mount San Antonio (aka Mount Baldy), the 10,000-foot focal point of the San Gabriel Mountains.

Before the 20th century, the San Antonio Creek flowed freely out from the mountains during the wet seasons, spreading out in the rocky, sage scrub-laden San Antonio Wash and infiltrating substantially into the groundwater of the Six Basins Aquifer. The creek is historically the primary source of water for this aquifer, which lies at the western edge of the Pomona Valley beneath Claremont, La Verne, Pomona, and northern Upland, and today accounts for 50% of the City of Claremont’s water supply. While San Antonio Creek’s surface waters are part of the Santa Ana River watershed to the east, the creek-fed Six Basins Aquifer is, below the ground, considered part of the San Gabriel River groundwater basin, draining to the west. (The surface watersheds are separated by the downhill slope of the ground, while underground the two basins are divided by the partial barriers of the San Jose and Chino faults) (California Department of Water Resources, 2003).

The Six Basin Aquifer’s groundwater was once plentiful enough that, nourished by San Antonio snowmelt, artesian springs bubbled to the surface in the City of Claremont, feeding perennial streams and swamplands. Since this time, the character of the watershed has undergone fundamental changes. Overdrafting of groundwater during the height of citrus cultivation in the Pomona Valley substantially lowered the water table, causing the springs to subside, re-emerging only in the heaviest of storms (Wright, 1999). The Pomona Valley Protective Association (PVPA) formed in 1910 to address the overdraft of groundwater—to protect property owners’ rights in the Six Basins Aquifer area and ensure a sustainable supply of irrigation for the citrus growers—as well as to mitigate the threat of flash floods inundating the growing towns. Claremont’s Willis S. Jones pioneered a plan for the conservation and infiltration of San Antonio floodwaters, to be accomplished through a combination of an upstream dam at the canyon mouth, an overflow spillway, and a system of spreading ditches (Figure 2) that would mitigate flood-water by allowing infiltration back into the groundwater before excess water was channeled away (Hackenberger, 2015). The bicarbonate-rich alluvium underlying the region is highly permeable, and thus is ideal for infiltrating water quickly from the surface back into the aquifer.

After much conflict between Pomona, Ontario, and Chino stakeholders over the fate of the redirected water, PVPA members jointly purchased about 700 acres around the San Antonio Wash. Spurred on by heavy flooding in 1916-1917 and the growing demand for water in the valley, they soon completed an infiltration system on the western (Los Angeles County) side of the San Antonio Wash. The expansive region featured a spindling, tree-like network of spreading ditches that conformed to the slope of the land. Later, in the 1930s, a second system was developed on the eastern (San Bernardino County) side, which involved a series of retention basins. The basins are separated by gabion check dams, slowing the water and allowing for more percolation through the porous beds into the alluvium and the water table below (Mitchelson, 1937). PVPA also constructed a smaller spreading  grounds  at Thompson Creek, a tributary of the San Gabriel River itself. The Thompson Creek Spreading Grounds remains a conserved area rich in native sage scrub.

The two spreading grounds in  northern Claremont were crucial in ensuring a stable water supply for the growing region, and were in many ways the direct predecessors to, and inspirations for, today’s state-of-the-art bioswales. In his 1917 report to the PVPA,  Jones  highlighted  the effectiveness of the native coastal sage scrub in infiltrat- ing water, in stark contrast to the heavy flow off orchard lands. “The wisdom of keeping a large acreage of this sage brush covered land in its virgin state,” he asserted, “will become more and more apparent as time goes on and lands are cleared for cultivation” (Hackenberger, 2015).

Indeed, as time went on, the paradigm of water management in the Pomona Valley shifted to one of large-scale waterworks. With the New Deal came the Flood Control Act of 1936, through which the Army Corps Dam was built and completed in 1956. The dam all but eradicated the threat of serious flooding in the valley, yet it further insulated residents from the watershed context. Rain falling below the dam would increasingly flow across paved surfaces—or through the concrete channels of the San Antonio and Thompson washes—rather than feeding the Six Basins Aquifer and the cities’ water supply. In recent years, with new construction projects like the Claremont Graduate University’s infiltration basin (built in 2009) and Pomona College’s bioswales and drought program (built in 2012) comes the opportunity for residents to re-engage with the local watershed and aquifer.

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References

Bortungno, E. J. & Spittler, T. E. (1986). Geologic Map of the San Bernardino Quadrangle. California Geological Survey, Regional Geologic Map No. 3A, 1:250,000 scale.

California Department of Water Resources. (2003). California’s Groundwater. Sacramento. Bulletin 118, Update 2003.

Cooper, L. [Photographer]. (1939). Aerial view of Claremont [photograph]. Claremont, California: Honnold Mudd Library, City of Claremont History Collection.  

Frampton, R. C. [Photographer]. (1946). Mt Baldy and Wash [photograph]. Claremont, California: Honnold Mudd Library, City of Claremont History Collection. 

Frampton, R. C. [Photographer]. (1949). Aerial [photograph]. Claremont, California: Honnold Mudd Library, City of Claremont History Collection. 

Hackenberger, B. C. (2015). “The San Antonio Wash: Addressing the Gap Between Claremont and Upland.” Pomona Senior Theses. 136. 

Hauksson, E. H. & Jones, L. M. (1991). The 1988 and 1990 Upland earthquakes: Left-lateral faulting adjacent to the central transverse ranges. Journal of Geophysical Research 96(B5), 8143-8165.

Mitchelson, A. T. & Muckel, D. C. (1937). Spreading Water for Storage Underground. Washington: U.S. Department of Agriculture, Technical Bulletin № 578.

Wright, J. (1999). Claremont: A Pictorial History (2nd ed.). Claremont, California: Claremont Historic Resources.