The next day they reached a "Laguna Grande" which today is covered by the Upper Crystal Springs Reservoir.

In de Anza's diary on March 29, 1776, he wrote: "Night having fallen, at a quarter past six I went down to the arroyo of San Andreas and to another, that of San Matheo, where it descends to empty into the estuary..."[5] Originally a small natural sag pond, the lake was expanded by the construction of a 100 foot (30 m) high earth dam in 1868.

[8] Construction of the 1868 dam would have trapped salmonids such as coho salmon (Oncorhynchus kisutch) and coastal rainbow trout (Oncorhynchus mykiss irideus) as both stream resident and steelhead life forms, both known to have been historically present in the San Mateo Creek watershed.

San Mateo Creek once hosted coho salmon as evidenced by specimens collected by Professor Alexander Agassiz of Harvard University in the 1850s and 1860s.

[9][10] The historical presence of coho salmon is also suggested in an 1877 description by Charles Hallock: "Pilarcitos, one of the Spring Valley Water Company's reservoirs, is now well filled with fair-sized trout, and San Andreas, chiefly with silver salmon of generally moderate size".