Approximately 1,130 preconstruction holes, with an average depth of about 4 m, were augered in the 2,025 km2 Westlands Water District, which is underlain mostly by Quaternary piedmont alluvium derived from the Coast Ranges. The alluvium consists of fluvial deposits of major ephemeral streams, separated by “interfan” alluvium of minor streams frequently deposited as “mudflows.” Average visual textural composition and average content of sand-size particles of alluvium were calculated for three depth intervals (0 to 1.5 m, 1.5 to 3 m and 3 to 4± m) for all fans and interfans in 461 the following three areas: 1) the western area, between the Coast Range foothills and San Luis Canal; 2) the central area between the San Luis Canal and the “outer rim” of piedmont alluvium; and 3) the eastern area or “outer rim” of the piedmont alluvium.

The overall average composition of near-surface piedmont alluvium is: gravel—traces; sand, mostly clayey or silty—20 percent; silt—1 percent; lean clay—68 percent; and fat clay—11 percent. The sand content of interfan alluvium is notably larger than that of the alluvium of major neighboring fans. There are also increases in sand content 1) from the valley trough toward the Coast Range foothills, and 2) from the northwest toward the southeast. The amount of fat clay increases toward the valley trough.

These patterns are explained by 1) differences in geology of source areas, 2) decreases in transportation forces away from the foothills, 3) variations in mode of deposition, 4) possible historical climatic changes; and 5) post-depositional ion exchange of clay minerals.

This study is a byproduct of preconstruction studies and has been useful for the planning of drainage and irrigation, application of gypsum, preparation of environmental reports and several specifications, etc. The information may also be helpful in planning future projects in similar piedmont terrain.

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