Abstract

Examination of medium-grained sand from southern Monterey Bay using Fourier grain-shape analysis and Q-mode multivariate techniques was performed to identify the contribution of sand from the Salinas River to the beaches along the bay following an episode of high stream discharge. Conventional textural and petrographic analyses also were used to aid in the interpretation of potential local sources for the sand in Monterey Bay. Fundamental sedimentologic properties along the littoral zone of Monterey Bay remained much the same during the 1978 flood period, but some distortion and greater sediment mixing occurred within certain segments of the shorezone. Statistically significant shape differences define various potential sand sources. A two-factor solution which explains 91 percent of the total sample variance shows two sources represented by Salinas River sand and southern beach sand (adjacent to a pre-Flandrian and Flandrian dune complex). The other potential sources, offshore and beach sand from north of the Salinas River, are mixtures of the two extracted factors. Offshore sand deposits in the southern bay accumulated by the seaward transport of adjacent beach sand. Northern beach sand, which is similar in shape to Salinas River sand, is deposited by longshore transport of Pajaro River and Soquel Creek sand. Both streams drain an area similar to that drained by the Salinas River, which accounts for the similarity of Salinas River and northern beach sand. During the 1978 flood season, the contribution of Salinas River sand to the southern Monterey Bay beaches ranged from 63 percent south of the river mouth to 0 percent near Fort Ord. Even during a period of high Salinas River discharge, the dominant source of the southern Monterey Bay beach sand is reworking of the adjacent Flandrian and pre-Flandrian dune complex. thus this sand must be considered a nonrenewable resource.

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