Abstract

Dryland river deposits are archives of past changes in fluvial-system form and process. Chronostratigraphic reconstruction of the late Holocene alluvial history of the Cuyama River in west-central California reveals past spatial and temporal variation in dryland channel form and process. The modern Cuyama River consists of a wide braided reach in the upper drainage basin, a narrower arroyo reach in the middle of the drainage basin, and a bedrock canyon reach in the lower drainage basin that drains to the Santa Maria coastal plain. Along the arroyo reach, late Holocene stratigraphy is well exposed and is the focus of this study. Between ca. 1700 and 350 yr B.P., two widespread deposits of tabular-bedded clay, silt, and fine sand were deposited, separated by a buried soil formed between 950 and 700 yr B.P. Channel incision occurred between 550 and 350 yr B.P. Next, deposition of massive to bedded sands occurred in a pattern alternating between poorly confined deposition on top of older deposits onto the broad valley floor, and channelized deposition along the valley axis inset into older deposits. The superposed deposits now underlie the main valley terrace, and the valley-axis deposits are preserved as inset fill terraces. Historical arroyo cutting then formed a 65-km-long arroyo ca. 150 yr B.P.

Based on correlation to regional paleoclimate records, channel aggradation occurred during periods of relative aridity and low peak discharge events, while wet periods, possibly floods after drought, led to fluvial incision. These cycles are superimposed on a transition from a wide, silt- and fine-sand–dominated fluvial system to a modern, narrow, sand- and gravel-dominated arroyo channel. The relationship between times of fluvial process change and climate change in Cuyama Valley bears considerable similarity to other well-studied dryland rivers in the southwestern United States; however, the complex sedimentology and geometry of preserved fluvial deposits suggest that a wider range of fluvial modes occurred along the Cuyama River than has been reported in simpler cut-and-fill–dominated channels elsewhere. These spatial and sedimentological complexities underscore the need to link fluvial deposits and their bounding unconformities along a channel in order to fully understand the spatial and temporal evolution of ancient fluvial systems.

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