Fluvial Response to Episodic Coastal Retreat and Implications for Flora at Gold Bluffs Beach

Merrill, Brian

Episodic coastal retreat brought on by repeated eustatic sea level rise has resulted in incision of formerly upland stream valleys in a region of very low tectonic uplift. Conversely, the most recent Holocene transgression has drowned several lowland stream valley mouths in the southern portion of the study area resulting in lagoons and estuaries that extend inland several hundred meters. A general trend of lower fluvial terrace elevations from north to south suggests a former lowland southwest of the study area possibly related to the presence of Redwood Creek. Absence of local marine terraces suggests a very low uplift rate, which is supported by drainage basin morphometry and recent marsh stratigraphy extracted from the East Creek and Major Creek estuaries. Soil development on the terraces suggests two distinct episodes of stream incision. The first was initiated by a sea-level highstand at approximately 124 ka during oxygen isotope stage 5. A second episode of incision was the result of a return of sea-level to near its present position in mid-Holocene. Furthermore, the incision of the strath terraces in Home Creek and Butler Creek appears to be influenced by zones of fractured bedrock related to high angle wrench faults coincident with the stream valleys. Mass wasting processes are also influenced by the fracturing and to some extent by the stratigraphy in the Plio-Pleistocene bedrock sediments. Hydrologic characteristics of Home Creek suggest that frequent high flows through the coastal canyons scour colluvium from the walls and deposit the sediments as broad fans on Gold Bluffs Beach. Specialized floral communities have exploited the geomorphic niches that have been created and maintained by a shift in the dynamic equilibrium of the fluvial geomorphology.

Fluvial Response to Episodic Coastal Retreat and Implications for Flora at Gold Bluffs Beach

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