We explore the response of bedrock streams to eustatic and tectonically induced fluctuations in base level. A numerical model coupling onshore fluvial erosion with offshore wave-base erosion is developed. The results of a series of simulations for simple transgressions with constant rate of sea-level change (SLR) show that response depends on the relative rates of rock uplift (U) and wave-base erosion (εm). Simple regression runs highlight the importance of nearshore bathymetry. Shoreline position during sea-level fall is set by the relative rate of base-level fall (U-SLR) and εm, and is constant horizontally when these two quantities are equal. The results of models forced by a realistic Late Quaternary sea-level curve are presented. These runs show that a stable shoreline position cannot be obtained if offshore uplift rates exceed εm. Only in the presence of a relatively stable shoreline position, fluvial profiles can begin to approximate a steady-state condition, with U balanced by fluvial erosion rate (ε∫). In the presence of a rapid offshore decrease in rock-uplift rate (U), short (~ 5 km) fluvial channels respond to significant changes in rock-uplift rate in just a few eustatic cycles. The results of the model are compared to real stream-profile data from the Mendocino triple junction region of northern California. The late Holocene sea-level stillstand response exhibited by the simulated channels is similar to the low-gradient mouths seen in the California streams.
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