CHANGING DEPOSITIONAL REGIMES IN THE HUMAN IMPACTED ESTUARY OF A SMALL, MOUNTAINOUS WATERSHED

Small (10^-1-10¹ km²) bar-built estuaries are typical geomorphic features of North American Pacific margin. Most of these estuaries were formed by the hydro-sedimentological interaction of small mountainous rivers and coastal marine forcings during the Holocene global rise in sea level, and have persisted for thousands of years. Long-term accretion rates equivalent to relative sea level rise in these systems likely interact with fluvial water and sediment supply to set estuarine channel migration vs. aggradation rates, and determine estuarine sedimentological architecture. However, extensive human development in the region over recent centuries has generally resulted in dramatic and complex physical alterations, with potential ramifications for their sedimentological development.

Here we examine changes in marsh platform accretion and channel position using sedimentation rates and sediment particle size distributions in the Scott Creek Estuary, California, from the late Holocene and Anthropocene. Over last 250 years the Scott Creek watershed (drainage area 190 km²) has experienced changes in land use, including intensive timber harvesting, and the lower Scott Creek channel was straightened and flanked with training berms in 1930’s to flow under the Hwy. 1 bridge. Long term late Holocene sedimentation rates on the marsh platform equivalent to rates of relative sea level rise were followed by an apparent increase in sedimentation rates associated with the European/US settlement. Effects of hydraulic modification on the estuarine sedimentation rates were eclipsed by a single crevasse splay deposit during a large rainfall-runoff event in the 1950s. Particle size analysis using Tanner domains and end member mixing models effectively discriminated between artificially abandoned channel fill and active channel alluviation, and were used to estimate the migration rate of the lower Scott Creek channel over the late Holocene. While human impacts on the geomorphic and sedimentological trajectory of bar built estuaries may be largely muted by the stochastic characteristics of North Pacific coastal fluvial forcings, long term implications of augmented or reduced sediment supplies may alter migration vs. aggradational time scales, with potential implications for estuarine channel stability.