INCREASED ANGULARITY OF SAND-SIZED GRAINS REFLECTING FLUVIAL PROCESSES OF TRABUCO CREEK, ORANGE COUNTY, CA: PRELIMINARY RESULTS FROM A COMMUNITY COLLEGE RESEARCH EXPERIENCE

As part of an early undergraduate research experience (early REU), we are conducting an ongoing analysis of Trabuco Creek, part of the San Juan Creek Watershed in Orange County, CA. The project involves cross-sectional and longitudinal stream profiling, sediment collection, grain size analysis, and roundness studies from 10 study areas dispersed along a roughly 20 km length of the creek. Sedimentary grains generally become more rounded due to random abrasion of sharp corners as they are transported further from their source. In our roundness studies, sediments in the 2.00mm through 63μm size fractions generally show increasing roundness with distance from the headwaters. However, an unexpected increase in angularity was observed ~ 3 kilometers downstream from the headward section of the study area. The sudden influx of angular grains reset the ratio of angular vs. rounded grains to that exhibited at the headwaters of our study area.

We formulated several working hypotheses in response to this unexpected trend in angularity: 1. Short, steep tributaries contribute sediment which has undergone less transport and is more angular into Trabuco Creek. We excluded a section of Trabuco Creek in our stream profiling and sampling due to lack of accessibility to the site. Renewed efforts to sample this area and conduct roundness studies for the 2.00mm to 63μm fraction will provide insight into the importance of tributary contribution. 2. Alternatively, expected trends of increasing roundedness downstream have been modified in Trabuco Creek because the system is susceptible to sporadic, high-energy events, and particles are subjected to collisions with one another. An increase in energy could be capable of transporting larger clasts via saltation, causing impacts among clasts and contributing to increased angularity. We expect that our studies will reveal a clearer answer as to what causes this unexpected increase in angularity.