The spatial and temporal variability of sediment transport processes in Oak Creek, OR was investigated and used to explore two study questions: 1) How do sediment transport processes influence benthic algal communities?, and 2) Can fluvial-hydraulic models make accurate predictions of bed load transport rates? Our study was conducted in a 96 m reach of Oak Creek, OR – a small gravel bed stream in the Oregon Coast range. The study site is located in the same reach that was used to collect the historic Oak Creek bed load dataset of Milhous .
To answer study question 1 we characterized the variability in sediment transport for a series of eight high flow events ranging from 0.64-3.4 m³/s using modeled shear stress (τ) from a field calibrated, high resolution (<0.1 m²) 2-dimensional hydrodynamic model (FaSTMECH) coupled with detailed measurements of the channel substrate. The stream bed was then categorized into regions of high and low disturbance based on potential mobility of the median grain size using a Shield’s stress approach. High resolution (<0.25m²), in-situ measurements benthic Chlorophyll-a (used as a metric of benthic algal production) were taken before and after high flow events in regions of contrasting disturbance to understand how benthic algal communities respond to sediment transport disturbance through both space and time. Growth factors including temperature, light, and nutrients were also measured. There was high spatial and temporal variability in both sediment transport and benthic Chl-a throughout the study period. We found significant differences (p<0.05) in benthic Chl-a concentrations between regions of high and low disturbance in half of the sampling events. The influence of sediment transport on benthic Chl-a was dependent on both the bed mobility and pre-disturbance Chl-a concentrations. There were also differences in benthic algal recovery rates in regions of contrasting disturbance following high flow events. The relationship between τ and Chl-a was highly variable, however the 95th percentile quantile regression of Chl-a was consistent with the bell-curve shape of the intermediate disturbance hypothesis for 10/18 sampling events. This study shows that sediment transport processes do influence benthic algal growth dynamics however the magnitude of that influence is also dependent on pre-disturbance productivity.
To answer study question 2 we made contemporary bed load measurements during 5 flow events ranging from 0.24 bankfull (Q[subscript bf]) to 0.52 Q[subscript bf] using a Helly-Smith bed load sampler in order to confirm the stability of sediment transport dynamics in Oak Creek. The contemporary measurements were consistent with the historical dataset for total load however, they had a finer grain size distribution (GSD). A 2-dimensional (2-D) hydrodynamic model (FaSTMECH) was used to calculate spatial distributions of τ for 5 flow levels ranging from 0.2 Qbf to Qbf (0.64-3.4 m³/s). Results indicate that τ is highly variable within the study reach and that mean normalized τ distributions are remarkably similar between flow levels. The τ distributions were then discretized and used to calculate bed load using surface and subsurface transport equations, and compared against the historical dataset for accuracy. Modeled bed load was consistently larger and coarser than the historical samples. Areas of τ greater than 2 times the mean comprised <2.5% of the bed and were responsible for transporting >32% of the bed load. We hypothesize that the inconsistency in our estimates may be due to combining 2-D τ with a reach averaged reference shields stress (τr*) value which may have caused such high transport rates from a small portion of the bed. Scaling τr* with τ throughout a reach may provide the basis for future work to incorporate spatially variable τ into commonly used bed load transport functions.