Sediment transport dynamics and its relation to primary production in mountain headwater streams

Abstract:

This study explores how the spatial and temporal variability of stream flow and sediment transport influence the growth and recovery rate of benthic primary producers in a rainfall dominated mountain stream over different seasons. Primary producers sustain higher trophic levels and thus understanding how sediment transport impacts their growth is important for efficient river management. The control of sediment transport processes on suitable habitat for these organisms depends on the frequency of high flows capable of mobilizing material and the spatial distribution of forces throughout the stream. However there other factors such as light, nutrient availability, and temperature that are also important. We hypothesize that the control physical processes exert on the growth dynamics of these organisms varies both according to the amount of localized bed disturbance caused by sediment transport and the flow field and seasonal variations in additional growth factors. This hypothesis is tested in a 160m reach of Oak Creek, OR draining 7 km². Habitat disturbance maps were developed based on the spatial and temporal variability of velocity and shear stress and estimated sediment transport rates. These maps are used to select habitat patches with contrasting disturbance levels for benthic chlorophyll a monitoring. Velocity and shear stress were modeled using the two dimensional hydraulic model FaSTMECH, developed by the USGS. Sediment transport rates were estimated using spatially variable magnitudes of shear stress and grain size distributions and validated using both field measurements and historical data. Preliminary results during the spring indicate an inverse correlation between disturbance and algal growth, however there were no flow events above the critical threshold for motion during this time. It is likely that increased nutrient fluxes in areas of higher velocity resulted in increased algal growth rates during these period.