CONCEPTUAL MODEL FOR THE PROVENANCE AND FLUX OF SUSPENDED SEDIMENT FOR A MID-ATLANTIC WATERSHED

We present a conceptual model for the provenance and flux of suspended sediment for a mid-Atlantic watershed in the context of its geology and anthropogenic influences. The Saucon Creek watershed in eastern Pennsylvania drains approximately 150 km² of mixed use land and variable bedrock that includes significant storage of legacy sediments. The watershed was formerly listed as impaired and underwent a TMDL study driven by the hypothesis that high sediment loads were responsible for the impairment. Over the past 5 years, we have instrumented the watershed to create a sediment budget in the context of its hydrology and response to storm events. In this time frame we have been able to capture extreme events including Hurricane Irene on 28 August 2011. Sediment provenance has been assessed through a fingerprinting approach using the fallout radionuclide ²¹⁰Pb. Sediment flux has been measured using ISCO-calibrated turbidity and discharge at two gaging stations, one near the headwaters and the other near the mouth of the trunk channel. Our results show that during most storms the rise and peak in suspended sediment precedes the rise and peak in discharge by ~1-2 hours, an observation shared by many similar studies in the mid-Atlantic region. However, we also note a population of storms, immediately preceded by a wet period, when the rise and peak in sediment flux tracks directly with the discharge hydrograph. Overall, sediment flux increases with discharge, but non-linearly strongly suggesting a sediment-limited system. The provenance of this sediment appears to be a mixed contribution from both overland and legacy sources. The overland sources dominate in the steeper headwater portions of the watershed and during the peak and falling limb of the hydrograph. In contrast, legacy sediments dominate in the lower reaches of the watershed and are mobilized during the rising limb of the hydrograph. These results are consistent with previous studies that attributed annual suspended sediment flux to channel widening and indicated the importance of channel wall slumping as providing in-channel sources of sediment during the waning phase of the discharge hydrograph. We estimate an annual sediment flux at the headwaters gaging station of ~5500 m³/yr which scales to a long-term erosion rate of ~40 m/Ma.