TEMPORAL AND SPATIAL VARIABILTY IN SEDIMENT SOURCE IN PLUM CREEK, OBERLIN, OH

Fallout radionuclide fingerprinting of detrital sediments provides a snapshot into the erosional patterns of an area. However, past work suggests that erosion patterns vary seasonally, thus, single samples may not be representative of upstream systems. The purpose of this study is to explore how sediment sources change seasonally in relation to land use and precipitation using ⁷Be, ¹³⁷Cs, and ²¹⁰Pb_ex. We sampled semi-monthly at 4 sites along Plum Creek, Oberlin, Ohio. This 35.7 km² watershed consists of agricultural, urban, and forested land in a region with little topographic (222-274 m), soil (silty and silty clay loams), or precipitation (mean monthly precipitation is 5.58-9.91 cm) variability.

We hypothesize that temporal and spatial changes in sediment source will be due to land use variability and recent precipitation events. Over time activity of ⁷Be and ¹³⁷Cs broadly decreases. Our site furthest upstream drains agricultural land and has low detectable levels of ¹³⁷Cs but no ⁷Be. These data suggest vertical erosion, which supports prior studies suggesting that agricultural drainage tiles accelerate the movement of water to rivers, increasing the intensity of floods and bank erosion. The next downstream site adds a golf course to the drainage area. It has detectable ⁷Be but little to no detectable ¹³⁷Cs. These data suggest that the creek is sourcing deep horizontal surfaces, such as gully bottoms, with some sediment mixed in from vertical sources. The third site is at the end of a forested stretch of creek and consistently has detectable ⁷Be, and ¹³⁷Cs is occasionally detectable (in two of five samples). This suggests that the sediment source varies temporally and combines both surface and gully erosion, with occasional substantial increases in surface runoff. The final site, which is the furthest downstream and incorporates an urban area, has high values of both ⁷Be and ¹³⁷Cs for the first two samples, and then has high ¹³⁷Cs and moderate ⁷Be for the second two samples. This suggests that the sediment source at this site deepened over the study period.

We find that sources of sediment vary both spatially and temporally. As sampling continues semimonthly and ²¹⁰Pb_ex analyses are still being completed, our data will continue to provide insight into how sediment source varies in a relatively homogenous, small watershed.