Bison and Cattle Grazing Impacts on Baseflow Suspended Sediment Concentrations within Grassland Streams

Prior to European colonization and the introduction of cattle, bison were the prominent grazing ungulates throughout North American grasslands. Yet, relative zoogeomorphic impacts between bison and cattle on grassland streams remain largely unknown. Utilizing a paired watershed study design, we compared baseflow suspended sediment concentrations across ten watersheds and four grazing treatments (ungrazed, bison-grazed, moderately stocked cattle-grazed, and intensively stocked cattle-grazed) in the Flint Hills subregion of the Great Plains. Additionally, we determined whether periods of increased thermal stress led to higher sediment concentrations within each treatment. Water samples were analyzed for total suspended solids (TSS, mg/L), nonvolatile suspended solids (NVSS, mg/L), and percentage organic matter (POM, percent). Intensively stocked cattle-grazed treatments produced significantly higher TSS and NVSS concentrations relative to ungrazed (TSS p = 0.012, NVSS p 0.10). Additionally, intensively and moderately stocked cattle-grazed treatments showed a significant increase in sediment concentrations with increasing temperature (p = 0.024 and p = 0.08, respectively), whereas bison-grazed and ungrazed treatments did not (p > 0.10). At the subdaily timescale, the highest sediment concentrations within cattle-grazed treatments and the greatest difference in sediment concentrations between cattle-grazed and ungrazed treatments coincided with the hottest daily temperatures, further highlighting that cattle-grazing impacts are influenced by thermal conditions.