CONTRIBUTIONS OF SUSPENDED LOAD TO THE MISSOURI RIVER DOWNSTREAM OF GAVINS POINT DAM, YANKTON, SOUTH DAKOTA

The installation of the mainstem dams on the Missouri River has reduced the suspended sediment transported along the fifty-nine mile stretch of the Missouri National Recreational River to approximately 0.2% of its former load (Jacobson et al., 2009). Reduction in sediment load influences sedimentation and habitat development along the Missouri. This project that started in 2012 is the beginning of a long-term study to assess the contributions of suspended load from major tributaries of the Missouri River located between Yankton, SD and Sioux City, IA. Using a depth integrated sampler, samples were collected along cross-channel transects of the Missouri River upstream and downstream of tributaries (James and Vermillion rivers) and from the tributaries themselves. Samples were analyzed for grain size, mineralogical composition, and organic matter content in the summer of 2012 and 2013. Preliminary results indicate that the grain size of the suspended particles in the Vermillion and James rivers are significantly smaller compared to the suspended particles in the Missouri. The differences in particle size are attributed primarily to differences in flow velocity. The levels of organic material and salt content in the suspended sediment of the James and Vermillion rivers were found to be higher than that of the sediment in the Missouri River in 2012. Discharge from the tributaries during 2012 was exceptionally low due to drought, therefore suspended load contribution from the tributaries did not notably influence the amount of suspended load carried by the Missouri at that time. Discharge of the tributaries in 2013 was higher, yet little change in the amount of suspended load was documented. However, slight increases in the suspended load content of the Missouri River immediately downstream of tributaries was detected. Our results suggest that tributaries may only affect suspended load of the Missouri River when tributary discharge is high. The significantly larger volume of the Missouri compared to the tributaries appears to effectively dilute contributions from tributaries. Additional study will be conducted to determine if seasonal changes in discharge may influence suspended load characteristics.