INTEGRATING TWO SEDIMENTATION RATE METHODS TO DETERMINE PAST CHANNEL ADJUSTMENT RATES

Channel adjustment and sedimentation rates are needed when determining stream management activities to reduce erosion and sedimentation. Channel adjustment rates are typically determined using historical channel geometry, channel profiles, and air photos, or dendrogeomorphic techniques. When this information is not available, sediment deposition rates of a downstream waterbody may be used as a surrogate for long-term channel adjustment rates in the watershed. Methods of determining sediment deposition rates include developing sediment budgets and/or radiometric ( 137 Cs) analysis. Using these two methods, short-term and long-term depositional rates within the Lower Cache River-Cypress Creek Wetland in southern Illinois were computed and compared. Results provide insights into the historical rate of upstream channel adjustments in the watershed. Specifically, based on sediment core data collected in 2000, the radiometric ( 137 Cs) analysis determined that the wetland has a long-term sediment deposition rate of 0.86 cm/yr (1963-2000). The sediment budget method, based upon sediment monitoring data collected between 1986 and 2002, computed a nearly identical short-term sediment deposition rate of 0.79 cm/yr. Since the depositional rates determined by the two methods are very similar, it is reasonable to assume that the sediment deposition in the wetland, as well as the channel adjustment rates in Big Creek, have been steady since at least 1963.