Effective Discharge Computations for an Intensively Monitored Illinois Stream

Substantial effort has been spent computing effective discharge (Q_e) values and their corresponding flow recurrence intervals for different stream sizes and within different geographical and ecological regions. Existing literature reports recurrence intervals varying from less than 1.1 years to more than 10 years with a commonly accepted average of between 1.5 and 2.0 years. However, little is known about the accuracy and variability associated with estimating Q_e. Using data from an intensely monitored watershed in Southern Illinois, an analysis was conducted to determine the influence that three existing methods of assigning discharge classes and loads carried by each discharge class have on Q_e recurrence intervals. The different methods estimated recurrence intervals ranging from less than 1.1 years to 9.5 years. Methods for reducing the subjective nature of estimating Q_e and validating Q_e estimates as dominant discharges capable of forming and maintaining a channel are needed.