Bankfull Discharge is (often) defined as the average high flow of a river that recurs about every two and a half years. Sometime bankfull discharge can be easily determined from the high water mark and local topography, whereas in other cases it is somewhat of an abstraction. Here's a chart of the yearly high flow (in CFS), for the Red River, near Questa NM, since 1915. It gives a good idea of the variability inherent to streamflow. What do you think the bankfull discharge is?