ABSTRACT
The changes in the Willow Drainage Ditch since 1919–1920 have been reconstructed by comparison of the original profile of the ditch, historical records, and a survey of the ditch in 1958. Since construction, the drainage ditch has filled where the ditch was constructed on the Missouri River Valley. The drainage ditch in the Willow River Valley has entrenched, however, and becomes progressively deeper upstream, attaining a maximum depth of 42 feet at the Monona-Harrison County line.
The drainage ditch in the Willow River Valley apparently deepened by channel scour and headward movement of knickpoints, but in a number of entrenchments rather than a single entrenchment followed by stabilization. Once a knickpoint has passed a point in the drainage ditch, stabilization of the channel does not necessarily follow but channel scour may deepen the ditch more than passage of the knickpoint.
The filling of the lower part of the drainage ditch probably is caused by the sharp decrease in gradient of the ditch, and by coinciding periods of high water in the Boyer Drainage Ditch or the Missouri River, to which the Willow is affluent. The entrenchment in the Willow River Valley probably was produced by a constructed increase in gradient of the drainage ditch as contrasted to that of the original river gradient in the same area. The entrenchment of the Willow Drainage Ditch has been responsible for much of, but cannot explain all of, the entrenchment of its tributary streams.
