Fundamentals of Fluvial Geomorphology and Channel Processes
increased slope (i.e., sediment transport capacity) by increasing its bed material load. This increased
sediment load will be derived from the bed and banks of the channel in the form of channel degradation and
bank erosion. As the bed continues to degrade, the zone of increased slope will migrate upstream and the
increased bed material load is transmitted downstream to drive aggradational instability.
Figure 3.21 Channelized Stream and Abandoned Old Channel
The manner in which degradation migrates through a channel system is a very complex process.
Before this process is discussed some of the relevant terminology must first be addressed. The following
definition of terms is based on the terminology used by Schumm et al. (1984). Channel degradation simply
refers to the lowering of the channel bed. Field indicators of degradation occur in the form of
knickpoints or knickzones. A knickpoint is a location on the thalweg of an abrupt change of elevation and
slope (Figure 3.22). This may also be visualized as a waterfall or vertical discontinuity in the stream bed.
A steep reach of channel representing the headward migrating zone is referred to as a knickzone (Figure
3.23). A knickzone is often composed of a series of small knickpoints. Knickpoints and knickzones are
often referred to as headcuts. While headcut is a commonly used term, it does generate some confusion
because it is also used as a description of the headward migration process of degradation. To avoid this
confusion the field indicators of degradation (knickpoints and knickzones) will not be referred to as
headcuts. Rather, a headcut (or headcutting) is defined as a headward migrating zone of degradation.
This headcutting may occur with or without the formation of knickpoints or knickzones which are purely
a function of the materials encountered.