A head cut is a physical feature found in a stream. It is an erosional feature found in both intermittent (flows only part of the year) and perennial (flows continuously all year) streams. A head cut occurs where there is an abrupt vertical drop in the streambed. They usually begin at a knickpoint (sharp change in channel slope) which can be as subtle as an over-steepened riffle or as obvious as a waterfall. At the base of a head cut a small plunge pool is usually found, caused by the high energy of falling water. As the streambed erodes and lowers the knickpoint the active head cut will migrate upstream. This is a problem because when a head cut moves up a stream it causes channel incision (the channel bed lowering or down cutting). This causes the stream to lose access to its floodplain. Which causes the stream channel to erode even faster because the waters energy is not being dispersed over the floodplain. The eroding banks lead to trees falling into the stream, therefore causing more erosion.
The past week on one of our project sites we saw a great example of a head cut. Below is a picture of the head cut which can be see with a waterfall flowing over the roots of a tree into a pool at the bottom. Also included is a picture of upstream of the head cut where you can see the stream is in fairly good shape and a picture downstream of the head cut where you can see severe eroding and several trees which are nearly falling in.