I recently spent some time slogging around a ravine with a small creek. The creek was incised down into a narrow channel with vertical sides of up to 5 feet over a 200-foot reach. Not a stable stream channel and thus suggesting a change had taken place.
Narrow stream with over steep banks
Stream has cut down under tree roots
Increased stream flow can cause this type of erosion, but in this case it is the stream breaking through a landslide deposit that had previously filled the ravine. The deep erosion channel was progressing up the stream from the lower end of the stream as the stream flowed down over the landslide deposit.
Buried wood in the landslide deposit exposed by stream erosion