The Horseshoe Falls, one of the three Niagara Falls. The falls are a knickpoint, formed by slower erosion above the falls than below.
A knickpoint is a term in geomorphology to describe a location of a river or channel where there is a sharp change in channel slope, such as a waterfall or lake. Knickpoints reflect different conditions and processes on the river often caused by previous erosion due to glaciation or variance in lithology.
Knickpoints are formed by the influence of tectonics, climate history, and/or lithology. For example, uplift along a fault over which a river is flowing will often result in an unusually steep reach along a channel, known as a knickzone. Glaciation resulting in a hanging valley are often prime spots for knickpoints. If lithology of the rock varies, such as shale amongst igneous rock, erosion will occur more steadily in the softer rock than the surrounding.
As is observed for many major waterfalls, knickpoints migrate upstream due to bedrock erosion leaving in their wake deep channels and abandoned floodplains, which then become terraces. Knickpoint retreat is easily demonstrated in some locations affected by postglacial isostatic response and relative sea-level drop such as in Scotland. In other areas, dating of exposed bedrock terraces is more consistent with spatially uniform incision and persistence of the knickzone at about the same location. The river, having gained more potential energy due to gravity, will then proceed to work the knickpoints out of its system by either erosion (in the case of waterfalls) or deposition (in the case of lakes) in order for the river to reattain its smooth concave graded profile.