Knickpoint

In geomorphology, a knickpoint or nickpoint is part 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. In the cycle of erosion model, knickpoints advance one cycle upstream, or inland, replacing an older cycle. A knickpoint that occurs at the head (furthest upstream extent) of a channel is called a headcut. Headcuts resulting in headward erosion are hallmarks of unstable expanding drainage features such as actively eroding gullies. Knickpoints also occur on other planetary bodies that previously had or currently have surface liquids, namely Mars and Titan. On Mars, the knickpoints have a common elevation that suggest a common sea level for a former Martian ocean. On Titan, mountain valleys adjacent to the present-day hydrocarbon seas show evidence of knickpoints and recent sea-level change. 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, tougher rock. Base level is the elevation of the surface of the water body into which a river ultimately drains, usually the ocean. A drop in base level causes a response by the river system to carve into the landscape. This incision begins at the formation of a knickpoint, and its upstream migration depends heavily upon the drainage area (and so the discharge of the river), material through which it cuts, and how large the drop in base level was. Knickpoints include both waterfalls and some lakes. These features are common in rivers with a sufficient slope, i.e.