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. 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. 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. enough change in elevation above sea level over their length to encourage degradation. Variations in stability of the underlying rock influence development of a bedrock-channeled river, as the waters erode different rock types at different rates. Victoria Falls, on the Zambezi River, is a spectacular example of this. The gorges visible by satellite imagery illustrate the erosional processes behind the formation of the falls. Here, much of the surface rock is a massive basalt sill, with large cracks filled with easily weathered sandstone made visible by the Zambezi's course across the land. The gorges downstream of the falls through which it flows were eroded over time by the action of the water. Throughout New Zealand, tectonic uplift and faulting are actively contributing to knickpoint initiation and recession. The Waipoua River system, on the North island, has been studied and used to create mathematical models to predict the behavior of knickpoints. The study showed a direct correlation between upstream drainage area and rate of migration, producing modeled data closely approximating the collected data. The Waipoua River system incises through sediments, for the most part, as opposed to bedrock. Sharp changes in slope are common in rivers flowing through the heavily carved landscape left behind when glaciers retreat. Glacial valleys, as well as isostatic rebound resulting from the removal of the mass of glacial ice contribute to this. Niagara Falls, on the border of the United States and Canada, is a characteristic example of knickpoint. The falls have slowed in migration from approximately 1m per year as of 1900 to their modern 10 cm per year. The falls, particularly Horseshoe Falls, are dramatically steep and caused by glaciation. The Great Lakes themselves lie in the depressions left behind by glaciers, as the crust is still rebounding. Bridalveil Fall, in Yosemite Valley, California, pours over the lip of a hanging valley.