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 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 lithology. For example, uplift along a fault over which a river is flowing will result in an unusually steep reach along a channel, known as a knickzone. Glaciation resulting in a hanging valley are prime spots for knickpoints. If lithology of the rock varies, such as shale amongst igneous rock, erosion will occur more 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 drains 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, its upstream migration depends upon the drainage area, material through which it cuts, how large the drop in base level was.
Knickpoints include 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 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 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 approximating the collected data. The Waipoua River system incises for the most part, as opposed to bedrock. Sharp changes in slope are common in rivers flowing through the 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 1m per year as of 1900 to their modern 10 cm per year. The falls Horseshoe Falls, are 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, pours over the lip of a hanging valley. Evidence of a knickpoint in the geologic past can be preserved in the shape of the bedrock below any subsequent depositions, as well as within sedimentary depositions left unchanged by human or other activity.
Lakes characteristically fill in with sediment over time, but waterfalls erode away. There are few dry examples still visible today of prehistoric knickpoints. Dry Falls, a 3.5 mi long precipice in central Washington, is an example of an ancient knickpoint. Geologic evidence suggests that the water which formed this feature flowed over the Channeled Scablands, bursting from the glacial lake Missoula during an event known as the Missoula Floods and into the Columbia River Gorge. On the Upper Cumberland River, there exist a series of hydrologically abandoned caves which still hold river-deposited sediments; these caves were the subject of an effort to measure the rate of knickpoint migration along the river, as well as to approximate the discharge of the river over time. In karst topography, a river dropping in level influences more than just its channel. Large drainages into the oceans the world over can be seen to have continued over land, once exposed, whether due to tectonic subsidence, sea level rise, or other factors.
Bathymetric imagery is available for much of the United States' western coast, in particular the ocean floor just offshore of rivers in the Pacific Northwest exhibit such underwater features. In certain locations there are still knickpoints preserved in these drowned river channels and valleys. A study conducted within the Mediterranean basin focused on such features. Here, incision was caused by the closing of the Mediterranean at the end of the Miocene; this sudden lack of ocean water influx allowed the basin to decrease in volume and increase in salinity, as a result of the drop in surface level many of the rivers which flow still today into the Mediterranean began to incise. As is observed for many major waterfalls, knickpoints migrate upstream due to bedrock erosion leaving in their wake deep channels and abandoned floodplains, which become terraces. Knickpoint retreat is 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 locat
International scale of river difficulty
The international scale of river difficulty is an American system used to rate the difficulty of navigating a stretch of river, or a single rapid. The scale was created by the American Whitewater Association to evaluate rivers throughout the world, hence international in the title, it should not be confused with the internationally used whitewater scale, published and adapted by a committee of the International Canoe Federation ICF. The grade reflects the technical difficulty and skill level required associated with the section of river; the scale is of use to various water sports and activities, such as rafting, whitewater canoeing, stand up paddle surfing, whitewater kayaking. There are six categories, each referred to; the scale is not linear, nor is it fixed. For instance, there can be difficult grade twos, easy grade threes, so on; the grade of a river may change with the level of flow. A river or rapid will be given a numerical grade, a plus or minus to indicate if it is in the higher or lower end of the difficulty level.
While a river section may be given an overall grading, it may contain sections above that grade noted as features, or conversely, it may contain sections of lower graded water as well. Details of portages may be given. A summary of river classifications as presented by the American Whitewater Association: Classifications can vary enormously, depending on the skill level and experience of the paddlers who rated the river. For example, at the 1999 International Conference on Outdoor Recreation and Education, an author of a paddling guide pointed out that there is too much variation in what is covered by the Class I designation, proposed making further distinctions within the Class I flat water designations and Class I+ moving water designations, with the goal of providing better information for canoeists, instructors leading trips, families with young children; the grade of a river or rapid is to change along with the level of the water. High water makes rapids more difficult and dangerous, although some rapids may be easier at high flows because features are covered or washed out.
At spate/flood stage rapids which are easy can contain lethal and unpredictable hazards. Conversely, some rapids may be easier with lower water levels when dangerous hydraulics become easier to manage; some rivers with high volumes of fast moving water may require little maneuvering, but will pose serious risk of injury or death in the event of a capsize. Degree of difficulty
A floodplain or flood plain is an area of land adjacent to a stream or river which stretches from the banks of its channel to the base of the enclosing valley walls, which experiences flooding during periods of high discharge. The soils consist of levees and sands deposited during floods. Levees are the heaviest materials and they are deposited first. Floodplains are formed; when a river breaks its banks, it leaves behind layers of alluvium. These build up to create the floor of the plain. Floodplains contain unconsolidated sediments extending below the bed of the stream; these are accumulations of sand, loam, and/or clay, are important aquifers, the water drawn from them being pre-filtered compared to the water in the river. Geologically ancient floodplains are represented in the landscape by fluvial terraces; these are old floodplains that remain high above the present floodplain and indicate former courses of a stream. Sections of the Missouri River floodplain taken by the United States Geological Survey show a great variety of material of varying coarseness, the stream bed having been scoured at one place and filled at another by currents and floods of varying swiftness, so that sometimes the deposits are of coarse gravel, sometimes of fine sand or of fine silt.
It is probable that any section of such an alluvial plain would show deposits of a similar character. The floodplain during its formation is marked by meandering or anastomotic streams, oxbow lakes and bayous, marshes or stagnant pools, is completely covered with water; when the drainage system has ceased to act or is diverted for any reason, the floodplain may become a level area of great fertility, similar in appearance to the floor of an old lake. The floodplain differs, because it is not altogether flat, it has a gentle slope downstream, for a distance, from the side towards the center. The floodplain is the natural place for a river to dissipate its energy. Meanders form over the floodplain to slow down the flow of water and when the channel is at capacity the water spills over the floodplain where it is temporarily stored. In terms of flood management the upper part of the floodplain is crucial as this is where the flood water control starts. Artificial canalisation of the river here will have a major impact on wider flooding.
This is the basis of sustainable flood management. Floodplains can support rich ecosystems, both in quantity and diversity. Tugay forests form an ecosystem associated with floodplains in Central Asia, they are a category of riparian systems. A floodplain can contain 100 or 1,000 times as many species as a river. Wetting of the floodplain soil releases an immediate surge of nutrients: those left over from the last flood, those that result from the rapid decomposition of organic matter that has accumulated since then. Microscopic organisms thrive and larger species enter a rapid breeding cycle. Opportunistic feeders move in to take advantage; the production of nutrients falls away quickly. This makes floodplains valuable for agriculture. River flow rates are undergoing change following suit with climate change; this change is a threat to other floodplain forests. These forests have over time synced their seedling deposits after the spring peaks in flow to best take advantage of the nutrient rich soil generated by peak flow.
Many towns have been built on floodplains, where they are susceptible to flooding, for a number of reasons: access to fresh water. The worst of these, the worst natural disaster were the 1931 China floods, estimated to have killed millions; this had been preceded by the 1887 Yellow River flood, which killed around one million people, is the second-worst natural disaster in history. The extent of floodplain inundation depends in part on the flood magnitude, defined by the return period. In the United States the Federal Emergency Management Agency manages the National Flood Insurance Program; the NFIP offers insurance to properties located within a flood prone area, as defined by the Flood Insurance Rate Map, which depicts various flood risks for a community. The FIRM focuses on delineation of the 100-year flood inundation area known within the NFIP as the Special Flood Hazard Area. Where a detailed study of a waterway has been done, the 100-year floodplain will include the floodway, the critical portion of the floodplain which includes the stream channel and any adjacent areas that must be kept free of encroachments that might block flood flows or restrict storage of flood waters.
Another encountered term is the Special Flood Hazard Area, any area subject to inundation by the 100-year flood. A problem is that any alteration of the watershed upstream of the point in question can affect the ability of the watershed to handle water, thus affects the levels of the periodic floods. A large shopping center and parking lot, for example, may raise the levels of the 5-year, 100-year, other floods, but the maps are adjusted, are rendered
An anabranch is a section of a river or stream that diverts from the main channel or stem of the watercourse and rejoins the main stem downstream. Local anabranches can be the result of small islands in the watercourse. In larger anabranches, the flow can diverge for a distance of several kilometers before rejoining the main channel; the term anabranch, in its hydrological meaning, is used more in Australia than in the rest of the English-speaking world. The term anabranching river describes a river with many anabranches, whilst an anastomosing river is an organic-rich subset of this river type; the term braided river describes watercourses which are divided by small islands into multiple channel threads within a single main channel, but the term does not describe the multiple channels of an anabranching river. A distributary is a branch of a river. A bayou is an anabranch. An anabranch that gets cut off from the main channel becomes an oxbow lake; the Magdalena River, in Colombia, bifurcates in El Banco in two branches: Brazo de Loba and Brazo de Mompox.
The first one, Brazo de Loba is 177 km long. Brazo de Mompox is thus the anabranch. There are several other branches that join those two main branches, interconnected by swamps, the main ones being Quitasol, Chicagua and La Victoria. Between Brazo de Loba and Brazo de Mompox is the island of Mompox or Margarita Island, a sunken geological block of 2.200 square km between the faults of Chicagua and El Romeral. In the Fraser River delta of British Columbia, North Arm Fraser River, Middle Arm Fraser River, South Arm Fraser River each fall into Georgia Strait separately. On the other hand, Annacis Island splits Fraser River into the Annieville Channel and the Annacis Channel, which rejoin below the island. In western New South Wales the Great Darling Anabranch is the ancestral path of the Darling River, dividing south of Menindee and extending for 460 km before joining the Murray River; the anabranch contains flowing water only in wetter years. In the Lower Simpson Desert, in South Australia, Kallakoopah Creek is an anabranch of the Warburton River that flows during major Warburton floods of over 5 m in depth.
The Kallakoopah is 400 km long and members of the Lake Eyre Yacht Club and others have used it to sail a circular route to Lake Eyre by traveling down the Kallakoopah and returning via the Warburton. The Bahr el Zeraf in southern Sudan splits from the Bahr al Jabal section of the White Nile and flows for 240 km, before rejoining the White Nile proper upriver from Malakal. 15 km out of Charters Towers is a section of the Burdekin River known as'the anabranch'. This strip of water is separate from the main flood way, is dry during the sunnier months. Once there is consistent rain, it will flood with fresh water, it is well known for barramundi fishing
A braided river, or braided channel, consists of a network of river channels separated by small temporary, islands called braid bars or, in British usage, aits or eyots. Braided streams occur in rivers with low slope, and/or large sediment load. Braided channels are typical of environments that decrease channel depth, channel velocity, such as river deltas, alluvial fans, peneplains. Braided rivers, as distinct from meandering rivers, occur when a threshold level of sediment load or slope is reached while a steep gradient is maintained. Geologically speaking, an increase in sediment load over time will increase the slope of the river, so these two conditions may be considered synonymous. A threshold slope was experimentally determined to be 0.016 for a 0.15 cu ft/s stream with poorly sorted coarse sand. Any slope over this threshold created a braided stream, while any slope under the threshold created a meandering stream or – for low slopes – a straight channel. So the main controlling factor on river development is the amount of sediment that the river carries.
Important to channel development is the proportion of suspended load sediment to bed load. An increase in suspended sediment allowed for the deposition of fine erosion-resistant material on the inside of a curve, which accentuated the curve and in some instances, caused a river to shift from a braided to a meandering profile; the channels and braid bars are highly mobile, with the river layout changing during flood events. Channels move sideways via differential velocity: On the outside of a curve, swift water picks up sediment, re-deposited in slow-moving water on the inside of a bend; the braided channels may flow within an area defined by stable banks or may occupy an entire valley floor. The Rakaia River in Canterbury, New Zealand has cut a channel 100 metres wide into the surrounding plains. Conditions associated with braided channel formation include: an abundant supply of sediment high stream gradient rapid and frequent variations in water discharge erodible banks a steep channel gradientHowever, the critical factor that determines whether a stream will meander or braid is bank erodibility.
A stream with cohesive banks that are resistant to erosion will form narrow, meandering channels, whereas a stream with erodible banks will form wide, shallow channels, sustaining helical flow and resulting in the formation of braided channels. Extensive braided river systems are found in Alaska, New Zealand's South Island, the Himalayas, which all contain young eroding mountains; the enormous Brahmaputra-Jamuna River in Asia is a classic example of a braided river. Braided river systems are present in Africa, for example in the Touat Valley. A notable example of a large braided stream in the contiguous United States is the Platte River in central and western Nebraska; the sediment of the arid Great Plains is augmented by the presence of the nearby Sandhills region north of the river. A portion of the lower Yellow River takes a braided form; the Sewanee Conglomerate, a Pennsylvanian coarse sandstone and conglomerate unit present on the Cumberland Plateau near the University of the South, may have been deposited by an ancient braided and meandering river that once existed in the eastern United States.
Others have interpreted the depositional environment for this unit as a tidal delta. Notable braided rivers in Europe: Italy Tagliamento Piave Brenta Cellina Meduna Fella Magra Poland and Belarus Narew Anastomosing rivers or streams are similar to braided rivers in that they consist of multiple interweaving channels. However, anastomosing rivers consist of a network of low-gradient, deep channels with stable banks, in contrast to braided rivers, which form on steeper gradients and display less bank stability. Ait – Islands found on the River Thames and its tributaries in England Anabranch – A section of a river or stream that diverts from the main channel and rejoins it downstream. Lagoon – A shallow body of water separated from a larger body of water by barrier islands or reefs Shoal – A natural landform that rises from the bed of a body of water to near the surface and is covered by unconsolidated material Hibbert, Barry. Braided River Field Guide. Christchurch, N. Z.: Deptartment of Conservation.
ISBN 9780478221213. Media related to Braided rivers at Wikimedia Commons
Whitewater is formed in a rapid, when a river's gradient increases enough to generate so much turbulence that air is entrained into the water body, that is, it forms a bubbly or aerated and unstable current. The term is loosely used to refer to less turbulent, but still agitated, flows; the term "whitewater" has a broader meaning, applying to any river or creek itself that has a significant number of rapids. The term is used as an adjective describing boating on such rivers, such as whitewater canoeing or whitewater kayaking. Four factors, separately or in combination, can create rapids: gradient, constriction and flow rate. Gradient and obstruction are streambed topography factors and are consistent. Flow rate is dependent upon both seasonal variation in precipitation and snowmelt and upon release rates of upstream dams. Streambed topography is the primary factor in creating rapids, is consistent over time. Increased flow, as during a flood or high rainfall season can make permanent changes to the streambed by displacing rocks and boulders, by deposition of alluvium or by creating new channels for flowing water.
The gradient of a river is the rate at which it loses elevation along its course. This loss determines the river's slope, to a large extent its rate of flow. Shallow gradients produce gentle, slow rivers while steep gradients are associated with raging torrents. Constrictions can form a rapid when a river's flow is forced into a narrower channel; this pressure causes the water to flow more and to react differently to riverbed events. A boulder or ledge in the middle of a river or near the side can obstruct the flow of the river, can create a "pillow". If the flow passes next to the obstruction, an eddy may form behind the obstruction; as with hydraulics, the power of eddies increases with the flow rate. In large rivers with high flow rates next to an obstruction, "eddy walls" can occur. An eddy wall is formed when the height of the river is higher than the level of the water in the eddy behind the obstruction; this can make it difficult for a boater, who has stopped in that particular eddy, to reenter the river due to a wall of water that can be several feet high at the point at which the eddy meets the river flow.
A marked increase or decrease in flow can create a rapid, "wash out" a rapid or make safe passage through previously-navigable rapids more difficult or impossible. Flow rate is measured in cubic metres per second, or in cubic feet per second, depending on the country; the most used grading system is the International Scale of River Difficulty, where whitewater is classed in six categories from class I to class VI. The grade reflects both the technical difficulty and the danger associated with a rapid, with grade I referring to flat or slow moving water with few hazards, grade VI referring to the hardest rapids which are dangerous for expert paddlers, are run. Grade-VI rapids are sometimes downgraded to grade-V or V+ if they have been run successfully. Harder rapids are portaged, a French term for carrying. A portaged rapid is where the boater lands and carries the boat around the hazard. A rapid's grade is not fixed, since it may vary depending on the water depth and speed of flow. Although some rapids may be easier at high flows because features are covered or "washed-out", high water makes rapids more difficult and dangerous.
At flood stage rapids which are easy can contain lethal and unpredictable hazards. Class 1: Very small rough areas, requires no maneuvering. Class 2: Some rough water, maybe some rocks, small drops, might require maneuvering. Class 3: Medium waves, maybe a 3–5 ft drop, but not much considerable danger. May require significant maneuvering. Class 4: Whitewater, large waves, long rapids, maybe a considerable drop, sharp maneuvers may be needed. Class 5: Whitewater, large waves, continuous rapids, large rocks and hazards, maybe a large drop, precise maneuvering. Characterized by "must make" moves, i.e. failure to execute a specific maneuver at a specific point may result in serious injury or death. Class 5 is sometimes expanded to Class 5+ that describes the most extreme, runnable rapids Class 6: While there is some debate over the term "Class 6", in practice it refers to rapids that are not passable and any attempt to do so would result in serious injury, near drowning or death. If a rapid is run, once thought to be impassible, it is reclassified as Class 5.
On any given rapid there can be a multitude of diff