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Whitewater is formed in a rapid, when a river's gradient increases enough to create so much turbulence that air is entrained into the water body, that is, it forms a bubbly or aerated and unstable current; the frothy water appears white. The term is also loosely used to refer to less turbulent, but still agitated, flows.
The term "whitewater" also has a broader meaning, applying to any river or creek itself that has a significant number of rapids. The term is also 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, obstruction and flow rate. Gradient, constriction and obstruction are streambed topography factors and are relatively 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, and is generally 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, and 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 rapidly (hence the name) and to react differently[dubious ] to riverbed events (rocks, drops, etc.).
A boulder or ledge in the middle of a river or near the side can obstruct the flow of the river, and can also create a "pillow"; when water flows backwards upstream of the obstruction, or a "pour over" (over the boulder); and "hydraulics" or "holes" where the river flows back on itself—perhaps back under the drop—often with fearful results for those caught in its grasp. (Holes, or hydraulics, are so-called because their foamy, aerated water provides less buoyancy and can feel like an actual hole in the river surface.) If the flow passes next to the obstruction, an eddy may form behind the obstruction; although eddies are typically sheltered areas where boaters can stop to rest, scout or leave the main current, they may be swirling and whirlpool-like. As with hydraulics (which pull downward rather than to the side and are essentially eddies turned at a 90-degree angle), 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 substantially 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 (where previously wasn't one), "wash out" a rapid (decreasing the hazard) or make safe passage through previously-navigable rapids more difficult or impossible. Flow rate is typically measured in cubic metres per second (m3/s), or in cubic feet per second (cfs), depending on the country.
The most widely used grading system is the International Scale of River Difficulty, where whitewater (either an individual rapid, or the entire river) is classed in six categories from class I (the easiest and safest) to class VI (the most difficult and most dangerous). 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, and grade VI referring to the hardest rapids which are very dangerous even for expert paddlers, and are rarely run. Grade-VI rapids are sometimes downgraded to grade-V or V+ if they have been run successfully. Harder rapids (for example a grade-V rapid on a mainly grade-III river) are often 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 greatly 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 usually makes rapids more difficult and dangerous. At flood stage, even rapids which are usually easy can contain lethal and unpredictable hazards (briefly adapted from the American version of the International Scale of River Difficulty).
On any given rapid there can be a multitude of different features which arise from the interplay between the shape of the riverbed and the velocity of the water in the stream.
Strainers are formed when an object blocks the passage of larger objects but allows the flow of water to continue - like a big food strainer or colander. These objects can be very dangerous, because the force of the water will pin an object or body against the strainer and then pile up, pushing it down under water. For a person caught in this position, it will be difficult or impossible to get to safety, often leading to a fatal outcome.
Strainers are formed by many natural or man-made objects, like storm grates over tunnels, trees that have fallen into a river ("log jam"), bushes by the side of the river that are flooded during high water, wire fence, rebar from broken concrete structures in the water, or other debris. Strainers occur naturally most often on the outside curves of rivers where the current undermines the shore exposing the roots of trees and causing them to fall into the river forming strainers.
In an emergency it is often best to try to climb on top of a strainer so as not to be pinned against the object under the water. In a river, swimming aggressively away from the strainer and into the main channel is recommended. If it is impossible to avoid the strainer, one should swim hard towards it and try to get as much of one's body up and over it as possible.
Sweepers are trees fallen in or heavily leaning over the river, still rooted on the shore and not fully submerged. Its trunk and branches may form an obstruction in the river like strainers. Since it is an obstruction from above, it often does not contribute to whitewater features but may create turbulence. In fast water sweepers can pose a serious hazard to paddlers.
Holes, or "hydraulics", (also known as "stoppers" or "souse-holes". See also Pillows), are formed when water pours over the top of a submerged object, or underwater ledges, causing the surface water to flow back upstream toward the object. Holes can be particularly dangerous - a boater may become stuck under the surface, in the recirculating water, or entertaining play-spots, where paddlers use the holes' features to perform various playboating moves. In high and low volume water flow, holes can subtly aerate the water, enough to allow craft to fall through the aerated water to the bottom of a deep 'hole'.
Some of the most dangerous types of holes are formed by low head dams (weirs), and similar types of obstruction. In a low head dam, the 'hole' has a very wide, uniform structure - there's no escape point - and the sides of the hydraulic (ends of the dam) are often blocked by a man-made wall, making it impossible to paddle around, or slip off the side of the hydraulic, where the bypass water flow would become normal (laminar). By (upside-down) analogy, this would be much like a surfer slipping out the end of the pipeline, where the wave no longer breaks. Low head dams are insidiously dangerous because their danger cannot be easily recognized by people who have not studied swift water. (Even 'experts' have died in them.) Floating debris (trees, kayaks, and so forth) is often trapped in these retro-flow 'grinders' for weeks at a time.
Waves are formed in a similar manner to hydraulics and are sometimes also considered hydraulics as well. Waves are noted by the large smooth face on the water rushing down. Sometimes a particularly large wave will also be followed by a "wave train", a long series of waves. These standing waves can be smooth or, particularly the larger ones, can be breaking waves (also called "whitecaps" or "haystacks").
Because of the rough and random pattern of a riverbed, waves are often not perpendicular to the river's current. This makes them challenging for boaters since a strong sideways or diagonal (also called a "lateral") wave can throw the craft off if the craft hits sideways or at an angle. The safest move for a whitewater boater approaching a lateral is to "square up" or turn the boat such that it hits the wave along the boats longest axis, reducing the chance of the boat flipping or capsizing. This is often counter-intuitive because it requires turning the boat such that it is no longer parallel to the current.
In fluid mechanics, waves are classified as laminar, but the whitewater world has also included waves with turbulence ("breaking waves") under the general heading of waves.
Pillows are formed when a large flow of water runs into a large obstruction, causing water to "pile up" or "boil" against the face of the obstruction. Pillows normally signal that a rock is not undercut. Pillows are also known as "pressure waves".
Eddies are formed, like hydraulics, on the downstream face of an obstruction. Unlike hydraulics, eddies swirl on the horizontal surface of the water. Typically, they are calm spots where the downward movement of water is partially or fully arrested—a nice place to rest or to make one's way upstream. However, in very powerful water, eddies can have powerful, swirling currents which can flip boats and from which escape can be very difficult.
Undercut rocks are rocks that have been worn down underneath the surface by the river, or loose boulders which cantilever out beyond their resting spots on the riverbed. They can be extremely dangerous features of a rapid because a person can get trapped underneath them underwater. This is especially true of rocks that are undercut on the upstream side. Here, a boater may become pinned against the rock underwater. Many whitewater deaths have occurred in this fashion. Undercuts sometimes have pillows, but other times the water just flows smoothly under them, which can indicate that the rock is undercut. Undercuts are most common in rivers where the riverbed cuts through sedimentary rocks like limestone rather than igneous rock like granite. In a steep canyon, the side walls of the canyon can also be undercut.
A particularly notorious undercut rock is Dimple Rock, in Dimple Rapid on the Lower Youghiogheny River, a very popular rafting and kayaking river in Pennsylvania. Of about nine people that have died at or near Dimple Rock, including three in 2000, several of the deaths were the result of people becoming entrapped after they were swept under the rock. 
Another major whitewater feature is a sieve, which is a narrow empty space that water flows through between two obstructions, usually rocks. Similar to strainers, water is forced through the sieve, resulting in higher flow velocity which forces water up and creates turbulence.
There are many types of whitewater craft that people use to make their way down a rapid, preferably with finesse and control. Here is a short list of them.
Whitewater kayaks differ from sea kayaks and recreational kayaks in that they are better specialized to deal with moving water. They are often shorter and more maneuverable than sea kayaks and are specially designed to deal with water flowing up onto their decks. Most whitewater kayaks are made of plastics these days, although some paddlers (especially racers and "squirt boaters") use kayaks made of fiberglass composites. Whitewater kayaks are fairly stable in turbulent water, once the paddler is skillful with them; if flipped upside-down, the skilled paddler can easily roll them back upright. This essential skill of whitewater kayaking is called the "Eskimo Roll", or simply "Roll." Kayaks are paddled in a low sitting position (legs extended forward), with a two-bladed paddle. See Whitewater kayaking.
Rafts are also often used as a whitewater craft; more stable than typical kayaks, they are less maneuverable. Rafts can carry large loads, so they are often used for expeditions. Typical whitewater rafts are inflatable craft, made from high strength fabric coated with PVC, Urethane, Neoprene or Hypalon; see rafting. While most rafts are large multi-passenger craft, the smallest rafts are single-person whitewater craft, see packraft. Rafts sometimes have inflatable floors, with holes around the edges, that allow water that splashes into the boat to easily flow to the side and out the bottom (these are typically called "self-bailers" because the occupants don't have to "bail" water out with a bucket). Others have simple fabric floors, without anyway for water to escape, these are called "bucket boats", both for their tendency to hold water like a bucket, and because the only way to get water out of them is by bailing with a bucket.
Catarafts are constructed from the same materials as rafts. They can either be paddled or rowed with oars. Typical catarafts are constructed from two inflatable pontoons on either side of the craft that are bridged by a frame. Oar-propelled catarafts have the occupants sitting on seats mounted on the frame. Virtually all oar-powered catarafts are operated by a boatsman with passengers having no direct responsibilities. Catarafts can be of all sizes; many are smaller and more maneuverable than a typical raft.
Canoes are often made of fiberglass, kevlar, plastic or a combination of the three for strength and durability. They may have a spraycover, resembling a kayak, or be "open," resembling the typical canoe. This type of canoe is usually referred to simply as an "open boat." Whitewater canoes are paddled in a low kneeling position, with a one-bladed paddle. Open whitewater canoes often have large airbags and in some cases foam, usually 2 lb density ethyl foam, firmly attached to the sides, to displace water in the boat when swamped by big waves and holes and to allow water to be spilled from the boat while still in the river by floating it up on its side using the foam and bags. Like kayaks, whitewater canoes can be righted after capsizing with an Eskimo Roll; however this requires more skill in a canoe.
C1s are similar in construction to whitewater kayaks. However, they are paddled in a low, kneeling position. They employ the use of a one-blade paddle, usually a little shorter than used in a more traditional canoe. They will have a spraycover, essentially the same type used in kayaking. Like kayaks, C1s can be righted after capsizing with an Eskimo Roll.
McKenzie River dory or "Drift Boat" by some. A more traditional "hard sided" boat. The design is characterized by a wide, flat bottom, flared sides, a narrow, flat bow, a pointed stern, and extreme rocker in the bow and stern to allow the boat to spin about its center for ease in maneuvering in rapids.
River bugs are small single person inflatable craft where a person's feet stick out of one end. River Bugging is done feet first with no paddle.
Running whitewater rivers is a popular recreational sport but is not without danger. In fast moving water there is always the potential for injury or death by drowning or hitting objects. Fatalities do occur; some 50 people die in whitewater accidents in the United States each year.
Strainers and sieves can pose a particular hazard. If the sieve is visible above water, a boater can be pinned against it and may eventually be forced underwater as the current passes through. If the sieve is completely submerged, it is especially insidious because it may not be discernible at all. In shallow water, bows of boats can get caught in submerged sieves, as the current pulls the nose down below the rocks where it can lodge. If this happens, it is likely that the whole boat will get pulled under water. Sieves pose a particular hazard to swimmers because even the smallest sieves can trap a person's foot if they stand up in the current. The force of the current then pushes the whole body underwater, becoming a deadly situation in a matter of seconds. It is for this reason that one of the first things whitewater boaters learn is never to stand up in more than ankle deep water where there is a current.
The dangers can be mitigated (but not eliminated) by training, experience, scouting, the use of safety equipment (such as personal flotation device, helmet, throw ropes), and using other persons as "spotters".
Scouting or examining the rapids before running them is crucial to familiarize oneself with the stream and anticipate the challenges. This is especially important during flood conditions when the highly increased flow have altered the normal conditions drastically.
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