Rapids
Three factors, separately or in combination, can create rapids: gradient, constriction, and obstruction.

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 to riverbed events (rocks, drops, etc.)

Lastly, there is obstruction. 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 "drop" (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 are, essentially, eddies turned at a 90-degree angle), the power of eddies increases with the flow rate.

Classification of whitewater

Main article: International Scale of River Difficulty

The most widely used grading system is the International Grading System, 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[1] of the International Scale of River Difficulty[2].)

• Class 1: no rapids, smooth flowing water

• Class 2: some rough water--the line is easy to see and pursue

• Class 3: whitewater but very safe for larger rafts; kayakers and canoeists should have good rolling or self-rescue skills

• Class 4: whitewater for experienced paddlers only; the route through the rapids may require quick maneuvering

• Class 5: whitewater for advanced paddlers; scouting the rapid may be required, and there may be hidden hazards which require precise maneuvering

• Class 6: impassable or exploratory; for teams of experts, taking all safety precautions

Features found in whitewater
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
Strainers are formed when an object blocks the passage of larger objects but allows the flow of water to continue - like a big strainer. 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. Strainers are formed by many different 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, or rebar from broken concrete structures in the water. In an emergency it is often best to try and climb on top of a strainer so as not to be pinned against the object under the water. If you are in a river, swimming aggressively away from the strainer and into the main channel is your best bet. If you cannot avoid the strainer, you should swim hard towards it and try to get as much of your body up and over it.

Sweepers
Sweepers are trees fallen 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 it should be mentioned here because in fast water sweepers can pose a serious hazard to paddlers.

Hydraulics
Hydraulics, often referred to as "holes" or "souse-holes", are formed when water pours over the top of a submerged object, causing the water downstream to flow back over the top of the water which rushes over the submerged object. Hydraulics can be dangerous: a boater may become stuck in the recirculating water. In high volume water, hydraulics will aerate the water, possibly to the point where it may even loose the capacity to carry any water crafts.

Some of the most dangerous types of hydraulics are formed by lowhead dams (weirs), underwater ledges, and similar types of obstruction. In lowhead dams, the hydraulic has a very symmetrical character - there's no weak point - and where the sides of the hydraulic are often blocked by a man-made wall, making it impossible to slip off the side of the hydraulic. Lowhead dams are insidiously dangerous because their danger cannot be easily recognized by people who have not studied whitewater.

Waves
Waves are formed in a similar nature 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 wave can throw the craft off.

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
Pillows are formed when a large flow of water runs into a large obstruction, causing water to boil up against the face of the obstruction. Pillows can be dangerous because sometimes the object that forms the pillow is undercut and so paddlers can be swept underwater - possibly to be entrapped.

Eddies
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.

Whitewater craft
There are many different 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 specialized to deal with moving water better. They are often shorter and more maneuverable then 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; but once flipped upside-down, the skilled paddler can roll them back upright. This essential skill of whitewater kayaking is called the "Eskimo Roll," or simply "Roll." Kayaks are paddled in a sitting position, with a two-bladed paddle.

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 plastic - much like an air mattress.

Catarafts are similar to rafts, constructed from two inflatable pontoons on either side of the craft which are bridged by a support carriage where the occupants sit. Catarafts can be smaller and more maneuverable than many typical rafts.

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 "open," resembling the typical canoe. Whitewater canoes are paddled in a kneeling position, with a one-bladed paddle. Open whitewater Canoes have large airbags to prevent the boat from being swamped by big waves and holes. Like kayaks, whitewater canoes can be righted after an upset with an Eskimo Roll.

Source - Internet