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

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..


- 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.).


- 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 "cushion"; 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 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..

River/Stream flow rate (Low or High Water)

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 meters per second (cumecs), or in cubic feet per second (cfs), depending on the country.. In the USA, cubic feet per second is the norm.  Usually a noting that there is "High Water" increases the power and danger, and the difficulty of rescue tremendously.  It is often misleading to judge river level at the put-in.  Scout a narrow critical passage.  Could a sudden rise from sun on a snow pack, rain, or a dam release occur on your trip.  This means being responsible for checking each time the weather forecast and dam operation forecast. 

Classification of whitewater

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[http://www.americanwhitewater.org/archive/safety/safety.html#rating%20scale] of the International Scale of River Difficulty[http://www.thameswey.co.uk/articles/grading.htm].)

Class 1: Very small rough areas, requires no maneuvering. (Skill Level: None)
Class 2: Some rough water, maybe some rocks, small drops, might require maneuvering. (Skill Level: Basic Paddling Skill)
Class 3: Whitewater, medium waves, maybe a 3-5 ft drop, but not much considerable danger. May require significant maneuvering. (Skill Level: Experienced paddling skills)
Class 4: Whitewater, large waves, rocks, maybe a considerable drop, sharp maneuvers may be needed. (Skill Level: Whitewater Experience)
Class 5: Whitewater, large waves, large rocks and hazards, maybe a large drop, precise maneuvering (Skill Level: Advanced Whitewater Experience)
Class 6: Whitewater, typically with huge waves, huge rocks and hazards, huge drops, but sometimes labeled thusly due to largely invisible dangers (e.g. , a smooth slide that creates a near-perfect, almost inescapable hydraulic, as at Woodall Shoals/Chattooga). Class 6 rapids are considered hazardous even for expert paddlers using state-of-the-art equipment, and come with the warning "danger to life or limb." (Skill Level: Expert)

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 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. There can be little or no whitewater to warn of the danger.  It's best to continually scan for tell tail signs of pieces that may just break the water surface or lie just underneath. 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, fences submerged, jetty jacks from stream/river flow modifications 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 if possible so as not to be pinned against the object under the water usually your boat. 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 as possible instead of going into it and under.


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 may create turbulence. In fast water sweepers can pose a serious hazard to paddlers.


Holes, or "hydraulics", (also known as "stoppers" or "souse-holes"), are formed when water pours over the top of a submerged object, causing the surface water to flow back upstream toward the object. Holes can be particularly dangerous—a boater may become stuck in the recirculating water—or entertaining playspots, where paddlers use the holes' features to perform various playboating moves. In high-volume water, holes dramatically aerate the water, possibly to the point where it may even lose the capacity to carry any water crafts. Some of the most dangerous types of holes are formed by lowhead dams (weirs), underwater ledges, and similar types of obstruction. In lowhead dams, the hole 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 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.


Weirs are diversions for moving water for irrigation or fishing purposes.  The drop of water over this obstacle can curl back on itself in a stationary wave called a "Reversal" and occurs also in a "Souse Hole".  The surface water is actually heading upstream, and this action will trap any floating object between the drop and the wave.  Very Dangerous and once trapped the only hope is to dive below the surface where the current is flowing downstream (and not get caught in any strainer) or try to swim out the end of the wave.


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 can be dangerous because sometimes the object that forms the pillow is undercut and so paddlers can be swept underwater - possibly to be entrapped. 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

Undercut rocks are rocks that have been worn down underneath the surface by the river. They can be extremely dangerous features of a rapid because a person can get trapped underneath them, under water. This is espcially 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 in Pennsylvania. Nine people have died here, including three in 2000.


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 pressures which forces water up and creates turbulence.

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 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.

Catarafts are constructed from the same materials as rafts. They can either be paddled or rowed with oars. A specialized cataraft, designed without any metal frame, is a Shredder. It was invented in 1982 by Tom Love is manufactured by his company Airtight Inflatables in Ohiopyle, Pa. A Shredder is specifically designed to be paddled. It is usually paddled by a two person crew, though highly skilled paddlers are able to negotiate extreme whitewater in a Shredder paddled as a solo boat. Typical catarafts are constructed from two inflatable pontoons on either side of the craft which 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 "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 have large airbags to prevent the boat from being swamped by big waves and holes. Like kayaks, whitewater canoes can be righted after capsizing with an Eskimo Roll, but requires more skill.

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, whitewater canoes 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.


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.

Leaders responsibilities and preparedness is to 1) review river conditions and have appropriate maps 2) assess participant awareness and qualifications for group safety and comfort 3) determine group equipment and individual checking of individual equipment at the put-in especially life jacket fit and their suitability 4) organizing group leaders and sweeps and stops 5) filing a float plan and checkpoint establishment/communications in the event of an emergency.

Individual responsibilities and preparedness is to 1) be a competent swimmer with an ability to handle oneself underwater 2) be familiar with your life jacket and its fit practicing maneuvers/rescues with it in pool situations before any major open water trip 3) be able to control your craft to stop and reach the shore before reaching any danger and especially not entering a rapid unless you are reasonably sure you can navigate it or swim the entire rapid in case of a capsize 4) be aware of the water or river hazards for the particular trip 4) not boat alone - try for a minimum of three craft and work not to get separated from your group 5) have a frank knowledge of your boating ability and don't attempt waters beyond your ability 6) be in good physical condition 7) be practiced in escape, self rescue for your craft, rescue procedures for others including First Aid and CPR 8) be Eskimo or C2C roll proficient for large rivers or places with large waves or continuous rapids where swimming to shore will not be a good option 9) wear a crash helmet where upsetting your craft is likely 10) be suitably equipped with shoes that stay on and will protect your feet during a bad swim or walk for help yet not interfere with swimming (again pool sessions for practice)  Read our other section on clothes.

Boat preparedness means: 1) test your new and unfamiliar equipment before trying it on the difficult 2) check your craft for hull conditions before starting (turn it over/clean it off and examine for holes/cracks) 3) have a craft if inflatable with Multiple Air Chambers and test it before traveling to the put-in 4) carry adequately sized paddle or oars for controlling the craft and carry sufficient spares for the trip duration  5) install floatation devices in non-inflatable craft secured and designed to displace as much water as possible 6) nothing to cause entanglement such as badly secured bow and stern lines 7) Provide lines for allowing you to hold onto your craft 8) respect capacity ratings and reduce them if necessary 9) carry repair materials 10) car top racks must be strong

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