Editing Whitewater (section)

== Features found in whitewater ==
On any given rapid, a multitude of different features can arise from the interplay between the shape of the riverbed and the velocity of the water in the stream.

=== Strainers or sifts ===
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, getting to safety will be difficult or impossible, often leading to a fatal outcome.

Strainers are formed by many natural or man-made objects, such as 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 and form strainers.

In an emergency, climbing on top of a strainer may be better 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 avoiding the strainer is not possible, one should swim hard towards it and try to get as much of one's body up and over it as possible.

=== Sweepers ===
Sweepers are trees fallen in or heavily leaning over the river, still rooted on the shore and not fully submerged. Their trunks 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 ===
{{See also|Swiftwater rescue|Canyoning}}
Holes, or "'''hydraulics'''", (also known as "stoppers" or "souse-holes" (see also [[Whitewater#Pillows|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 or watercraft 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-volume water flows, 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 ([[weir]]s), and similar types of obstructions.  In a low-head dam, the 'hole' has a very wide, uniform structure with no escape point, and the sides of the hydraulic (ends of the dam) are often blocked by a man-made wall, making  paddling around, or slipping off, the side of the hydraulic, where the bypass water flow would become normal (laminar), difficult.  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, etc.) is often trapped in these retroflow 'grinders' for weeks at a time.<ref>[http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=1006 NFPA-1006 Standard for Technical Rescuer]</ref>

=== Waves ===
[[Wave]]s 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 also is followed by a "wave train", a long series of waves. These [[standing wave]]s 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 boat's longest axis, reducing the chance of the boat flipping or capsizing. This is often counterintuitive 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 ===
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 ===
[[Eddy (fluid dynamics)|Eddies]] are formed, like hydraulics, on the downstream face of an obstruction. Unlike hydraulics, which swirl vertically in the water column,  eddies revolve on the horizontal surface of the water. Typically, they are calm spots where the downward movement of water is partially or fully arrested—a place to rest or to make one's way upstream. However, in very powerful water, eddies can have powerful, swirling currents that trap or even can flip boats{{citation needed|date=October 2020}} and from which escape can be very difficult.

=== Eddy Lines ===
Located between the eddy and the main current, the eddy line is a swirling seam of green and sometimes white water. Eddy lines vary in size based on the size of the water column, the gradient of the section, and the obstacle creating the eddy. Often containing boils and whirlpools, eddy lines can spin and grab your watercraft in unexpected ways, but if used correctly, they can be a really playful spot. Full slice and half slice boaters are able to perform tricks like stern squirts and cartwheels, but nobody uses eddy lines as well as squirt boaters(link to squirt boating wiki), who use the swirling water and crossing currents to dance below the surface of the river.

=== Undercut rocks ===
Undercut rocks have been worn down underneath the surface by the river, or are 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 under water.  This is especially true of rocks that are undercut on the upstream side.  Here, a boater may become pinned against the rock under water.  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 such as [[limestone]] rather than [[igneous rock]] such as [[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 who 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.<ref>{{cite web |url= http://www.post-gazette.com/pg/06100/680887-192.stm |title= Editorial: Rock on / Dimple Rock is left alone to be dangerous
|access-date= 2008-02-29 |date= 2006-04-10 |publisher= Pittsburgh Post-Gazette}}</ref>
<ref>{{cite web |url= http://www.americanwhitewater.org/content/River/detail/id/1687/#tab-news |title= American Whitewate, Lower Yough accident reports|access-date=30 December 2010|publisher=Americanwhitewater.org}}</ref>

=== Sieves ===
Another major whitewater feature is a sieve, which is a narrow, empty space through which water flows between two obstructions, usually rocks.  Similar to strainers, water is forced through the sieve, resulting in higher velocity flow, which forces water up and creates turbulence.