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Whitewater
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==Fast rivers== 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. ====Gradient==== The [[stream gradient|gradient]] of a river is the rate at which it changes [[elevation]] along its course. This loss determines the river's slope, and to a large extent its rate of flow (velocity). Shallow gradients produce gentle, slow rivers, while steep gradients are associated with raging torrents. ====Constriction==== 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 and to react to riverbed events (rocks, drops, etc.). ====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 "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 (fluid dynamics)|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° angle), the power of eddies increases with the flow rate.{{Citation needed|date=September 2008}} 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 re-enter 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. ===Stream flow rate=== A marked increase or decrease in flow can create a rapid, "wash out" a rapid (decreasing the hazard), or make safe passage through previously navigable rapids more difficult or impossible. Flow rate is measured in volume per unit of time. The stream flow rate may be faster for different parts of a river, such as if there's an undercurrent.<ref>{{cite web |title=How to Survive a Fast River Current |url=https://www.theactivetimes.com/how-survive-fast-river-current |website=The Active Times |access-date=12 March 2021 |language=en |date=10 June 2014}}</ref>
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