Characteristics of Rapids
Rapids are created by a volume of water flowing down a considerable "gradient", meaning the downhill slope, over obstructions. These obstructions, such as rocks and ledges, create “Features” depending on how and in what volume the water hits the obstruction. A feature is the effect an obstruction has on the rapid. Here is a list of features:
• Waves- Water that is rushing off of an obstruction and forms a swell in the river. Breaking waves can be surfed, due to the hydraulic created.
• Hydraulic- Water that is circulating on top of itself. This may be in a hole or big breaking wave. The water is forced down and has to fill in the gap left creating a never-ending cycle in the case of a hole; however, breaking waves have water rushing back down onto the upstream side of the wave due to he loss of momentum at the crest creating a circular hydraulic.
• Holes- Holes are created when water pours over the edge of a rock or ledge and creates a void. The void is then filled with surrounding water, but pulled down by the flow coming over the obstruction.
• Eddies- Eddies are created when water rushes around an obstruction creating a void in the river. Water then flows from the side to fill the gap. However, once the gap is filled, the current rushes past and creates a very strong line of moving water meeting the still water. Eddies are good places to stop if a paddler needs a break, but they can be difficult to get out of.
• Pourover- Water that flows over a rock or ledge. This causes a vertical drop that creates a powerful hole upon meeting the water below.
• Constriction Waves- When water is compressed by passing between obstructions (i.e. a boulder and the bank of a river) the water gains velocity and results in a series of waves called a Wave Train. These usually aren’t Breaking Waves, since the water is moving very quickly.
• Breaking Wave- A wave that has the top of the swell crashing back down onto the upstream side of the wave.
• Drop- Any part of a rapid where water is rushing over an object and falling straight down till it again meets with the surface of the water (i. e. a waterfall).
• Pillow- A pillow occurs when a strong, swift current crashes against an immovable obstruction, such as a boulder,that is not entirely submerged. The current becomes compressed and as most of the water flows around the obstruction, a big swell of whitewater forms where the current hits.
Here is the Pillow at Pillow Rock on the Gauley River. The kayaker is right of the center of the Pillow.