River Reading And Hydraulics

Chutes

River reading involves recognition of obstacles and evaluation of the water flow so that an appropriate course can be chosen. A good route through obstructions in a rapids is called a chute. Chutes are usually characterized by smooth water in which surface riffles suggest the letter "V" with the point downstream. Conversely, surface ripples that suggest the letter "V" with the point upstream indicate a rock at the point of the "V" and must be avoided.

Eddies

Rocks in the river bed, unless they protrude above the surface, are recognized by the nature of the turbulence downstream. The turbulence (hydraulics) below rocks is related to the gradient, discharge, and size of the obstruction. During periods of low discharge, large rocks protrude well above the surface and are readily obvious from upstream. Eddies occur below both of these obstructions. An eddy is an area immediately downstream of an obstacle in a river where the current is relatively calm or may actually flow upstream. The eddy results from water filling the void behind the obstacle as the main current flows past. Eddies provide a place for rest stops in the midst of even the most difficult rapids. Rocks submerged just below the surface of the water can be recognized by a smooth hump that is formed by the fast-moving water as it follows the contour of the rock. This smooth hump of dark water is often followed by white turbulence downstream. Be careful since the turbulence may be several feet downstream of the rock. Such rocks are hard to spot. In slow-moving water, the hump may not show. In commonly paddled areas these rocks can sometimes be identified by their shiny coat of aluminum left by previous boaters.

Souse Holes and Backrollers

The water flows over a rock it piles up on itself, creating small waves. If the water falls steeply enough, the first wave may curl back on itself creating a back wave or backroller. Water will also flow around the rock creating an eddy. The combination of eddy and backroller creates the hydraulic known as the souse hole. In a souse hole the water enters the depression from all sidesÑ from upstream over the rock, from downstream (backroller), and from the sides (eddy action). At low , the mini-souse hole is merely a mildly turbulent eddy. Large souse holes can be hazardous. Experts and only experts should even approach let alone willingly enter one.

Standing Waves

Standing waves result from the slowing down of a fast current and are usually associated with deep water. Because of the increased depth required to produce standing waves, they provide a good rock-free ride through a rapids. Occasionally troublesome insufficiently submerged rocks may be concealed among the standing waves. Their presence can usually be identified by the irregularity of the waves. Standing waves also form downstream of any constriction in the width of a river channel. For example, standing waves are frequently encountered at a point where the river widens below a narrow canyon. Conditions necessary to form standing waves do not require much gradient if there is adequate discharge. River reading is an art that must be mastered slowly and sometimes with much bumping and scraping. The topography of the river bed is revealed much more clearly from the bottom of the rapids than from the top. It is often helpful to look back upstream after running a rapids and try to relate these two completely different views or to scout a rapids from below before running it.

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