Technical AR Principles: Reading White Water

Reading white water – Learning how to read water is like learning a foreign language. Just as being relaxed and confident helps your cross-cultural communication skills, being relaxed will also enable you to pick the best routes through rapids while in your boat. A cool head allows you to clearly see obstacles, drops, and paths, whereas anxiety clouds your vision so that you see only an undecipherable mass of white froth.

Volume – Volume is the amount of water flowing past a point in the river per unit of time. In the United States, volume is almost always expressed in cubic feet per second (cfs), though occasionally it is given in some other dimension, such as cubic meters per second or cubic yards per minute. Depth gauges usually give the river level in feet, which is meaningful only if you know the individual river and therefore have a reference level or some other basis for understanding the relevance of the gauge reading.
A river’s power and speed always increase with an increase in volume. This usually means a difficult river will become harder with more water, but sometimes the opposite is the case. Sometimes high water causes a rapid to wash out and diminish in intensity. Experience on a particular river is really the key to knowing what to expect.
High water can create other problems. Flooding can cause trees to fall into the river or be picked up from the banks, increasing the danger to boaters. Extremely high water also tends to wash out most of a river’s eddies, those still-water havens behind rocks that are used for scouting and resting. On a difficult, flooded river, there’s little room for error. The water can be so powerful that your strongest attempts at maneuvering will be ineffective. If you want to learn all sorts of fun facts about rivers in flood, see William Nealy’s book, Kayak.

Gradient – River gradient refers to the average steepness of the riverbed. A rapid forms when a stretch of river has a higher gradient than the river’s average. A consistently steep river forms one very long rapid. Conversely, pools form where the incline levels out and the velocity of the water decreases. Most rivers are characterized as pool-drop rivers, with the riverbed alternating between relatively steep sections and level sections. These are the type of rivers most commonly run because they allow kayakers to rest after each rapid and provide them with an opportunity for checking out upcoming drops.
Gradient is measured in feet per mile. The most popular stretches for river running have gradients ranging from 10 to 100 feet per mile. The relation between gradient and difficulty depends, of course, on the individual river. Some rivers drop so evenly that they have miles of continuous low-grade riffles, despite high gradients. Conversely, a river with a low gradient may have miles of flat water with only one drop—a 200 foot waterfall.
As an extremely rough rule of thumb, most rivers that have gradients from 5 to 30 are usually Class 1 to 2 rivers, and gradients from 30 to 60 are likely to be Class 3 or 4. But as always, this depends on the particular river. Rivers with gradients up to 300 feet per mile are run, but only by experts, crazed maniacs, or those who can’t read topographical maps.

Water Velocity – Water velocity depends on where it’s being measured. In a straight section of river, the fastest current is in the middle where the river is deepest (See photo above). Water velocity decreases toward the banks and near the river bottom. Friction accounts for much of this loss of speed.

When a river bends, a majority of the water travels on the outside of the bend. This causes a deeper and faster channel to form on the bend’s outer curve. Because the inside of the bend has slower-moving water, not much of a channel is cut into the river bottom and the water is shallow. Often, the riverbank is higher on the bend’s outside. If there are waves going around a bend, then the biggest and best waves are where the water is deepest—toward the out bank (See photo to the right). But be wary—the faster water at the outside of the turn can undercut the outer bank, causing trees to topple into the water. In addition, the outside bank may be covered with overhanging brush. The water wants to take you there, but this is not where you want to go, unless you enjoy being slapped and snagged by shrubbery.
When water hits an obstacle (midstream rock, bank outcropping, and so on), it piles up on the upstream side of the obstruction and then accelerates as it continues downstream. Consequently, water speed is somewhat faster downstream and to the side of (but not behind) and solid obstacle.

Water speed increases past obstacle and near eddy line

Eddies – An eddy is any spot in the river where the water is moving more slowly or in a different direction than the main current. Rocks in the river are the most common cause of eddies, but eddies also form behind logs, bridge pilings, and bedrock outcroppings, as well as on the inside of bends and along the riverbank where friction slows the water. The water in an eddy can move at a slower speed than the main current, be completely still, or most often, move back upstream. An eddy’s character depends on the shape of the obstacle and on the level of the water. The faster the current is moving when it hits an obstacle, the stronger the current moving upstream behind the rock. This upstream current is always fastest just below the obstacle, making the eddy strongest near its top. The phenomenon of eddy water flowing counter to the direction of the main flow is caused by downstream water pulling eddy water downstream out the top of the eddy. Water from the downstream end of the eddy then moves upstream to fill the void. This leads to a constantly circulating flow of eddy water upstream and then downstream with the main current. A big powerful eddy on the side of the river with a lot of upstream current is, in essence, a giant whirlpool. The center is somewhat like the eye of a hurricane and is sunken in relation to the periphery.