Rapids and River Features

There was a post on CanoeTripping.com about running holes. It got me thinking about how rapids are formed, and what features you will encounter on a typical class II/III rapid.

How do Rapids Form

There are four main factors that combine to create rapids: water level, gradient, constriction and obstruction. Gradient, constriction, and obstruction are dependent on streambed topography.

Water Level

Water level is dependent upon seasonal variation in precipitation and snowmelt, or upon release rates of upstream dams. Combine the right water level with right streambed topography, and rapids will form.

Gradient

The gradient of a river is the rate at which it changes elevation along its course. This determines the river's slope, and to a large extent its rate of flow or velocity. Shallow gradients produce gentle, slow rivers, while steep gradients are associated with whitewater rapids.

Constriction

Constrictions can form a rapid when a river's flow is forced into a narrower channel. This pressure can also cause the water to flow more rapidly increasing its velocity and its reaction to riverbed objects like rocks and drops.

Obstruction

Rocks and ledges in the river will obstruct the flow of the river creating river features like eddies, pour overs, holes and waves. The more water and the faster it flows, the larger these features become.

The combination of the water level and streambed topography determine the types of rapids you will find on the river. For example, rapids on the Kennebec are formed when water gets funneled through narrow gorges creating long, towering wave trains. Seeboomic runs over a series of ledges creating pool drops. The Dead is mostly boulder-type rapids with lots of holes and pour-overs.

Zoar Gap at dam release level. The river constricts and velocity increases as it drops over rocks and ledges creating eddies, pour overs, holes, shoots and waves.

Whitewater Features

Understanding the features of the river and how they will affect your boat is critical to running whitewater safely.

Rocks

You will find rocks on almost every river. Depending on the water level, some rocks may be above the water creating eddies, and others may be submerged creating pour-overs, holes and waves.

Eddies

Water moving downstream that hits an obstruction and goes around it rather than over it forms an eddy in the space downstream. Eddies can form in the middle of the river behind rocks, or on the sides due to changes in the shore line.

Located between the eddy and the main current is the eddy line. Eddy lines vary in size based on the level and speed of the water, and the size of the obstruction causing the eddy. Eddies have current that flows opposite of the main flow, but they are usually calmer than the surrounding current making them good places to wait and regroup before the next rapid.

Pour Overs/Holes

Water moving downstream that flows over a rock can cause a pour over. It resembles a small waterfall and often creates a hole on the downstream side of the rock. Holes form when water falls over an obstruction and recirculates (reversing direction on itself) back upstream towards the rock at the surface. A hole can be friendly and make for a great surfing spot, or very unfriendly and a place to avoid.

The nature of a hole depends on the shape of the drop, the depth of the pool below it, and the speed and volume of the current. Water falling downstream over an obstacle will push downward until it hits the bottom. While the main current is pushing to the river bottom, the surrounding water slides upstream toward the foot of the drop to fill in the depression. This is called the backwash. Where the current surfaces again is called the boil line.

If the hole is big enough, things that float (like boats or people) get pulled back up into the seam between the drop and the boil line. These are known as hydraulics or reversals. If the boil line is a long distance from the pour over, it is more likely that floating objects will not escape, but cycle back into the hole repeatedly.

The shape of the hole will also determine its retentiveness. Smiley holes with the corners further downstream than the drop are generally safer than frowning hole with the corners upstream than the rest of the hole. Some of the most dangerous types of holes are formed by river wide ledges or low-head dams since the width make getting out of the hole very difficult.

Pour Overs/Waves

Waves are formed in a similar manner to holes – fast moving water runs over or down an obstruction forms a pile of whitewater where the wave is falling over on itself known as a breaking wave. If the breaking wave is big enough it can stop you (also known as a “stopper”), fill you up, or and flip you over.

One notable difference between a wave and a hole is that the wave is less likely to be retentive and will generally flush a swimmer or boat after a flip. Riding the current into the face of a crashing wave, the current going under the backwash is exiting at surface level on the backside of the crest.

The Downstream V

When running rapids you generally want to follow the dark water through the rocks and other features. Dark water means there is a deep channels and no obstructions. It will often will resemble a tongue, a shoot or “Downstream V”.

Running the downstream V is the principal strategy for safely navigating Class II and III rapids. It marks the path of least resistance where the river bed is deepest, helping you avoid hitting rocks or getting stuck in shallow areas.

Because the V points exactly where the water is flowing most cleanly, aiming your boat's bow for the point of the V ensures you are following the strongest, most stable current. Downstream V's often lead into "haystacks" or "standing waves".

Haystacks (Standing Waves)

Sometimes, when fast moving water runs into slower moving current, a long series of waves develops known as haystacks or standing waves. These standing waves or wave trains can be smooth, or particularly the larger ones, can be breaking waves or whitecaps.

Unlike ocean waves that move across the face of the water, haystacks do not move downstream. They stay fixed in the spot where the underwater features created them, and the river moves through the wave. 

When running standing waves it is crucial to stay centered in the boat as you move up and down the face of the waves, and maintain a steady paddling cadence to maintain downriver speed and avoid getting stalled in the waves.

Picking a line through the rocky drops on the Wonalancet River