Rapids, River Features and River Running Techniques

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, and skills you will need to run 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 on seasonal variation in precipitation and snowmelt, or on release rates of upstream dams. Combine the right water level with right streambed topography, and rapids will form. Too little water the rapids can be "boney". Too much water and the rapids "wash out".

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. First Trestle, Mile-Long and the Gorge on the Upper Millers are examples of rapids where a significant drop in gradient increases the length and complexity of the 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. Zoar Gap on the Fife Book Section of the Deerfield and and the Funnel on the Lower Millers are examples of rapids where constriction greatly increases the flow and difficulty of the rapid.

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. Boulder type rapids are typical here in the northeast.

It is the combination of the water level and streambed topography that determines the types of rapids you will find on the river. The 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 has 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.

River 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 flow in the opposite direction 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. 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 forming a wave known as a breaking wave. If the breaking wave is big enough it can stop you (known as a “stopper”), fill you up, or and flip you over.

One notable difference between a wave and a hole is that a 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 or a shoot.

Running the "dark water" or downstream V's 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. 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 or wind-blown waves that move across the face of the water, haystacks do not move downstream. They stay fixed 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 

River Running Techniques

Avoiding Rocks - Sideslips

One of the common mistakes that new paddlers make in rapids is trying to avoid rocks by turning broadside in the current. With enough space and enough forward momentum this can be successful, but often the canoe ends up getting pushed broadside on to the rock.

If you do end up broadside on the rock, lean into the rock ("love the rock") which will reduce the risk of wrapping the boat, and hopefully the current will eventually push you off.

A better approach to maneuvering around rocks is to sideslip around them. To sideslip your boat, you will use either static, active or sculling draws and pries to move the boat laterally across the river without turning broadside to the current.

A sideslip is efficient since it maintains forward speed and avoids the drag associated with turning or back paddling the boat. A back ferry (discussed later) can also be used to move the boat laterally, but is more difficult to set up and does slow the boat's momentum.

Eddy Turns and Peel Outs

Eddy turns and peal outs are other important river running skill. An eddy turn allows you to move from the main current into an eddy, and a peal out allows you to move from the eddy back into the main current. To do an eddy turn or peal out, you need to know your good friend "SAL" – Speed, Angle and Lean.

Speed - every eddy turn and peel out needs a bit of speed to get you across the squirrelly eddy line separating the downstream current in the main flow from the upstream current in the eddy.

Angle - you need to cross that eddy line at an angle – 45 degrees is typical, but you made need to increase or decrease the angle depending on flow.

Lean - when we say lean, we are talking about edging or a J-lean, not body lean. As always in a whitewater boat, you need to lean (or edge) the boat into the turn as you cross the eddy line, and maintain that lean as you complete the turn.  This is often called raising the "side of opposition". 

To do an eddy turn, you will accelerate your boat at around a 45 degree angle to the eddy line. Once the bow crosses the eddy line, you plant a Dufek (turning high brace) in the eddy and lean the boat into the turn. With the bow planted in the eddy, the stern will swing around, and you will now be in the eddy facing upstream.

To do a peal out the process is similar. Accelerate your boat at around a 45 degree angle to the eddy line. Once the bow crosses the eddy line, plant a Dufek (turning high brace) in the main current and lean the boat into the turn. With the bow moving downstream in the current, the stern will swing around and follow, and you will be moving back downstream in the current. 

The faster the current is moving, the more acceleration you will need to cross the eddy line, and the steeper the lean.

Ferrying

Another critical skill in safely running rapids is the ferry. Ferrying is the act of moving laterally across the river with minimal downstream drift. It can be done facing upstream or downstream, and allows you to slow your downstream motion to choose a path, and move laterally across the river to set up above a downstream V or deepwater channel. 

The ferry is initiated by paddling your boat at an angle somewhere between directly upstream and perpendicular to the main current in the direction that you want to move. The speed of the current will determine the amount of that angle.

As a general rule, the less angle you have (the closer you are to pointing directly upstream), the slower you'll move laterally and the more your paddle strokes will be working to keep you from drifting downstream. On the other hand, the more angle that you have (the closer you are to pointing directly across the main current), the more quickly you'll move across the river and the less your strokes will be working to keep you from drifting downstream. With experience, you'll learn what angle is most effective for what you are trying to do.

Running Holes and Waves

As methodical as you are trying to pick your way down a rapid, eventually you are going to have to run a drop or punch though a hole or wave. While small holes/waves can easily be run, large holes/waves can be your most formidable challenge when running rapids.

To punch a hole you need speed, and a well-placed stroke. Speed is important because you're going to be hitting a wall of aerated water at the bottom of the drop that will try to stop you dead. Take your last stroke just as you enter the hole/wave and use it to pull yourself through the aerated water. It's a good idea to punch a hole with your boat tilted slightly towards your paddle side. This will raise the "side of opposition" and help avoid a surprise flip to your non-paddle side.

Running the Gorge Drop on the Knightville section of the Westfield River