River reading
River reading is examining a rapid and figuring out what the water is doing, the impact it will have on your boat, and finding the best way through it.
River hydrology
Canoes don't drive like cars. There are forces in rapids that push the canoe sideways, backwards, up and down. The more you know about river reading, the better you can understand what will happen to your boat in the water and best select a line through it that matches your skill level.
"Hydrology is the study of the distribution and movement of water both on and below the Earth's surface, as well as the impact of human activity on water availability and conditions" (source).
Special thinks to Boreal River Rescue for making these videos freely available on their YouTube channel.
Rapids classification system
Rapids are classified by a six-point system that is not an exact science and will differ among different guidebooks.
Since the classification system is about danger, a rapid can only really be classified in the moment with the current conditions and water level. It is also dependent on the temperature of the water as well as what comes next. Some rapids are run-pool (which means there are calm sections between short rapids), and other rapids are long sets where an upset could mean a very long and punishing swim, which can also be brutal for an overturned canoe.
You can read all about the International classification system here, but I've summarized it below.
Class 1 / Swifts
Areas of the river where waves pile up due to compression in the width of the river or a drop in elevation but do not have enough speed or volume to be considered risky.
Swifts do not contain obstacles so they can be run straight through without thinking much about the path.
Class 1 is similar to a swift but requires some non-complex maneuvering to get through.
Swifts are not dangerous and rescue within a swift would not be a challenge.
Capsized paddlers can easily swim ashore without concern, or sometimes walk out.
Even in shallow water, there is some risk of canoes being pinned on rocks in Class 1, but otherwise, self rescue is not difficult.
As long as there is line of sight to the end, swifts and Class 1 rapids often do not need to be scouted.
Class 2
This is where you have to start paying attention more. The waves are a bit larger and more obstacles within the rapid require some maneuvering to get through.
Some risk of canoe pinning.
Possibly larger waves that may push the boat around.
Self-rescue could be a challenge but not super dangerous. It may have a "pool" of water after it making it easier to rescue capsized canoes.
Novice paddlers should scout. Experienced paddlers familiar with the rapid may not need to scout.
Class 3
This is where it really gets fun but with increased danger. This is typically the limit of casual whitewater paddlers.
The drops may be larger and higher and the pathway more difficult to navigate.
There is a greater chance of a capsize or swamping.
A swim is much trickier and a boat rescue difficult.
You may find ledges, strong eddies, and large waves.
Self-rescue is still possible but may require support from others, such as using a throw rope, to avoid long swims. (This is where the money is for whitewater paddling).
Class 4
More of the above, but with greater intensity.
Can be run in an open canoe by very experienced and skilled paddlers.
Powerful but predictable rapids that require precise maneuvering.
The rapid may contain dangerous obstacles, sweepers, souses holes, large ledges that will need to be avoided.
If the rapid is long, group assistance may be required for swimmers.
4+ can be used for the upper end of this category.
Class 5
This used to be the "off the charts, crazy big that no one can run" category, but people in those tiny kayaks started running them so this is the category that's ridiculously wild, but some how some experts can still paddle them.
Large and violent rapids that require paddling expertise and usually only whitewater kayaks, but sometimes covered canoes.
These are considered off the scale for our paddling trips.
Class 6
This is the new "off the charts, crazy big that no one can run" category. Niagara Falls, for example, is a class 6.