DIFFICULTY RATING:
It is not easy to rate a rapid, except from saying "this is easy" or "this is difficult", or maybe "this is not runnable". But even the rating "unrunnable" gets regularly disproven as kayakers get better and better and decide to run ever more challenging rapids. However, there is a rating system within the sport, and here is a brief outline of it.
Class I: Flat water, non-moving current. Swimming is not hazardous.
Class II: Moving current, swimming is not hazardous.
Class III: Flowing current, obstacles in the rives like rocks and trees can make swimming hazardous.
Class IV: Technical whitewater with obstacles in the riverbed like rocks, trees, etc. Line choice through rapids is crucial. Swimming can be dangerous.
Class V: The most techincal whitewater we can run in a kayak. Consequenses are often serious, and the rapid is very challenging and dangerous. Only for experts.
Class VI: Unrunnable whitewater where the risk of death is particularly high.
There are grey areas in between all of these levels, and you can add a "plus" or a "minus" to each of them, thus making a rapid a "class III+", or "Class V-".
Getting started - River classification