Who grades rivers in terms of their technical difficulty to navigate in a raft or kayak? Is the grading system internationally recognised? Will a grade three river in the UK be of the same technical difficulty as in the USA?
Having been rafting in the US and Canada, they both used the scale 1-6 in difficulty. 1 being nearly dead water, mild rapid, and 6 being unnavigable…I do not know if there is an international regulatory committee for the difficulty of rapids. But of the two countries I have rafted they were the same.
And more importantly, are the ones that receive failing grades held back by dams?
** International ** Scale of River Difficulty
** Class 1 **
Easy. Fast-moving water with riffles and small waves. Few obstructions, all obvious and easily missed with little training. Risk to swimmers is slight; self-rescue is easy.
** Class 2 **
Novice. Straightforward rapids with wide, clear channels that are evident without scouting. Occasional manoeuvering may be required, but rocks and medium-sized waves are easily missed by trained paddlers. Swimmers are seldom injured and group assistance, while helpful, is seldom needed.
** Class 3 **
Intermediate. Rapids with moderate, irregular waves that may be difficult to avoid and can swamp an open canoe. Complex maneouvers in fast current and good boat control in tight passages or around ledges are often required; large waves or strainers may be present but are easily avoided. Strong eddies and powerful current effects can be found, particularly on a large volume river. Scouting is advisable for inexperienced parties. Injuries while swimming are rare; self-rescue is usually easy, but group assistance may be required to avoid long swims.
** Class 4 **
Advanced. Intense, powerful, but predictable rapids requiring precise boat handling in turbulent water. Depending on the character of the river, it may feature large, unavoidable waves and holes or constricted passages demanding fast manoeuvers under pressure. A fast, reliable eddy turn may be needed to initiate maneouvers, scout rapids, or rest. Rapids may require “must” moves above dangerous hazards. Scouting is necessary the first time down. Risk of injury to swimmers is moderate to high, and water conditions may make self-rescue difficult. Group assistance for rescue is often essential but requires practiced skills.
** Class 5 **
Expert. Extremely long, obstructed, or very violent rapids that expose a paddler to above average endangerment. Drops may contain large, unavoidable waves and holes, or steep, congested chutes with complex, demanding routes. Rapids may continue for long distances between pools, demanding a high level of fitness. What eddies exist may be small, turbulent, or difficult to reach. At the high end of the scale, several of these factors may be combined. Scouting is mandatory but often difficult. Swims are dangerous, and rescue is difficult even for experts. Proper equipment, extensive experience, and practiced rescue skills are essential for survival.
** Class 6 **
Extreme. One grade more difficult than Class 5. These runs often exemplify the extremes of difficulty, unpredictability and danger. The consequences of errors are very severe and rescue may be impossible. For teams of experts only, at favourable water levels, after close personal inspection and taking all precautions. This class does not represent drops thought to be unrunnable, but may include rapids that are only occasionally run.
I’d be gladto rate a river for you, provided you pay my flight and have a boat waiting for me. 
For the most part, local paddlers rate the rivers which they paddle. Since experienced paddlers tend to paddle both inside and outside of their local areas, there is a fair bit of cross-pollination, but there is still a bit of challenge trying to compare technical eastern runs with big water western runs.
In the USA and Canada (I can’t speak for other countries where I have not paddled) we use the International Scale of River Difficulty (I-VI), which American Whitewater has modified with +/- degrees. Something truly useful is American Whitewater’s development of a list of benchmark rapids, but a lot of work still needs to be done on this: American Whitewater
Cheers,
Richard Culpeper
http://my.tbaytel.net/culpeper/
Although a III is a III is a III, keep in mind that a great deal depends on water levels.
Also, if you are going to make any western big water runs, expect less technical work but generally bigger waves and longer runs than what you are used to for a given river rating.