Wording taken from UKRiversGuideBook (www.ukriversguidebook.co.uk)
Moving water, unobstructed and without technical difficulties. There may be small waves and riffles to challenge the paddler.
Example of Grade 1: The section of the lower River Nevis which is shown here.
Waves, small stoppers and other minor obstructions to avoid. Eddies and cushion waves may be strong.
Example of Grade 2: The River Washburn, shown here.
Waves, stoppers and technical difficulties are more severe. There may be drops and powerful constrictions. The main distinguishing factor of Grade 3 water is that the paddler will have to follow a recognisable route to avoid obstacles and hazards.
Example of Grade 3: The River Tryweryn, pictured here.
Severe waves, drops, stoppers and other obstructions. The route is not easily recognisable and will usually require careful inspection from the boat or bank.
Grade 4 encompasses a wide range of rivers, from those with pool-drop rapids to those with extended continuous rapids; so there is a huge variation in difficulty. It is common to distinguish easier grade 4 rapids by grading them as 4- and harder rapids as 4+ (or in some cases, 3/4 or 4/5).
Example of Grade 4: The River Erme in Devon, shown here.
Extremely difficult rapids with precise and technically demanding routes to be followed. Stoppers, currents and waves will be powerful and inspection is essential.
Example of Grade 5: The River Moriston in Scotland, part of which is shown here.
All of the above carried to extremes. Grade 6 usually means unrunnable rapids, which may just be possible in certain conditions.
Example of Grade 6: This warm-up rapid on the Abhainn Righ in Scotland probably conforms to most people's idea of 'unrunnable'. The paddler wound up in hospital...