This classification is based on the international rapid grading system. Grades are approximate, rather than definitive measures of a rapid's seriousness or difficulty. The difficulty and seriousness of any section of river can alter dramatically within a short space of time, through factors such as increased or decreased flow, new obstacles and changes in river bed topography.
- Grade 1
- Rapids are small regular waves. The passage is clear and easy to recognise and negotiate. Care may be needed with obstacles like fallen trees and bridge piers.
- Grade 2
- Rapids have regular, medium sized waves (less than 1 metre), low ledges or drops, easy eddies and gradual bends. The passage is easy to recognise and is generally unobstructed although there may be rocks in the main current, overhanging branches or log jams.
- Grade 3
- Rapids with fairly high waves (1-2 metres), broken water, stoppers, and strong eddies, exposed rocks and small falls. The passage may be difficult to recognise from the river and manoeuvring to negotiate the rapid is required.
- Grade 4
- Difficult rapids with high, powerful, irregular waves, broken water, often boiling eddies, strong stoppers, ledges, drops and dangerous exposed rocks. The passage is often difficult to recognise and precise and sequential manoeuvring is required.
- Grade 5
- Very powerful rapids with very confused and broken water, large drops, violent and fast currents, abrupt turns, difficult powerful stoppers and fast boiling eddies; numerous obstacles in the main current. Complex, precise and powerful sequential manoeuvring is required. A definite risk to personal safety exists.
- Grade 6
- All previous difficulties increased to the point of practicability. Very confused and violent water, so difficult that controlled navigation by raft is virtually imposssible. Significantly life threatening if swimming and unrunnable by all but a few experts.