Assessing the Grade of a River or Rapid is not an exact science. Different people may pick different grades for the same section of water. How you grade a Rapid normally depends on your experience with other Rapids, and thier grades. Following is a breif description of each grade.

Grade Zero
Flat water with no current or waves. A lake on a calm day.

Grade One
Flat water with a weak current and/or small waves. A lake on a windy day, or a wide slow moving river such as the Waikato River.

Grade Two
Moving water with some large waves and small holes. There are very few obstacles, and those that are present are not in the main current flow. There is no main line. Paddlers can pretty much go anywhere on a rapid safetly. Grade 2 rapids are generally safe to swim in, with deep water and no rocks.

Grade Three
Fast moving water with large waves, holes, obstacles and some small drops. There is an obvious easy line that can be followed with little problem, however experienced paddlers can pick and choose alternate lines for more fun. Grade 3 rapids are often shallow rock gardens. Only paddle a Grade 3 river with people who have done it before and know the lines, unless you are a very experienced paddeler.

Grade Four
Very fasty moving water with large waves, large holes, waterfalls and many obstacles. There is a defined line that should be followed. If you miss the main line it can be difficult to complete the rapid, however there is little danger to experienced paddlers. Grade 4 rapids should be scouted to check for changes to the rapid, especially trees, and logs. Also scout these rapids to determine the best line.

Grade Five
Very fast moving water with large waves, large holes and high waterfalls. The main line is usually difficult to achieve and the rapid poses reasonable danger if the main line is missed. Always scout Grade 5 rapids.

Grade Six
Grade 6 rapids are rapids that could be possible but no-one(or very few) has been brave (or stupid) enough to paddle it. These pose considerable danger. These are usually vary large waterfalls such as McLarens Falls and Huka Falls, or tight sieves such as Tree Trunk Gorge. Only the best of the best paddle grade six rapids, and only with a large amount of safety equipment about.