Paddlers have been tackling rapids for centuries, dating back to the days when paddling was primarily a form of transportation rather than pleasure. And probably for just as long, they've been seeking advance information about the degree of difficulty and the potential hazards of stretches of whitewater. In the early 1800s, explorers Meriwether Lewis and William Clark, who traveled the American wilderness largely by canoe, were careful to seek advice from local Native American guides about the location, size and difficulty of rapids that lay ahead. Alas, those highly informal ratings weren't always reliable; in Lewis' and Clark's written history of the expedition, for example, they describe encountering one stretch filled with treacherous rocks that "we found more difficult than the report of the Indians had led us to believe" [source: Lewis and Clark].
Even after paddling evolved into a recreational sport in the 1900s, the difficulty of getting dependable guidance about whitewater continued to bedevil rafters, kayakers and canoeists. Other paddlers tended to describe rapids though subjective comparisons -- i.e., "Lumpy Falls is almost as hard as Heavy Falls…with a little bit of Twister Rapids thrown in" -- that were pretty much useless unless you'd actually paddled in those places, too [source: Belknap].
Finally, in the 1950s, the newly formed American Whitewater association came up with a solution: a numerical rating system. The AWA's brainstorm evolved into the International Scale of River Difficulty, variations of which became popular around the world [source: AmericanWhitewater.org].
In the late 1990s, AW compiled the most recent update of the scale, based upon information solicited from 100 expert paddlers. About 80 actually wrote ratings for 3,000 different rapids, while others added comments. In addition to describing in detail the features and hazards of various stretches of whitewater, the experts were asked to rate their difficulty numerically against a list of benchmarks -- well-known rapids whose degree of difficulty was generally agreed upon [source: Belknap].
The experts were asked to compare a particular rapid to the benchmark example in terms of specific characteristics, such as the speed and power of the water flow, hazards like bridge pilings and strong currents that swirl around rocky obstacles, water temperature, and the time required to make it through the rapid. American Whitewater also asked the experts to focus on the technical difficulty of navigating a rapid, rather than the potential danger involved [source: Belknap]. (That makes sense, since it stands to reason that expert paddlers, who have the skill to handle rapids that would put the fear of God into ordinary mortals, are likely to have a very different perception of risk.)
In the next section, we'll talk about the categories in the system and what they mean.