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How to Understand River Flows
Baseball has "RBIs," basketball has "PPGs," and boaters have "cfs." Cubic feet per second (cfs) is one of the key indicators of river or creek conditions. The mathematical formula is: river width x depth x speed (flow and gradient) = cfs. Cfs readings are taken at select points along rivers, and readings vary with location. As a general rule, the higher the cfs reading, the more water is flowing past that point at that time
More is not necessarily better. Water volume data, including cfs readings, are collected by agencies like the USGS, Fish and Game, Bureau of Reclamation, and other federal and state agencies. For boaters, too much or too little flow can affect the runnability as well as the difficulty posed by a river. The Middle Fork of the Salmon, for example, is best run at 1,500-5,000 cfs, while the Lower Salmon River is best run around 15,000-20,000 cfs.
River volume is also sometimes measured in gauge feet. To get a handle on this concept, imagine a big ruler attached to a rock wall or stuck in the river; the gauge-feet flow is represented by where the waterline hits that ruler, (e.g. the 4' hash mark). Sometimes gauge feet can be equated with cfs; at other times, though, gauge feet is the only available indicator of water volume, and local historical knowledge (or your neighborhood paddle shop) will be needed to determine when that river or creek is best run. Other good sources for information are government websites, guidebooks, local boat shops, friends, or paddle clubs. It's a good idea to learn all you can beforehand about specific characteristics of the rivers you want to run this coming season.