When addressing the discharge of a stream, most of the reporting sites do in fact gage the discharge in cfs (cubic feet per second). This is the quantity of discharge going through the controlling gates, weirs, or generating turbines. The swiftness of the water, current, depends then on the configuration of the stream bed, i.e., how wide, how deep, etc. A stream bed of 100 feet width and a depth of one foot with a discharge of 500 cubic feet per second would have a current moving at approximately 5 feet per second (about 3.5 miles per hour). Again, with a discharge of 500 cft, assume then the stream bed width is 50 feet wide and only one foot deep, then the water will be traveling at twice that current speed, or 10 feet per second (about 7 mph). There are many conditions downstream of the discharge control point that will affect the current speed, this example is only meant to help one understand the difference in discharge (cfs) and current speed (fps).
This would mean one should know something about the particular stream and what the conditions are at 100 cfs, 500 cfs, and so on. In other words, a discharge of 500 cfs at Lower Mountain Fork might result in a current much less than 100 cfs at another stream. I hope I haven't made this too confusing. The point is, you must know what affect on current the different discharge amounts have on each stream.