skycries,
yep, it's cubic feet per second. It a hydrological measurement of volume of flow past a set point usually taken at a stream gauging station from the USGS. Check out the USGS.gov site for loads of good info on how it's done. Cool stuff, really. I've had the opportunity to take the measurements a bunch of times. The thing is, you need to know your river. Each river is different and has a standard baseflow and floodflow, that sort of thing. So there's no magic number. Get to know the levels of your favorite streams. One stream at 600cfs could be really low while 600cfs on another could be a serious safety issue!

I sorta go by low, high...qualitative measurements usually. I'm sure some of the other guys will repsond specifically about the Yak and give you a good range and what they mean.

Whitewater paddling clubs often have good sites showing flows for rivers but they're catered to the paddler so 'good' for paddling might be absolute crap for fishing.

 

I might be old---but I'm good.

Usually when you pull it up,the graph will show you what is the ideal flow. That is what I get anyway.

Jim