Bob,
The gauge reads water in cubic feet per second (CFS). What's fishable and what's not depends on the river and, in some cases, what part of the river. For instance, the Gunpowder's optimal level is around 85-100 cfs in the upper portions of the catch and release section but is 150-180 or so in the lower portions of the Catch and release near Bluemount Rd. If you look at the usgs page,
http://waterdata.usgs.gov/nwis/rt, it will show you the average cfs for different time periods over the past year. You can then see what the average level for the river should be. As another example, the North Branch of the Potomac at Barnum, WV has an optimal of about 250-300 cfs. As mentioned, that would make the Gunpowder unfishable.