Gauge Description:

Different lines are available at different water levels; in particular, the Fish Ladder can be run higher.

The gage is located at Little Falls (aka Brookmont) Dam, where the river is very wide.  Consequently, an inch on the gage can translate to a foot at Great Falls.  The gage is also 8-9 miles downstream, so if the river is rising or falling rapidly there could be a discrepancy between the gage reading and the actual level.  Scout the rapids visually if there is any doubt.  (You were going to do that anyway, right?)

It's never too low for the MD Lines.  Below 2.6' is ELF, but there are runnable channels at least down to 2.4', and most summers the river never gets that low.  So when everyone else is whining about the drought, you still have a class V playground in your backyard.  Count your blessings.  I'd say 2.6' to 2.7' is a good first time level.  Above 2.8' the holes get mean, and above 3.0' they're vicious.  The MD Lines can certainly be run higher than 3.1', but if you're considering it you don't need this guide.

The Maryland side was run April 2004 at 20,000 cfs.

The VA Lines are most commonly run between 2.9 and 3.1, but they can be run lower and much higher if you have enough skill and knowledge of the river.  The limiting factor is the Spout.  There's a rock behind the curtain called the Big Toe that comes into play as the level drops; one local shattered his elbow on it below 2.8.  Above 3.2 the hole at the base of the Spout gets beefy.

The Center Lines are typically run between 3.3 and 3.8.

Below 3.3 the landing zone below the Fingers gets really shallow; however, you can run Grace and then ferry over to the MD side for Z-Turn and Horseshoe.  This is called the Alpine Line.  Above 3.8 Grace gets beefy and there's very little time to recover if something goes wrong.