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 pool drop, but there are runnable channels at least down to 2.3', 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 2.8' is a good first time level. Horseshoe is the toughest drop. 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 (4.7), and again in 2015. There is also the high water "Brine" line that is gaining popularity
The VA Lines were 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 used to be 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. In Modern times, kayakers discovered that plastic boats with rocker made the right line easier than the traditional line. U-Hole is the most difficult rapid on this section.
The Center Lines are typically run between 3.5 and 3.8.
|POTOMAC RIVER NEAR WASH, DC LITTLE FALLS PUMP STA|
|usgs-01646500||2.40 - 3.90 ft||V+||01h04m||3.19 ft (running)|
Reports give the public a chance to report on river conditions throughout the country as well as log the history of a river.