Potomac

To go further with what Blas said, Little Falls changes pretty dramatically as a rapid once the river goes over 4.5-5ft. At 5ft, the center island starts going underwater, and a long wave train develops. Parallel to the observation deck, a 2nd wave train develops at low tide with some irregular waves and holes. These continue down to Chain Bridge at low tide, but don't appear at high tide.

From 5ft to about 6.5ft, the wave train gets a little larger and the waves start to crash more. It really has the feeling of a rapid on the Gauley, though, the waves are more irregular and chaotic. The lines are still fairly wide, and the river right contains eddies you can use to bypass most of the chaos, but it is still very much a Class IV rapid.

Above 6.5ft the wave train starts to wash out and over 7-7.5ft, two new dangers come into play. The first is that the river is flowing over the observation deck, creating a pretty nasty hole that doesn't go away until the river is over 9ft. The second is the water starts piling high on Chain Bridge, and some nasty eddylines and whirlpools form. Also, at these levels, Z-Channel changes directions, forcing you to attain up it, rather than paddle down it. Experience boaters can probably find a line over the break in the canal by Lock 5

Once you are in the 9+ft range, the wave train and hole are mostly gone, but some really powerful eddylines, whirlpools, and exploding waves take their place. You will be running the main rapid next to entire trees, and there isn't a predictable line. Also, you'll have to paddle into the river right from Lock 5, as Z-Channel and the Feeder Canal are flowing downstream much too quickly to attain. It's a really fun experience for an accomplished boater, but, not something that should be attempted by anyone without solid skills.