Pickerel-weed is an emergent aquatic, so you’ll find it along shorelines, forming large colonies of four-foot tall plants that bloom from summer into autumn.
The flowers have two lips, an upper lip with two lobes and a lower lip with three lobes.
I’ve never seen it in the upper Potomac Gorge, but found masses of plants along the shores of the river downstream of Chain Bridge.
The flower spikes and heart-shaped leaves are typically about six inches long.
And the skippers adore it. I believe the one on the left is a male zabulon skipper (Poanes zabulon); the one on the right is either a female zabulon or a clouded skipper (Lerema accius).
Pickerel-weed ranges from Texas to Minnesota and east (and is also found in Oregon and British Columbia). It’s threatened in Kentucky yet considered weedy by some authorities.
By the way, pickerel-weed flowers are generally a blue-violet color. All the pictures on this page (except for the colony) are of the same spike; I think some combination of shifting sunlight and camera position accounts for the apparent discrepancy in color.