Hypericum hypericoides; Clusiaceae (garcinia family)

A very close relative of the St. Johnsworts, but note that the blossom has only four petals.  The plant is low-growing, with pleasantly light green foliage and dainty flowers. You might have to click on this next picture to see the flower in the lower right: Apparently it can be found in many different habitats, from wet to dry, but I’ve only ever seen it in the drier, rockier soils near Carderock (never close to the river).