N 38° 53.918 W 077° 03.989
Found as described. Upon reaching Roosevelt Island by way of the foot bridge, turn hard left (NNW) and follow the path closest to the Little River. The mark is set in the top of a boulder in the first multi-boulder outcrop north of the footbridge, and about four feet off-shore.
[Photos:]HV1700 - the Mark
HV1700 on Roosevelt Island, Washington, DC
Looking west at the rock outcropping from the shore. Mark is in the top of the farthest rock, center.
OK, I have to admit, I'm proud I found this one. The description is pretty vague, and the coordinates were scaled from a map with just 6 second accuracy, so it was a bit of a hunting expedition, but it was a gorgeous afternoon and maybe, after some morning benchmarking failures, my luck would improve.
The island once named Analostan is now Theodore Roosevelt Island, and in addition to nature trails and such, it has an impressive memorial to the Rough Rider.
The benchmark disk is not stamped with the station number. It just has the standard wording Public Buildings & Public Parks / Write the Director for Information / Bench Mark / $250 Fine for Disturbing. I'm not an expert, but I'd say the metal is bronze. My GPS gave coordinates around 38°53'54.8-55.0 and 077°03'49.2-59.9
My directions: after crossing the pedestrian bridge, turn left and walk nearly to the end of the island to an information sign on the left about the causeway. Backtrack about 60 paces, and the station is atop a good size boulder at the water's edge.
[Photos:]Closeup of HV1700, Washington, DC
Station disk, bottom center, and Potomac River, looking upstream, with Key Bridge and Georgetown University in distance