Eyes are like Tachytes or Tachysphex, but maybe something else (e.g., Liris)? There is something odd in the area of the frons: see images 1 and 2 (and their enlargements, images 4 and 5). In image 1 and 4, I get the impression of a pit with a rim around it, between the compound eyes, above the antennal sockets and below the vertex. [Update: looking more carefully, I do see the same structure of the face in various Tachytes photographs, or at least something similar.]

The fore and mid femora and the hind tibiae all seemed to have interesting shapes, but perhaps all the Tachytes I've seen this summer were similar and I just didn't notice.

She (10 flagellomeres -- see images 2 and 5) was hanging about on the gravel where the trail goes over a coffer dam to reach the far side of a slough of the Mississippi River. She would sit in place briefly, but kept nervously flying from place to place (for example, see the last image). I'm not sure if she was hunting or scouting for nest sites.