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.
Sbee,30,Origanum vulgare
I saw what was likely the same bee two times throughout the day. I saw her once in the morning doing typical nest searching behavior likely to parasitize another Bombus sp. I got one bad photo, so I'm leaving it out.
I saw this bee AGAIN in the afternoon, primarily nest searching. Luckily she stopped on the Sundial Lupine to forage so I could get some photos. She then investigated a hole near the plant she was foraging, only for about 30 seconds. The last two photos is her coming out of said hole.
Used the Nomada Key on Discoverlife.org. I have believe to narrowed this individual down to either N. florilega, N. depressa, or N. composita. Seen parasitizing Andrena bradleyi broods.