Here's our thoughts: we went to Northern Prairie and checked the distribution maps. It looks like Thompson Falls is almost Idaho, so it looks as though your choices are anna, boreale, carunculatum, civile, cyathigerum, ebrium, and hageni. Luckily most Enallagma are eastern beasts!
Also we're lucky in that boreale, civile, cyathigerum and hageni would all have much more blue than this. Ebrium would also have more blue, but there is a dark form - luckily it's rare.
So we're down to two: River Bluet, Enallagma anna; and Tule Bluet, E. carunculatum. River has long appendages in the male and are usually quite visible in photos and tends to have narrow black shoulder stripes. So we're leaning to Tule.
Everything works fine for Tule, the amount of blue coloring, the width of the shoulder stripes, the green color of the female. Ed Lam says Tule has small to medium eyespots. We think we can rate these as medium. So our best guess after all this is Tule. Hope you find time to puzzle it out for yourself. We'd be curious to find out if you end at the same place!