Distinguished from most other dragonflies (at least in the region) by the paddle-shaped abdominal appendages (male only); from other Aeshna by the dark 10th (last) abdominal segment, and lower/forward blue spot on the 7th segment larger than the upper/rearward one. Based on (1).

Although, is A. umbrosa supposed to have spots between the lateral stripes on the thorax, like A. palmata? After two years' study ;-), I think those spots are part of the forward stripe's upper "flag".

I posted this a few hours before changing my mind about ID, on examining another individual.

For more resolution: Flickr (click the down arrow at the image's lower right to download).