I will compare this image with the all the female specimens I have in Somatochlora at home. We may be able to narrow this further one down by other clues than the ones in the keys, but will need to be careful. In the meanwhile, I am in the process of posting several more closeups of the terminalia of Somatochlora species (including females) with which you can compare too. The shape the tail and its terminalia here, as well as the apparent overall shape/size and absence of dark areas at the base of the hind wings certainly eliminate a number of possibilities. For example, I do not believe this is S walshii, incurvata, franklini or minor for those reasons, but other larger Somatochloras will have to be eliminated one by one by other means if possible. A quick scan of other images of females in bug guide has also lead me to believe that there are presently some errors in the females, especially in Clamp-tipped vs Williamsons. I will try to address those individually too. At the present time, as Ben suggests, this is most likely Clamp-tipped, but cannot eliminate Williamson's or S elongata yet.