This was an impressively long Ichneumonid! (The antennae and ovipositor extended beyond the edges of the image.) Unfortunately it was very flightly and this was the best image I got.

From its size and look, I'm guessing it's a Megarhyssa, and from photos of the four nearctic species found on the web, it think this is M. nortoni. Corrections and/or comments appreciated.

It was photographed in a wild marsh surrounded by montane coniferous forest in the northern Sierra Nevada.