Left some river water in an uncovered petri dish and found these in it the next time I looked at it. Probably all first instar. I tried to rear them to adults for species ID, but they all starved to death or became a hydra's dinner.

Collected from water in treehole.

Larva from same water sample as these: https://www.inaturalist.org/observations/42041326

So, probably Ochlerotatus triseriatus--and it seems to key out to that:
http://www.mvcac.org/amg/wp-content/uploads/Rapid-ID-of-container-mosquito-larvae-in-USA-JAMCA-20131.pdf

Observation set of mosquitoes obtained from the same treehole water sample: https://www.inaturalist.org/observations?verifiable=any&place_id=any&field:Similar%20observation%20set=42041326