The area around Columbia State Park was hydraulically mined during the gold rush, and many dug-out areas remain. These fill with water during the winter, becoming vernal pools. Floodwater mosquitoes like these pools.
Here are photos and videos showing how infested these pools can become. Included below is a compilation of mosquitoes suffocating videos, after a treatment with the BVA 2 oil.
11 May 2017.