"No streams or lakes within 7 or 8 miles" -- sounds like a relative desert, and my copy of DeLorme's Gazetteer doesn't show very many areas around Spokane that wouldn't have any water sources within a 16 mile radius. Mayflies are rather primitive insects and their wings are best adapted to flying up and down; controlled directional flight, not so good. Once airborne they are pretty much at the mercy of the wind and can sometimes be carried for long distances.