Ok so I've thought this through for a minute or two while smoking.

Water evaporates from the river, cooling the air. The cooling makes the air denser, but the humidity makes the air less dense, so there is a battle. I propose that the humidity will tend to rise perpendicular to the ground, as it is less dense, while cool less humid air moves outward, parallel to the ground, sucked into a lower pressure area away from the river. Warm dry air will be sucked in at a 45 degree angle to continue the cycle.