But my question is does fresh water cause corrosion that quickly it was under water for at least 10 seconds
'Fresh' water isn't. There are all sorts of minerals, fine particulates and chemicals in 'fresh' water. Yes, it will cause corrosion. Plus, the water that got inside didn't disappear when you pulled it out of the river - it stayed in there until it evaporated.
FWIW, there's a subatantial difference between being splashed (or rained on) and being submerged, as far as water resistance goes.