I've never heard of a river going thru "turnover" (what the old men refer to as "flipped"). Generally it's a deal that happens in some lakes & ponds (that have low flowage) when weather creates a cold layer of water over a warm layer ... and the cold layer, being heavier, reaches a point where it gets heavy enough to sink down thru the lower warm layer & mixes.
As for the pH ... it could have been changed by agricultural runoff.
Even so ... I think the gentlemen were just making excuses for themselves, possibly as a reason why they weren't catching any fish
... cp