Ottawa River Sunrise 2
My theory is based on something one of my neighbours told me four years ago (and he was in a position to know). At that time the twelve or fourteen, upstream hydro-electrical generating stations had their mandate changed from 'base load' operation to 'peak load' operation, that means they would only be used at the worst time for the grid, which is during very hot summer days.
Three days before the fish appeared the damns were open due to the summer demand. The water level went up about 12" and the shore line moved back about thirty feet, to the point it was at on July 1st, one month earlier.