The issue is that top of the dam is (way) over 30 feet above the water level behind the dam, which prevents the suction from overcoming gravity. A siphon won't work with that much of a rise in the middle no matter what the difference is between the elevations of the ends of the tube.
Also what you're describing is literally how all siphons must work--you can't siphon to a higher elevation.
It would only work if the difference between the water level and the top of the wall is less than rouhgly 30ft (as per xkcd). The bottom line is, the siphon will not work if it has a vertical water column taller than 30ft (water weight per unit area becomes greater than opposing atmospheric pressure)
Also what you're describing is literally how all siphons must work--you can't siphon to a higher elevation.