syphons are very interesting. as ewest notes they can be very dangerous in situations like pinch points when adding pipe joints (especially a cap)or when large diameter pipes are used it is the pressure differential (psi) * pipe area so for a 10 psi differential and an 8 inch pipe there is 500 pounds of force sucking you in, be very carefull snorkelling around the inlet. and the pressure spikes when stopping are much greater than steady state pressure differentials
i have played with syphons several times and have lots of ideas for them. you sure dont need a bulldozer to excavate a pond
that is some big pond you guys are playing with good luck.
plus i wanted to try posting a picture so here is one of my syphons
syphon fountain
