Chris brings up a good point about the vent tube. The piping system that is being referred to is not actually a syphon, but is a bottom water drain system, UNLESS the vent tube gets blocked. The pupose of the vent is to prevent a syphon from beggining. Ryans drawing is great and is a true syphon. If you use the Pond Dam System, make sureyou use a trash guard on the vent tube so you DON'T ever create a syphon. You can always create a syphon system by blocking off the vent and then opening it when you get to the point were you want to draw the water down to. BTW, this is the easiest and least expensive way to retrofit a drain into a pond 20' deep or less and if 100% sealed there is no chance of leakage causing a dam failure, like a conventional "through the dam" drain.
For GW. If you want a dual level pond, setup your syphon for the highest pool level, you can add an extension to the vent tube to lower it to your desired "low" level by creating the syphon action at any time. There would be no need for two seperate systems. To maintain the "low" pool level, you would need to restart the syphon each time the pool level raised, because the syphon will not self-start until the "high" level is reached.
