Dave,
I don't think you would define it as a "flow through", though it is spring-fed at the deepest part (25') with a small constant runoff depending on water level. The pond is three acres, and I figure an average of 12' deep, so that's a volume of about 1.5 Million cubic feet, or 11.7 Million Gallons. I would estimate the typical exit flow rate at about 30 - 50 GPM, which is 43,200 - 72,000 Gal/Day, or .4-.6% of the total volume of the pond. If there is a rainstorm and it ratchets up to something crazy like 500 GPM, that's still only 6% of the volume in a day. Also, the spring source is on the same end of the pond as the runoff, so it's not like the entire pond is getting flow through it.