I have a nearly acre pond w the regular bass, cats, bream. UPSTREAM from that about 50yds. I have another much smaller pond waterfront to the cabin. I'm trying to keep only koi/ goldfish in the smaller pond. It's not continual flow but can produce a decent stream between the 2 during snowmelt / spring storms. Seems every spring a few bass make the run and end up in the koi pond. I hv a friend w/unlimited supply of goldfish he'll give from his pond. Any ideas how to block the runners? How high would a wall at spillway have to be?

I would not do a wall. I would use something like chicken wire or mesh and stake it down well. Seems funny if the bass can go up the stream that the goldfish are not going down the stream.

Bryce

You need an overflow pipe that goes up to the level you want. Buried in the berm and out.

Stretch a piece of fence across the run between the ponds.

Seems like it would be much easier to just catch and move the occasional bass that gets upstream.