I think RAH has the right idea. You need as much surface area on the screen as possible. If you just weld a screen over the spillpipe you will constantly be clearing debris off of it.
What about putting some expanded metal out around the pipe, rather than attached directly to it? Kind of an upside down "U", where the open ends contact the dam, and the curve travels out around the open end of the pipe? Let it stick up above the normal water level a couple of feet, or whatever you deem neccessary.
As far as hole sizes, how big are the fish you wish to retain? On the expanded metal, I don't think I would want any smaller holes than 1/2", or maybe 3/4"
