I'm planning a new pond and stream and have the following problem
figuring out how to clean out the gunk carried away from the bottom
drain. In short, it's going to be 5' underground where I can't just
open a valve and drain it off.

The distance from the bottom drain to a bio filter at the head of the
stream will be about 45'. With the bottom of the pond at at least 3'
and the bottom drain pipe dropping at least 1/4" per foot, that puts
the output of the drain pipe at least 4' below ground level. I plan to
have a gravity-fed drum there from which water will be pumped into the
bio filter. The drum will act as a settling tank for the crud carried
away from the bottom drain. Should I put a small pump in the drum that
can tollerate the gunk in the water to occasionally pump it out, or is
there a better solution?