Actually it's an upflow the black pipe you see on the lower left is the feed in the top white pipes with the holes on the gravel is the main out the other pipe is an over flow / by pass/clean out. I'll try to explain the set up with paint
it will take me longer to explain in paint as i don't use it. hope these pics help.
picture 20170716_-182245
bottom "h" of bigger pipes is my in. the smaller pipes on the bottom in a grid is for me to "blow out the bog" with a shop vac.
The large pipe with the holes and the valve goes to the waterfalls.
The other pipe with the elbow its my over flow. Normal operations i have this set to above my normal waterfall but still in the water fall for a visual cue that the bog has issues/clogs. as operates when i need to blow out the bog or by pass the stream for winter.
the huge vertical pipe is just for inspection.
picture 2017017_161125 shows the bypass pipe roughed in for winter.
picture 2017020_145504 shows overall bog less gravel. the whit pipe outside the bog with the ball valve goes to my by pass for winter and bog blow out. ( made 2 different pipes with a union attachment)
I hope i explained it well.
My bog was a quick DYI thing trying to keep cost down and keep with my wifes grandfathers doings.
look up "the pond digger" on you tube and "bog" I pretty much mimicked that project.on the bog end.