What a mess. Today is Day 6 of trying to recover a bunch of river rock from the front yard. Was buried under grass that moved in over bark dust and bark chips. And all different sizes from about 1/2 inch to 6". Needed to be taken out of the front yard anyway, but has been kind of a disaster trying to make use of it in the bog filter.
Was able to manually sort out the bigger stuff to add to the bottom and about 1/2 of the middle-sized stuff for the middle layer. But the rest of it—about 1.5 yards—is still just too dirty with clay chunks and mixed up with plant material and bark chips to be of any use without extremely tedious manual labor. And that's AFTER sifting it with my homemade trommel. Think I'm going to abandon it and just move onto filling with pea gravel rather than mess with.
Will end up with 6" of large cobble at bottom, 6" of 1-2" round drain rock in the middle and about 12" of pea gravel on top. Hoping 12" of pea gravel doesn't make it impossible to properly backflush, but we'll see!