Those black flies sound horrendous, I'll stick to our swarms of skeeters. At least they can be taken care of with bug spray for the most part. As far as the den, it's not a den to my knowledge, but an old dig. Likely a vein of gold or copper there that was dug out in the early 20th century. When I got down to the other side, I could see a couple of other small digs on the opposite bank. Pics coming.