What Is This Little Building On A Stream?

Sometimes when I'm out birding I don't get any bird photos at all but there is always something to look at anyway. I saw this tiny building along the stream and wondered what it was? A pump house? Something to do with a waterwheel? An abandoned Hobbit complex? I'll have to ask around because curiosity gets the best of me.

I suppose I could look this fungus up in a book. I don't really need to know but it's nice to familiarize yourself with things in the woods. I'll just call it a rare roast beef tree mushroom for now.