Leech

I found this huge leech moving across the muddy bottom of a beaver pond. This is the largest I've ever seen and I have no idea what species it is. For perspective, my girly hand is 3 inches across and this leech was scrunched up as much as possible when I took the picture. Stretched to full length, it would probably be between 4-5 inches long. All black with no distinguishing features. Any leech experts out there?