Big flat worm!

Who is this guy?

I recently had a bunch of plants die in my jar (collected in Arizona) so I grabbed some more from the river and replanted this weekend. I also sifted the sand a lot to get rid of most of the silt and dead plant matter. I kept the old water and as many of the snails as I could.

One of the plants I brought back was apparently harboring a very large, swimming flat worm, and I've never seen this kind at this river. I spend a lot of time in the water and I haven't been bitten by any leeches. I have a bunch of tubifex and planaria but I have no idea what this guy is. Any ideas?