Yes definitely Najas/guppy grass. i see quite a bit of it in flowing streams in Northwest Louisiana, not too far from where you are.
A few years I was at an American Livebearer Association convention andat the end of the HUGE auction, I heard bids for "Guppy Grass". i was too far from the front to see it, and the stuff was going for about $1 a bag anyway, so I bid on it. When I won some and went up front to get it, I laughed when i realized that it was the same Najas I got out of ditches back home! However, the stuff i get from ditches is a darker green than the stuff i won. That is the first time that I heard it being called "Guppy grass" It certainly lives up to its name; it is a great plant for livebearer tanks; babies can hide in it and it provides O2 and takes ammonia compounds out.