This grows in clumps in shallow running water in John's Mountain Wildlife management area in northwest Georgia. I only see this near the head of the creek drainage up in the hills and always in running water, never in the lower regions. It also grows many clumps where thick moss grows on the rocks with water running over them, especially where the grade is sufficient to prevent the mosses from being predated on by various species. It appears that these mosses form a prime soil substrate for this grass. I think it is predation (stonerollers and such) of these mosses that prevents it from growing in the larger sections of the creek further down. It has a small ridge all the way up the center of the blades and is fairly soft to the feel.
The ridge on the leaf may indicate that it is sedge. I remember our prof of botany teaching us to distinguish grasses from sedges by the leaf section: V-section corresponds to grasses, W-section - to sedges. Don't know how much universal it is.
Yes. I will be making return trips to see what seeding/flowering occurs. Here is two more photos showing the apparently preferred root substrate and blade ridge detail. It does appear to be a sedge from Andrey's information. The grade of the falls is deceptively steep. Too steep for fish, snails, etc., to hang out in it. The photo makes it look flatter than it is. Yet the mosses are also thick enough to significantly reduce the flow rate of the water. The water volume is nowhere near as high as the creek further down. Edit: Below the mosses on that waterfall photo is solid rock, so the sedges cannot be getting nutrients from soil underneath the mosses.