Where's The Grass In Upper Potomac?

I went fishing out of Algonkian Park for the first time this season in the upper Potomac and was amazed by the lack of grass mats. What's going on? I mean, no grass anywhere, for as far as the eye could see. Normally the river has grass mats all over the place by mid-August. I also went over the Point Of Rocks bridge recently, and there was no grass there either.