We’ve been experiencing the same thing on pool 14 for the last 10 years or so. Pool 13 has a tons of shallow grass flat acres, and the weeds and grass usually start in late September and go on all fall, on and off.
I believe another reason for the increase here is the water quality of the river. With better farming practices and better pollution control up and down the river. Talking to some really old timers around here, this grass flow was normal here even before the lock and dam system was in place. Herbicide runoff and other pollutants where blamed by them for “killing off” the river. Now the chemicals used are better and less transient, the application rates more controlled, and the river greener.
Don’t get me wrong, I get very frustrated with the grass and weeds, but take it as part of what mother nature has provided, not as a nusiance caused by duck enthusiasts.