Curly pondweed (Potamogeton crispus):

Curly pondweed is a plant easily identified by its elongated, reddish-green, wavy leaves with finely toothed margins. It forms thick mats on the surface of lakes and can interfere with recreation and navigation. Curly pondweed is one of the most severe invasives in the Midwest. A native of Eurasia, Africa and Australia, it is now found in nearly every US state. Curly pondweed is difficult to manage because just before it dies back, it forms vegetative propagules (turions) that disperse by water movement. Control usually consists of mechanical harvesting.