Carex stricta #1 (Tussock Sedge)

Tussock Sedge is a strongly clumping, evergreen perennial that spreads rapidly and even aggressively by rhizomes, in favorable conditions. It thrives in very wet sites, especially in shallow swamps where the vertical rhizomes form many short stems that weave together, along with the old dead stems, fashioning little islands of foliage that rise above the surface of the water. This growth habit provides many benefits to wildlife in the form of shelter, nesting sites and plentiful food. Turtles and muskrats have been observed feeding on the roots and seeds of these wetland sedges, while various birds feast on the seeds. The larval stages of the Eyed Brown, Mulberry Wing, and Black Dash host on the leaves.

If Tussock Sedge is planted in drier conditions, it has been known to spread more aggressively by rhizomes and is less likely to make the distinctive, dense clumps.

Sources:
Native Ferns, Moss, & Grasses by William Cullina
Missouri Botanical Garden
Lady Bird Johnson Wildflower Center
Illinois Wildflowers