Carex lacustris

Carex lacustris, known as lake sedge (lucastris is from the Latin lacus, or lake), is a tufted grass-like perennial of the sedge family (Cyperaceae), native to southern Canada and the northern United States. C. lacustris us an herbaceous surface-piercing plant that grows in water up to 50 cm (1.6 ft) deep, and grows 50–150 cm (1.6–4.9 ft) tall. It grows well in marshes and swampy woods of the boreal forest, along river and lake shores, in ditches, marshes, swamps, and other wetland habitat. It grows on muck, sedge peat, wet sand or silt, in filtered or full sunlight.

It's a common sedge that dominates many native wetlands, or intermixes with other sedges and grasses, and its ability to spread by rhizomes makes it a good colonizer for a large area.