Triangle Marsh is a small wetland area dominated by cattails located along the Lakeshore Path near northern end of Walnut Street.
Before 1900, Triangle Marsh was part of an extensive wetland extending well inland of University Bay Drive and east of Willow Creek Woods. The original wetlands were mostly drained, leaving only the lakeshore remnants including Triangle Marsh, and not until 1969 was restoration begun on part of the drained inland area that became Class of 1918 Marsh.