Lake Ianthe is a large body of open water mostly surrounded by hill slopes of indigenous podocarp forest, with a large flax swamp at its southern end. The lake supports a substantial diversity of wetland birds, including perhaps Westland’s largest population of Great Crested Grebe, an endangered species in New Zealand. The lake occupies the depression created by the lobe of a former glacial trough, and is surrounded on three sides by a moraine extending from the Wanganui River valley. The southern shores are bordered by swampy alluvial flats of recent geological origin. Two small creeks flow into the northern end of the lake, and the lake and swamp drain via Ianthe Creek into the Wanganui River. The lake has a maximum recorded depth of approximately 32 m towards the northern end, but more than half of the lake is less than 6 m deep.