Lake Mapourika (also written Māpouriki) sits at an altitude of 52 metres on New Zealand's West Coast, roughly 8.6 kilometres north of Franz Josef village — about a seven-minute drive. It is the largest lake within the national park, covering approximately 889.7 hectares, and lies within a UNESCO World Heritage site. Five in-flowing streams feed the lake, with McDonald's Creek being the primary one and the main spawning river for Quinnat salmon.

Unlike many glacially fed lakes in the region, Mapourika is no longer connected to a glacier, which means its water temperatures are considerably warmer than typical alpine lakes. Water temperatures can reach around 20°C in summer, making it comfortable for both paddling and swimming. The water has a distinctive dark appearance due to tannins leaching from the surrounding rainforest — a characteristic that gives it a tea-coloured quality rather than the turquoise typical of glacial lakes. The lake is encircled by native rainforest, with views extending to the snow-capped peaks of the Southern Alps and glaciers.

The main Jetty Bay access point at the southern end of the lake is Jetty Bay, where a concrete boat launching ramp sits adjacent to a wharf. Basic facilities at Jetty Bay include a toilet and a picnic table. Swimmers are advised not to dive from the end of the jetty due to shallow water. There is no camping at Jetty Bay; the established DOC campsite (Otto's/McDonald's) is located at the northern end of the lake.

Paddlers can explore hidden creeks feeding into the lake and have opportunities to spot native birdlife. The calm water surface reflects the surrounding peaks and forest. Guided kayak tours operate on the lake, with Franz Josef Wilderness Tours being one operator offering trips that include exploration of a protected rainforest conservation area adjacent to the lake.

Summer (December to February) is generally considered the best season for paddling in the region, with warmer temperatures and more daylight hours. Autumn (March to May) offers quieter conditions with fewer visitors. The West Coast receives significant rainfall overall, so weather variability is a practical consideration when planning any trip to the area.