Where does the water go? © Parks Canada / R. Bray
Summer visitors assume that Medicine is a normal mountain lake, but it isn't.
During the summer, glacier melt waters flood the lake, sometimes overflowing it. In fall and winter the lake disappears, becoming a mudflat with scattered pools of water connected by a stream. But there is no visible channel draining the lake – so where then does the water go?
The answer is, "out the bottom", like a bathtub without a plug. The Maligne River pours into the lake from the south and drains out through sinkholes in the bottom. The water then streams through a cave system formed in the slightly soluble limestone rock, surfacing again in the area of Maligne Canyon 16 kilometers downstream. This is one of the largest known sinking rivers in the Western Hemisphere and may be the largest inaccessible cave system anywhere in the world!
Summer melt water coming into the lake exceeds the capacity of the sinkholes to drain it. Decreased melt water in the late summer and fall means that the lake's sinkholes can drain the lake faster then the Maligne River can fill it. This creates the disappearing lake phenomena. Aboriginal peoples called the lake Medicine because of its seemingly magical powers, and the United Nations created the Rocky Mountain Parks World Heritage Site partly because of this unique drainage system.