Lake Manly and its sister lakes started to dry up about 10,000 years ago as the alpine glaciers that fed the rivers that filled the lakes disappeared and the region became increasingly arid. Fish that had migrated into the lake system from the Colorado River started to die off; the only survivors are the minnow-sized Death Valley pupfish and related species that adapted to living in springs. Ancient weak shorelines called strandlines from Lake Manly can easily be seen on a former island in the lake called Shoreline Butte.