The Great Lakes watershed is one of the world's largest freshwater ecosystems. 4,000 kilometers of coastline and are home to more than 3,500 plant and animal species. Each of the lakes' ecosystems has a unique inter-relationship with wildlife. Beavers and wolves jointly control the purity and flow of water into Lake Superior. Loons hunt fish in its clear waters. Lake Michigan has the world's largest freshwater sand dunes and the endangered piping plover. Scientists work to prevent Michigan River's invasive silver carp from entering this ecosystem. Lake Huron has one of the largest concentrations of shipwrecks in the world. Lake Erie is a bi-annual stopping point for millions of migrating birds. Pelee Island shores shelter the endangered blue racer snake. Niagara Falls, the most powerful waterfall in the world, drains the lake. Lake Ontario has the world's largest population of cormorants. Huge amounts of clean water enter it from the Ottawa River (the 6th of the Great Lakes). In the river's vast underwater cave system, millions of mussels filter the water. It will take 200 years for a drop of water to flow from the source of Lake Superior to the mouth of the St. Lawrence River, the largest estuary on Earth.