What are the Great Lakes, and how much water do they hold?

The five Great Lakes on the border of the United States and Canada hold roughly 21 percent of the world's surface fresh water and about 84 percent of North America's, an enormous reservoir supplying drinking water to tens of millions of people. From west to east they are Superior, Michigan, Huron, Erie, and Ontario, which the mnemonic HOMES covers. Superior is the largest by surface area of any freshwater lake in the world and holds more water than the other four combined. They were carved by advancing and retreating glaciers during the last ice age and filled with meltwater as the ice withdrew.

Michigan and Huron are hydrologically a single lake, joined at the Straits of Mackinac at the same surface elevation with water flowing freely between them, and they are treated as one body for water-level measurement while being named separately for historical reasons. Water flows generally eastward through the system, from Superior down through the others and out via the Saint Lawrence River to the Atlantic, dropping most dramatically at Niagara Falls between Erie and Ontario. Residence time is very long, so pollutants entering the system persist for decades.

Two environmental threads dominate their management. Invasive species have transformed the ecosystem repeatedly, with sea lampreys arriving through shipping canals and devastating native fish, and zebra and quagga mussels introduced through ballast water altering water clarity and food webs profoundly. And nutrient runoff, principally phosphorus from agriculture, drives harmful algal blooms in Lake Erie that have at times shut down municipal drinking water supplies, most notably in Toledo in 2014. Binational management operates under the Great Lakes Water Quality Agreement between the United States and Canada.