Yet another perspective: From http://water.usgs.gov/edu/wateruse-total.html in 2010 306 billion gallons of fresh water per day used in the united states. Volume of Lake Superior is very roughly 3,000,000,000,000,000 gallons.
V Lake Superior / (306 * 10^12 * 365) = 26.86 years
About 27 years of entire US water supply assuming no water cycle. Now that I do the math (hopefully I didn't screw it up), I'm not sure that's a big number or a small one actually.
Its big... take in account the inefficient water management that we have and that a martian colony would be numbered in the thousands, at best. This is basically unlimited reserves!
That lake is not just huge, but crazy deep. It's bigger than the state of Maine in terms of area and in places is deeper than the Empire State Building is tall.
Or in other words, imagine turning Maine into a fishtank forty stories high that's full of fresh water.
North and South America have a surface area of about 42.55 million sq km or 4.58x10^14 sq ft (1 foot deep water is obviously 4.58x10^14 cubic feet).
Lake Superior has a volume of 12,100 cubic km or 4.2x10^14 cubic feet.
That's unbelievable!