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(from the article.)

“For freshwater, this will be a quantum leap in terms of our knowledge,” said Daniel Esteban-Fernandez, KaRIn instrument manager at NASA’s Jet Propulsion Laboratory in Southern California. "Researchers currently have good data on only a few thousand lakes around the world; SWOT will increase that number to at least a million."
According to JPL, KaRIn will measure the height of water in the ocean, “seeing” features like currents and eddies that are less than 13 miles (20 kilometers) across – up to 10 times smaller than those detectable with other sea-level satellites. It will also collect data on lakes and reservoirs larger than 15 acres (62,500 square meters) and rivers wider than 330 feet (100 meters) across.