Altimetry satellites has been optimized to measure the level of the oceans. However, inland seas and lakes are also easily observable, offering an expanse of water for the instrument to view and monitor.

The Great Lakes, situated in the North-East of the United States, are among the largest fresh water lakes on Earth. Flown over by Topex/Poseidon, we have been able to monitor surface water levels and their seasonal variations since 1992. At the end of 1998, some of the lakes (e.g. Lake Ontario) experienced lower than normal minimum levels possibly due to the El Niño/La Niña phenomenon.