Much of the Arctic coastal plain in Alaska is covered by shallow lakes. Those in the Barrow area, which are believed to be representative of most of the lakes in the coastal plain, are generally either two to three feet or six to nine feet deep. The shallow lakes can often provide a suitable summer water supply, but only the deeper lakes provide a significant water supply throughout the year.

The water in the individual lakes is in an essentially isothermal state during the ice-free period that lasts from late June until September. The maximum temperature recorded in a lake near Barrow in 1954 was about 12°C. When ice formation begins, the temperature of the body of water as a whole may be only a few tenths of a degree above 0°C. Once the lake surface is iced over, the water temperatures may rise rapidly as much as 2°C, apparently owing to radiated heat received through the ice. The heating is terminated by a sudden cooling that coincides with the covering of the ice by a thin blanket of snow. This cooling is followed in turn by a second warming of the bottom and near-bottom water that takes place gradually over a period of weeks. The bottom sediments beneath the lake are the source of heat. The magnitude of the second rise and the time during which it takes place depend on the distance from shore and the depth of water. A gradual cooling takes place during the balance of the winter. Permafrost underlies the shallow lakes but an unfrozen basin several hundred feet deep may extend beneath the deeper lakes.