Growth slumps: melting permafrost shapes Alaskan lakes.

Thousands of fast-growing, egg-shaped lakes--most with their pointy ends aimed toward the northwest--pepper the landscape of northern Alaska. A new study suggests that these enigmatic waters expand when their permafrost banks melt, rather than when wind-driven currents cause erosion, as scientists had proposed for decades. Geologists refer to these shallow bodies of water as oriented-thaw lakes because their long axes are aligned and they freeze each winter and melt each spring, says Jon D. Pelletier, a geologist at the University of Arizona in Tucson. They occur throughout an area about the size of New Hampshire and Vermont combined. An individual lake occasionally measures dozens of kilometers in length.

Prevailing winds in northern Alaska blow toward the east-northeast, almost perpendicular to the long axes of most of the oriented-thaw lakes. Researchers had suggested that those winds set up lake currents that erode the banks more quickly at the ends of the lakes than along the sides, says Pelletier. The swirling of such currents would erode coarse-grained soils faster than fine-grained sediments, so that model predicts that larger lakes would turn up in areas with coarse soils. However, Pelletier discovered unexpected patterns, which he reports in an upcoming Journal of Geophysical Research (Earth Surface).

Pelletier compiled information about 1,400 oriented-thaw lakes. He found that in parts of north-central Alaska, the lakes point almost parallel to the prevailing winds, not perpendicular to them--a strike against wind-spawned currents as the major cause of lake growth. Also, many large lakes are surrounded by fine-grained soils.

Pelletier's new analysis indicates that the sharper end of each egg-shaped Alaskan lake points toward the lower terrain. On the downhill side of each lake, the banks are shorter, Pelletier explains. During spring, permafrost thaws more completely on short banks than on tall ones. If the permafrost thaws too rapidly for water to drain from the soil, the saturated sediment slumps into the lake. Slumps can occur on a bank sloped as little as 2[degrees]. Over decades, the lake thus grows faster in the downhill than in the uphill direction, says Pelletier.

Not all scientists agree with the new hypothesis. Field studies of Alaska's oriented-thaw lakes, as well as of similar bodies of water in northern Canada and Siberia, only rarely show signs of slumping along lakeshores, says Kenneth M. Hinkel, a geologist at the University of Cincinnati.

Also, the lake orientation in parts of north-central Alaska is probably influenced more by the shape of dunes laid down in the area thousands of years ago than by other factors, says Hinkel.