The Orderly Orientation of Arctic Lakes
My girlfriend stopped me last spring as we walked through a hallway in the Geophysical Institute's Elvey Building. She pointed to an aerial photograph of the North Slope near Prudhoe Bay. "Look at those lakes," she said. "They all point the same way."
I looked at the photograph, taken from a U2 surveillance plane. Sure enough, every lake in the photo was long, narrow, and pointed in the same general direction. Like dozens of salmon returning to their spawning stream, all of the blue and black lakes lined up northwest and southeast, parallel to each other.
Since the natural world doesn't often appear with such symmetry, I thought someone must have a hypothesis for the shared shape and orientation of the lakes. Someone did. The mystery intrigued Charles Carson so much that he earned his Ph.D. from Iowa State University in 1962 by exploring the orientation of the lakes.
Carson traveled to Barrow in the late 1950s and early 1960s and spent many hours on the treeless tundra examining the cigar-shaped lakes. He armed himself with speculations of other researchers who had also pondered the northwest-pointing lakes. One researcher said the sun was the driving factor; direct sunlight might soften up the frozen peat in the south-facing banks in the north end of the lakes, and the lakes might grow northward as the peat fell in. Another scientist said the lakes were the result of meltwater filling in cracks in the ground's surface that happened to occur in the pattern the lakes are oriented. Yet another theory was that strong winds elongate the lakes by bashing lake ice into the northwest or southeast shorelines.
Before making his own educated guess, Carson got to know the character of the lakes. From the soil sediments around the lake, he deduced they weren't formed by glaciers. He discovered the lakes are actually thaw basins, low areas in the tundra where water from melting snow and ice collect. Carson hypothesized how thaw basins grow: after snow melts and a puddle forms, the relatively warm water thaws the frozen ground below. In the fall, the water freezes and expands, shredding the vegetation beneath it. In following summers, the shredded tundra is moved around by winds, allowing the ground to thaw even further, and the lake to grow bigger. All of the lakes Carson studied were very shallow; even though the lakes could be several thousand feet long, most were no deeper than 10 feet.
After the lakes formed, did the sun create their orderly appearance? Carson thought it was quite plausible that the south-facing banks at each lake's northern end could become mushy in the summer and dissolve into the lake, thereby extending the lake in a northward direction. To study the effects of the sun, Carson rigged up a set of copper plates that faced the sun in July and August, the only months when Carson figured it was warm enough in the Arctic for the lakes to grow. The copper plates absorbed a lot of heat on clear days, but it was almost always foggy during the thaw season. The clouds blocked enough solar radiation that Carson concluded direct sunlight has little to do with the way the lakes are oriented.
With the sun out of the lake-formation picture, Carson focused on a force he couldn't ignore-arctic winds. He measured winds that averaged 30 miles-per-hour at Ikroavik Lake near Barrow, with frequent gusts to 60 miles-per-hour. Oddly, the prevailing direction of the winds was northeast, perpendicular to the long lakes. Carson found a possible key to the crosswind mystery by measuring the currents the wind produced in the shallow lakes. When the wind stirred the surface of the lakes, it created eddies that produced larger waves on the sides of the lake at right angles to the wind. With that discovery, Carson came up with a hypothesis that the wind was indeed the shaper of the lakes: the waves caused by crosswinds struck the northwest and southeast shores, perhaps eating away at the peat more aggressively there and creating a pattern of elongation that all arctic lakes share.
