WOTR: A lake in retreat
September 19, 2012
I’m standing on the shores of Summer Lake, or, to be more accurate, what used to be a lakeshore but is now a dry lakebed in Oregon’s high desert. I’m here with a group of writers, scientists and artists, all of us gathered to talk about changes in the northern Great Basin.
Sharp environmental contrasts, through both time and space, have always been a feature of life in the Great Basin. We know this because the region’s arid conditions that make living here a challenge are also ideal for preserving the remains of past life.
In lake sediments, packrat stockpiles and even in exquisitely air-dried human dung (coprolites), ecological change has been revealed by the painstaking work of geologists, paleontologists and archeologists. That knowledge has recently been synthesized in a masterful book, “The Great Basin: A Natural Prehistory,” by Donald K. Grayson of the University of Washington, and it’s required reading for anyone interested in the West.
The most dramatic changes in the Great Basin over the past 100,000 years are the appearance and disappearance of lakes. The Great Basin is comprised of many lesser basins that are connected to some degree, but which have no outlet. In the late Pleistocene, about 15,000 years ago, the Great Basin was a labyrinth of lakes, covering almost 28 million acres by Grayson’s calculation. The largest of these, Lake Bonneville, was almost the size of Lake Michigan and reached depths well over 1,000 feet. Its shriveled remnant is the Great Salt Lake.
Summer Lake, too, was part of a much larger Pleistocene lake, called Lake Chewaucan, which covered 480 square miles and reached a depth of 375 feet. At its springtime maximum these days, Summer Lake is lucky to cover 70 square miles at a maximum depth of three feet. By late summer, the lake has retreated to a puddle, a thin dark smudge almost lost in the heat waves.
The Pleistocene lakes existed when the continental ice sheets deflected the jet stream southward. This brought both high precipitation and cool temperatures, which together filled the basins of the Great Basin. In contrast, the greatest retreat of Great Basin lakes occurred during a period of high temperatures and drought sometimes called the Altithermal, from about 7,500-4,500 years ago. The causes for this climatic shift in the middle Holocene are not well understood, and its effects were not equally severe everywhere. Nevertheless, the implications for the future of the region are sobering.
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The Altithermal appears to have been characterized by temperatures 5-15 degrees higher than today; in other words, within the range of predicted Great Basin temperatures by the end of this century. And what were the effects of these temperatures? Many Great Basinlakes and marshes virtually disappeared. The frequency of fires increased, as shown by studies near Lake Tahoe. A variety of mammals associated with sagebrush were replaced by species adapted to drought-tolerant saltbush. The hardy woodrats, whose middens are such a reliable source of data on environmental conditions, disappeared from many sites, to return only after the end of the drought.
Even those most adaptable of creatures, human beings, suffered population declines during the Altithermal. Those who survived were forced to adapt to a diet heavy in small seeds that were extremely labor-intensive to gather and process, and we know this from their, um, coprolites. All in all, Grayson singles out the Altithermal as the least hospitable time for humans in the Great Basin over the past 10,000 years.
This bad time seems to be returning. And the challenges ahead will not be due to climate change alone. Human population density in the Great Basin is far greater than it has ever been, with major cities in Las Vegas, Salt Lake City and Reno. These urban populations, especially those in Las Vegas, rely on water sources that may not last the century, and most rural residents also depend on readily available water for farming and ranching. It is hard to imagine how these populations can be sustained in the face of conditions approaching those of the Altithermal.
The dry bed, or playa, of Summer Lake, has a stark beauty. Every evening, our group gathers on its salty edge to watch the shadow of Winter Ridge roll smoothly across it as the sun sets. The history of the Great Basin assures us that this lake will be brim-full again – in a thousand, or 10,000 years. But that is cold comfort for the hot days ahead.
Pepper Trail is a contributor to Writers on the Range, a service of High Country News (hcn.org). He is naturalist and writer from Ashland, Ore., who wrote this piece during a residency at Playa, a retreat center for artists, writers and scientists on the shores of Oregon’s Summer Lake.