Over 10,000 years ago a giant lake covered much of southeast Oregon; today, all that remains of Lake Chewaucan is a huge salt flat and two smaller alkaline lakes.
Once a lush green plain fed by the freshwater of Lake Chewaucan, the lands around Summer Lake and Lake Abert are now a desert known as Oregon’s Outback.
This region is part of the Great Basin, a cool, dry expanse of desert the size of Spain that is a closed drainage system, meaning the water that drains into the Great Basin never flows out to the sea.
Instead, this water is held in lakes, such as the ancient Lake Chewaucan at the northern tip of the Great Basin, and its successors, the Summer and Abert Lakes.
As Lake Chewaucan dried up about 13,000 years ago, only a few remnants of the lake remained; as it shrank, dried salt flats were left behind and salts and alkali were concentrated in its remaining waters.
These lakes still remain highly alkaline as water evaporates in the lake faster than it is replenished by rain or rivers, leaving behind high concentrations of minerals.
The Chewaucan Basin provides a spectacular example of the basin and range topography that is seen across the Great Basin with mountainous peaks, sharp escarpments (slopes) and depressions that often contain lakes.
Basin and range topography is created when movements in Earth’s crust cause it to stretch apart, cracking the crust along fault lines into a series of blocks.
The movement in the crust causes the blocks to slip past one another subsequently dropping and tilting at an angle.
The raised edge of a block forms a mountain with a steep escarpment on one side, and the lowered edge forms a basin where water running off the escarpment gathers as a lake.
Some of the largest escarpments in North America are found in the Chewaucan Basin with Abert Rim adjacent to Lake Abert, at 50 kilometres long and 600 metres high, considered the longest exposed escarpment in the United States.