Geological History

Malheur, Mud, and Harney Lakes

Malheur, Mud, and Harney Lakes are three shallow playa lakes located in the lowest part of the Harney Basin. They receive life-producing water from the surrounding hills and mountains. Most of this water arrives in the spring, when melting snow flows southward through the Silvies River, northward along the Donner und Blitzen River, and through the Silver Creek drainage from the northwest.

A Basin Shaped by Extreme Water Conditions

The Harney Basin receives an average of only nine inches of rain and snow each year. During drought years, extremely dry conditions can reduce the lakes to a small fraction of their former size or leave them as alkali-covered playas. Because the land surrounding the lakes is relatively flat, even a one-inch rise in water level can submerge nearly three square miles of adjacent land.

In exceptionally wet years, heavy rain and snowfall can push water beyond the Refuge’s boundaries and double or triple the size of the marsh. During the mid-1980s, three years of above-normal snowfall caused Malheur Lake to expand from 67 square miles to more than 160 square miles. The opposite occurred in 1992, when Malheur Lake shrank to just 200 acres.

The Formation of Steens Mountain

The climatic and geologic conditions in this part of Oregon have changed considerably over time. Nearly ten million years ago, tectonic faults and regional uplift began forming Steens Mountain on the southern side of the Harney Basin.

Steens Mountain eventually rose 9,700 feet above the surrounding valleys. About one million years ago, a vast ice field covered its upper reaches. More recent glaciers carved the mountain’s spectacular U-shaped gorges. As the glaciers moved slowly downhill, their weight and motion ground the underlying rock into a fine powder called loess.

The Formation of the Valley Dunes

Streams flowing from beneath the glaciers carried the loess down the Donner und Blitzen River and other creeks on the mountain’s western flank. The material was deposited across the Blitzen Valley floodplain, where strong downslope winds moved it around the valley floor. Over time, these deposits formed a series of low, vegetation-covered dunes at the southern end of the river valley.