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Geology and Geomorphology

Dunes and Playa on Harney LakeMalheur National Wildlife Refuge is an oasis in the high desert of southeastern Oregon for the multitude of birds and other wildlife who make it their home. Situated in the wide open spaces of the Harney Basin on the northern edge of the Great Basin, the refuge encompasses a mere 292 square miles of the 5,300 square miles covered by the basin. The refuge includes vast cattail and tule wetlands, lakes, dry alkali playas, ponds, greasewood covered flats, lush native grass meadows, long corridors of riparian vegetation, and sagebrush covered hills bordered by impressive basalt rims. This great variety of wildlife habitats encompassed by the refuge begins in the lowest elevations of the basin and expands southward along the Donner und Blitzen River to the base of Steens Mountain and northwest into the lower reaches of the Silver Creek drainage.

Three shallow playa lakes, Malheur, Mud and Harney, are located in the lowest portion of this vast basin and receive life producing water from the surrounding hills and mountains. Most of the water reaching the lakes arrives in the spring as snow melts and flows southward down the Silvies River, northward in the Donner und Blitzen River, and through the Silver Creek drainage from the northwest. With an average annual rain/snow fall of only nine inches, a drought year can result in extremely dry conditions, reducing the lakes to a mere fraction of their former size or becoming alkali-covered playas. The area surrounding the lakes is relatively flat, so a one inch rise in the water level will put almost three square miles of adjacent land under water. An extremely abundant year of rain and snow can force water to rise beyond the boundaries of the refuge to cover surrounding lands - doubling or tripling the size of the marsh. In the mid-1980’s three years of above normal snow forced Malheur Lake beyond the refuge boundary; the lake grew from 67 square miles to more than 160 square miles. The reverse is also possible: in 1992 Malheur Lake shrank to 200 acres.

The climatic and geologic conditions in this portion of Oregon have changed significantly over time. Nearly ten million years ago tectonic faults and regional uplifting began the formation of Steens Mountain on the south side of the Harney Basin. Eventually rising 9,700 feet above the surrounding valleys, Steens Mountain developed a vast ice field covering the upper reaches of the mountain around one million years ago. More recent glaciers carved the spectacular U-shaped gorges on the flanks of the mountain. As the glaciers slowly moved downhill, their weight and movement ground the rock below into a fine powder, or loess. This loess was captured in the numerous streams flowing from beneath the glaciers and carried down the Donner und Blitzen River and other creeks on the western flank of the mountain to be deposited on the flood plain of the Blitzen Valley. Turbulent down slope winds pushed these deposits of loess around the valley floor, eventually forming a series of low, vegetation covered dunes at the south end of the river valley.

Corresponding with the cooler and moister conditions of the late Pleistocene epoch, 1.8 million to 11,550 years ago, vast amounts of water flowed into the lowest elevations of the basin; from there it drained down the Malheur River and then to the ocean. This connection to the ocean gave salmon and other fish species access to the basin. However, this continuous access to the ocean ended around 32,000 years ago, when basalt flows erupted in the southeast corner of the basin, blocking the outlet from the lake into the river. As climatic conditions varied over the centuries with shifts in seasonal rain fall and temperature, the right combination of conditions created an enormous lake covering the floor of the basin. At several different times conditions were wet enough and water levels rose until the lake was over 25 feet deep, when this happened the water was high enough to spill over the top of the blocked outlet and reconnect the basin to the ocean. Salmon bones discovered in spawning gravels near the connection between Malheur and Mud lakes attest to one of these overflow events around 22,000 years ago.

 Pluvial Lake Map of the Harney Basin

With the end of the cold conditions of the Pleistocene, subtle warming conditions combined with abundant rain fall transformed the lower elevations of the Harney Basin into an expansive pluvial lake. With no outlet to the ocean until the level of the lake crested at over 30 feet, a major portion of the Harney Basin was covered with water by 9,300 years ago. This rise in lake levels would combine Malheur, Mud and Harney lakes into a deep lake at least four times over the next 9,000 years. A lush, narrow band of wetland vegetation bordered the edges of this lake. As the water expanded southward into the Blitzen Valley shallower areas developed in to extensive cattail and tule marshes. Remnant shore lines of these pluvial lake occurrences are still evident along the south side of Harney and Mud lakes. Large gravel bars, formed under these large lakes, can be seen at various locations along the south side of the lakes and at the mouth of the Blitzen Valley.

As the level of the lakes fluctuated over the centuries, the marshes within the lakes were renewed and revitalized. During dry episodes silts and salts on the surface of the lake beds were tumbled and whipped by the wind into dunes, these dunes would be favored later as nesting areas by colonial nesting birds. The return of deeper water replenished the marsh, benefiting wildlife by providing open areas for foraging and islands of vegetation suitable for nesting. In addition, the deeper water provided safety from predators for those species nesting on the dune islands.

In the southwestern corner of the Harney Basin, abundant springs bubble up from an area at the base of basalt cliffs. This area was essential to wildlife during the dry episodes when the lakes shrank or disappeared. Small, narrow waterways lined with marsh vegetation extended along the base of the cliffs and water seeped into neighboring grassy meadows. This presence of marsh and meadow provided ample nesting habitat for birds using the area. During wetter periods, dunes in the area grew lush stands of native grasses, providing abundant cover. In addition, low lying playas filled with water, creating areas important for shorebird feeding and nesting.

These same climatic changes also had a profound influence on the Blitzen Valley. In times of plentiful snow on the upper elevations of Steens Mountain, the Donner und Blitzen River burst out of the narrow canyon at the south end of the valley, depositing nutrient-loaded sand and silt on the flood plain. Water overflowing the banks of the river settled into old channels of the river, creating small, linear ponds and providing water for nearby native grass meadows. Creeks entering the valley from the east side funneled water across these grassy meadows, creating numerous stringers of lush riparian vegetation. As water decreased in the river in the late spring or during periods of lower snow fall, these creeks became increasingly important habitat for wildlife using the area.

Throughout the eons this lush oasis supported an incredible profusion of birds and wildlife. They adapted to repeated and extreme changes in climate, in the shrinking and swelling of lakes, in the expansion and contraction of marshes, and in the abundance or scarcity of available food. Eventually the birds and wildlife would also have to learn to adapt to the changes brought about by human use of the area.
Last Updated: Nov 20, 2014
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