Written by W.G. Duffy, FOMR Board Member
Redband trout Photo Credit: ODFW
Part I: Geologic history of the northern Great Basin
The redband trout (Oncorhynchus mykiss newberri) is a remarkable fish. A colorful, seemingly delicate relative of rainbow trout, it conjures up images of cold mountain streams. But they are creatures of basins in the Interior West, occupying streams, rivers and lakes from western Montana to eastern Oregon and northeastern California. This hot, dry environment would seem an inhospitable place for a subspecies of trout to prosper – yet the redband trout has done just that. Furthermore, they have persisted in the Northern Great Basin for the past 10,000 to 60,000 years during a period of cataclysmic change wrought by volcanism and massive floods as well as intervals of drought and climate change. To better understand the redband trout of the Harney Basin and Malheur National Wildlife Refuge, we should take a moment to understand the geologic and climactic phenomenon that have helped shape this fish and contributed to its continued survival.
About 16 million years ago, eastern Oregon was a region of substantial volcanic activity. During this period magma rose to the surface throughout the region leading to lava flows, some traveling up to 400 miles and covering as much as 10,000 square miles with as much as 100 feet of lava. The Steens Basalt flow, dated to 15 million years ago, covered about 19,000 square miles of the Oregon Plateau with basalt, in places up to 3,000 feet thick. Eventually, these lava flows covered half the state of Oregon, creating a formation known as the Columbia River Basalt Group, the geologic foundation beneath much of the high desert region. Between 15 and 2 million years ago, volcanic activity declined but continued to create new volcanoes that added additional layers of lava on top of the Columbia River Basalt. The deposition of magma along with differential uplift restructured the earth’s surface, isolating the Great Basin from the Snake River as well as isolating multiple basins within the Great Basin.
During the last 2 million years, a series of ice ages altered the landscape. As each ice sheet melted, runoff and increased rainfall filled many of the region’s closed basins, forming large pluvial lakes, some covering as much as 1,000 square miles. The Goose Lake, Harney and Klamath basins were filled during this period, along with many other smaller basins. As the climate became drier, however, these large lakes shrank away. Goose Lake, Harney Lake, Malheur Lake, Summer Lake, Lake Abert and the Warner Lakes are remnants of ancient pluvial lakes. Archeological evidence of the past presence of several fish species in the Harney Basin that now occur only in the Columbia River suggests the basin may have been connected to that river about 18,000 years ago. Loss of this hydrologic connection would suggest redband trout in the Harney Basin also lost their potential genetic exchange with anadromous salmonids such as steelhead at that time. Periodic droughts during this time would also have led to the desiccation of the Harney Basin pluvial lake resulting in the eventual formation of Harney and Malheur lakes.
Human activity during the past 175 years also influenced redband trout and their habitat. Grazing, timber harvest and road construction have altered the hydrology of Great Basin lakes and streams. Stocking of hatchery trout into redband trout waters has diluted the genetic integrity of some populations and the introduction of exotic fish species such as European carp has, in some instance, altered ecosystems.
Malheur National Wildlife Refuge is located within the Harney Basin, a terminal or closed basin in southeastern Oregon. The basin contains freshwater Malheur Lake that receives the discharge of both the Silvies River and Donner and Blitzen River. During periods of high-water Malheur Lake discharges into the saline-alkaline Harney Lake, that also receives any discharge from Silver Creek. During wet years both lakes have considerable open water while during dry years wetlands predominate. Redband trout within Malheur National Wildlife Refuge
Larger rivers flowing along the periphery of the Great Basin also influenced redband trout. Research of genetic diversity in redband trout has found that three major river systems that discharge to the Pacific Ocean have had an influence on redband trout diversity in the northern Great Basin. These studies suggest that redband trout of the Goose Lake, Warner Valley, Chewaucan and Catlow Valley basins were distinct genetic races and were influenced by fish from the Sacramento River. White River and Fort Rock redband trout were associated with the Columbia River. Harney Basin redband trout were a unique genetic group most closely associated with Columbia River redband trout. The influence of geological processes on these fish is demonstrated on south Steens Mountain where a ridge separates Sacramento River redband trout in the Catlow Valley from Columbia redband trout of the upper Blitzen River about one mile away. or you could say something like the Sacramento and Columbia fish are separated by a ridge on south Steens Mountain. The Harney Basin was part of the Columbia River Basin until about 18,000 years ago when lava flows isolated the basin (Figure 2).
The redband trout of the Harney Basin are comprised of three large populations that once comprised a large metapopulation (group of spatially separated populations that interact through migration and genetic exchange). These large populations include the Donner und Blitzen River subbasin, Silvies River subbasin, and the Silver Creek subbasin. Smaller populations also exist in small disjunct streams on the north side of the basin including Rattlesnake Creek, Cow Creek, Coffeepot Creek, Prater Creek, Poison Creek, and similarly sized streams that drain the north side of Steens Mountain including Smyth, Riddle and Paul creeks. Historically, fish in all these streams had access to the large lake in the Harney Basin. It was a closed basin most of the time, with occasional connection to the Columbia River Basin during periods of very wet weather cycles.
