Lower Salmon River - Natural History
The Lower Salmon River winds through the volcanic rocks and metamorphosed sediments of the Seven Devils Group and the lava flows of the Columbia River Basalt.
About 200 million years ago the Seven Devils Mountains, now located just west of Riggins, were a chain of volcanic islands called the "Wallowa Terrane." The Wallowa Terrane was located in the Pacific Ocean, near the modern Aleutian Islands. Movement of crustal plates brought the Wallowa Terrane to the west coast of North America where it "slammed" against the continent at the speed of a few centimeters per year. Over time, the slow but relentless collision caused the rocks to fold and push up, nearly vertical in some places.
About 15 million years ago, molten basalt flowed repeatedly from large rifts, or cracks in the the earth's surface. Most of the rifts occurred near what is now the Columbia River in the southeastern corner of Washington state. Because molten basalt is very fluid, these enormous flows covered huge areas - nearly half of Washington, large areas of northern Oregon, and northern and central Idaho - before they hardened. As the basalt cooled, columns were formed. These columns are visible today and in many places along the Salmon, particularly in the vicinity of Wapshilla Rapids. The width and orientation of the columns was determined by the way the hot lava that formed them cooled. Wide columns indicate slow, even cooling while narrow columns signify rapid cooling. Vertical columns formed when the lava cooled from the surface. Curved and horizontal columns resulted when water entered the lava through cracks and cooling proceeded for the center out.
Other features of the Lower Salmon River also provide clues about the area's geology. Where the canyon walls are steep and confining, the rock is generally hard and resistant to the erosive action of water. The "pool and drop" character of the river, or alternating between stretches of deep, slow water and rapids, indicates that some layers of the Seven Devils bedrock are more resistant than others. Places where the canyon widens and the river slows, making lots of riffles and a few mild rapids, signify passing through the Columbia River basalt. Basalt is very susceptible to erosion and the resistance of every layer is about the same.
Most of the Lower Salmon river rolls through arid grassland, a relatively small yet distinctive vegetative region of the Pacific Northwest. The semi-arid climate features hot, dry summers and mild, moist winters with the longest growing season and most frost free days of any region in Idaho. Elevations within the river canyon range from 900 to over 5,000 feet enabling many plant communities to thrive. Native species common to the Lower Salmon River include bluebunch wheatgrass, prickly pear cactus, poison ivy, lupine, arrowleaf balsamroot, yarrow, mullein, willow, curl leaf mahogany, netleaf, hackberry and ponderosa pine.
Much of the easily accessible land surrounding the river canyon has been disturbed by grazing, logging or fire, facilitating the invasion of non-native or introduced plant species. Non-native species include yellowstar thistle, cheatgrass, teasel, knapweed, and horticultural species such as apricot, apple and walnut trees.
Floods, which vary widely in frequency and duration on free flowing rivers like the Salmon, have created distinct bands of lichen and moss on the canyon walls. Four distinct zones are normally apparent. The low water zone, usually underwater, contains lichen and algae. The normal flood zone, covered by water only during normal high flow periods, contains whitish-gray lichen and eddy moss. The high flood zone, covered by water only during extreme high flow periods, contain two types of flood moss. This zone occurs more consistently than any other. The extreme flood zone supports terrestrial vegetation and is predominantly barren of lichen or moss.