The Bruneau Dunes – more properly, the Bruneau Dune – are an accident of geomorphology. The Bruneau Dunes are in the Snake River Canyon in south-central Idaho. It’s a stretch of the Snake River that cuts through the lake bottom sediments from the long-vanished Lake Idaho instead of the ubiquitous basalt that’s more common along the southern side of the Snake River Plain. The Dunes are located in Eagle Cove.1 Geologists disagree on whether the Eagle Cove is a former meander channel of the Snake River, or was carved by a giant eddy during the Bonneville Flood. If it was a meander, the Flood scoured the cove floor down to bedrock before slowing currents dumped hundreds of thousands of cubic yards of sand and silt there.
The lake sediments are soft and easily eroded. WC can believe the violence of the Bonneville Flood, which after all moved giant boulders miles downstream, carved Eagle Cove as well. The relative softness of the sedimentary rock, called the Glenn Ferry Formation, is evident in the post-Flood arroyos carved into the margins of Eagle Cove. All of those side valleys are less than 17,400 years old.
The combination of the location of the river channel, the soft rock of the Glenns Ferry Formation, the Bonneville Flood, and the orientation of the valley in relation to the prevailing winds combined to create a very large deposit of coarse sand in a semi-sheltered valley. The prevailing winds on the Snake River Plain blow from the southeast half the year, and from the northwest the other half. So instead of being blown away, the coarse sands are blown back and forth. The silts all blew away. The result is a pretty impressive sand dune, as much as 470 feet high, covers about 600 acres and is more than a mile in length.2
The sand itself seems to be about 50-50 dark basalt and paler rhyolite, giving it a salt and pepper look up close, and a somewhat darker-toned sand from a distance.
The Snake River Valley, throughout the entire course of its run across the bottom of Idaho, is a profoundly human-altered area. In the area around Bruneau Dunes, one of the major impacts is the CJ Strike Reservoir, an impoundment created by one of the 15 or so dams along the course of the Idaho stretch of the Snake River. The reservoir raised the water table. That, combined with the use of flood irrigation in farms along the south side of the reservoir, created two small lakes/large ponds at the base of the dune. Better irrigation practices beginning in the late 1970s caused those lakes to shrink. To preserve the lakes for sport fishing, water is now pumped up from the CJ Strike Reservoir to keep the lakes intact. The lakes/ponds, especially Big Lake, were fairly low, as the photo shows.
In an arid to semi-arid environment like Bruneau Dunes, a water source like these two small lakes is a critical element of the habitat, especially for migrating birds. WC saw Tundra Swans, American White Pelicans, Double-crested Cormorants and hundreds of geese and ducks. Bruneau Dunes, surprisingly to folks who haven’t been there, is a site on the Idaho Birding Trail. Geology and birding in the same place. What could be better?
Well, better preservation of the habitat would be a start. A combination of historic overgrazing, wildfire damage and invasive species have hammered the sagebrush steppe habitat surrounding the dunes and lakes/ponds. Extensive sections are no longer recognizable as sagebrush steppe at all. This is an Idaho state park, and, frankly, it’s a little embarrassing. The parts anywhere near the roadways, at least, are an exhibition on how not to care for the land.
There’s another problem. Bruneau Dunes State Park adjoins Mountain Home Air Force Base, and the sky is full of gas hawks.3 While there are many fine aspects associated with the U.S. Air Force, “Quiet” isn’t one of them. The aircraft are low, loud and sometimes ubiquitous. The Air Force wants to make the problem worse, lowering the operating altitudes and increasing the number of supersonic flights, but that will have to be the subject of a future blog post.
Still and all, this is an interesting, accessible park and an interesting bit of geology. Depending on your tolerance for getting sand into absolutely everything, you can even slide or ski down the sand dunes.
1 The Oxford English Dictionary defines (paywalled) a “cove” as “a small, sheltered place or recess among hills, woods, etc.” Eagle Cove is a a semi-circular, 3.5 mile wide divot in the wall of the Snake River Canyon, covering about 48,000 acres. It’s not small. This isn’t the only very large “cove” in Idaho; Weiser Cove is also overly large to call a “cove.” Use of the place description “cove” in the Mountain West seems to be a significant expansion (sorry) on the traditional use of the word.
2 For perspective, the Great Kobuk Sand Dunes in Alaska cover 16,000 acres. Oregon Dunes National Recreation Area, with some 31,500 acres, is even larger, although it is plagued with dune buggies. Mercifully, Bruneau State Park bans ORVs.
3 Birder slang for any heavier-than-air aircraft, especially military jets.