Notes on the Pahsimeroi Basin


When WC moved to Idaho a few years ago, WC knew it was a state that has an extensive basin and range province. “Basin and range,” for non-geologists, is a patter of alternating steep mountains and broad, mostly flat valleys. The mountains and valleys all extend northwesterly, mostly parallel, and are major geoform of the Great Basin. One geologist described the view from high above as hordes of caterpillars crawling towards British Columbia.

As is WC’s habit, he read extensively about them both before moving and since. There is much that isn’t clearly understood. Including the underlying forces that have created these alternating mountains and valleys. Sure, geologists talk about the landscape being stretched – the extensional effect has widened this part of the United States by as much as 50% – but what is doing the stretching and why are pretty much unknown. There are approximately as many theories as there are geologists.

Boise, WC’s adopted home town, sits by the Boise Front, itself a part of the basin and range province. Of course, that rhythmic pattern of mountains valleys was disrupted in southwestern Idaho when the geological blowtorch of the Yellowstone hot spot passed under these parts a few million years ago. But north and east of Boise, the geographic pattern resumes.

Looking southwest across the middle Pahsimeroi Valley at the Lost River Range

Looking southwest across the middle Pahsimeroi Valley at the Lost River Range

Last weekend, WC visited the Pahsimeroi Valley, one of some 200 plus basins in the West. It’s sandwiched between the Lemhi Range on east and the Lost River Range on the west. It’s drained by the Pahsimeroi River, a much-abused stream, running north to the Salmon River. You can be prepared intellectually, but the reality hits you between the eyes. It’s vast. Desert weather erodes the steep-sided mountains, but there’s not enough water to transport the sediment and debris far. So there are big alluvial fans at the base of each side valley, called bajadas. the lobes of the bajadas overlap, creating a strange topography. There were mountain glaciers in both the Lemhi and Lost River Ranges during the last ice age; the glaciers contributed a lot of the debris to the extensive bajadas.

Mt. Borah, the highest point in Idaho, 12,667 feet, part of the Lost River Range

Mt. Borah, the highest point in Idaho, 12,667 feet, part of the Lost River Range

On the west side, Mt. Borah, at 12,667 feet the highest point in Idaho, sits on the west side of the valley. The valley itself is some 60 miles long, but the saddle at the south end is really an extension of the valley.1

There is farming, most hay and alfalfa, where there is water. In the West it’s always about water.

U.S. Geological Survey drainage map of the Pahsimeroi Valley, 1923, Pamphlet No. 4, February 1924

U.S. Geological Survey drainage map of the Pahsimeroi Valley, 1923, Pamphlet No. 4, February 1924

Elsewhere, the land is grazed but much of the sagebrush is still intact. pronghorn, elk and deer are around in decent numbers. But most the place is astonishingly big.

Pronghorn – not an "antelope" – in the Pahsimeroi Valley

Pronghorn – not an “antelope” – in the Pahsimeroi Valley

And this is just one of hundreds of basins, each a little different from the others, a product of latitude, rainfall, geology and time. It would take months to really explore one basin. A lifetime to really know it.

 


  1. The other side of Mt. Borah was the epicenter of the Mt. Borah earthquake of 1983, an ML 7.3 event. The basin and range is still an active system. Mt. Borah was raised about five feet. They still talk about the quake in Idaho. Not a serious earthquake – although there were fatalities – but enough of a jolt to get your attention. 
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3 thoughts on “Notes on the Pahsimeroi Basin

  1. Thanks for the trip down memory lane. The Pashimeroi not that long ago never reached the Salmon during much of the year. The river was diverted during low summer flows. Through the efforts of many including The Nature Conservancy, Idaho Fish and Game and many more there has been drastic improvement but it needs constant monitoring and support.

    • He is indeed. My brother-in-law Pete and I took and extended backpacking and fishing journey one year and it included the Pashimeroi valley since he had never seen it. We caught fish in 55 out of 57 streams and lakes he recorded and the Pahimeroi was one of those we did not. Pete lived in Oregon at the time and was eventually president of the North Umpqua Foundation and after moving to Boise and now the East Fork of the Big Wood River very active in Trout Unlimited and Idaho Rivers United. After that trip and seeing the dire situation of the river we both started working behind the scenes to help. Our efforts could not hold a candle to what others did but it took efforts from a lot of different directions to raise awareness to the issue.

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