Travels Along the South Fork of the Boise River


Green Creek, Soldier Mountains, Elmore County, Idaho

Green Creek, Soldier Mountains, Elmore County, Idaho

As WC mentioned, he was recently in the southern portions of the Idaho batholith, that gigantic blob of igneous rock that comprises the center of Idaho. The batholith is some 65-80 million years old, but it’s only been exposed above the surface for a fraction of that time. But since exposed, it has been deeply carved by Idaho’s rivers: the Boise, the Payette, the Salmon and the Clearwater, to name a few. While the batholith is all one giant chunk of rock, humans have given different parts of it different names. WC was in the Soldier Mountains, one of several oddly named portions on the southern edge of the batholith.

The river that has done most of the eroding there is the South Fork of Boise. The combination of erosion and the durable granite of the batholith has created rugged, steep-sided terrain, deep gorges and dramatic landscapes. Glaciation doesn’t seem to have touched this area.

Nor is the batholith is a completely uniform block of exposed granite. A batholith this large is an assemblage of multiple plutons, gigantic mushroom-shaped blobs of molten rock that congealed underground. The seams, where the plutons touch, are slightly more easily eroded. This part of the batholith was also impacted by the long volcanic event called the Challis Volcanics, resulting in intrusive dikes poking through the granite. And, nearer the margins of the native rocks – the stuff that was there when the plutons rose up – got melted into the margins, and a little further away got converted to by the heat and pressure to metamorphic rock.

So while the area is mostly granite, it’s not all granite. Geology is messy.

The area is all above 4,500 feet, which in these parts means there is forest, mixed Douglas Fir, Ponderosa Pine and, at higher altitudes, Englemann Spruce. Unhappily, near-drought conditions in 2013 resulted in the Elk Complex Fire, which burned some 130,000 acres, a big fire for Idaho. The fire killed all but the Ponderosa Pines, and much of them.

On the other hand, the brushy understory regrowth and dead trees provide decent habitat for birds. Come on, you knew this was eventually going to be about birds. Here’s a few of the species we saw.

Pine Siskin, South Fork of the Boise River, Idaho

Pine Siskin, South Fork of the Boise River, Idaho

WC sometimes sees this species in the winter at his feeders in Boise. But in the summer, they migrate to higher elevations, like this. Readers will note this bird is noshing on dandelion seeds.

Swainson's Thrush, South Fork of the Boise River, Idaho

Swainson’s Thrush, South Fork of the Boise River, Idaho

WC hasn’t seen many Swainson’s Thrushes in Idaho. They were a very common species during spring and summer in Interior Alaska. This was one of the first WC has photographed in Idaho.

Western Tanager, South Fork Boise River, Idaho

Western Tanager, South Fork Boise River, Idaho

There seem to be Western Tanagers everywhere in the intermountain West this year. There were certainly a lot of them along the Forest Service Roads and trails.

Turkey Vulture, Pine, Idaho

Turkey Vulture, Pine, Idaho

Vultures get no respect. But, even so, we probably should not end this post with a carrion eater.

Mountain Bluebird, Baumgartner Campground, Elmore Country, Idaho

Mountain Bluebird, Baumgartner Campground, Elmore Country, Idaho

Geology is the overwhelmingly most important factor in an environment.1 The elevations determines the temperatures, climate and precipitation. The rock type determines the soils. The topography determines the amount of moisture available for plants. And that determines the kind and number of birds. Which is why geology and birds are overlapping interests.

 


  1. Admittedly, geologists hold this opinion much more strongly than other kinds of scientists. 
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