|
Big Southern
Butte, the Twin Buttes, and numerous small volcanic cones are
roughly aligned along a broad volcanic ridge extending from
Craters of the Moon, which lies some 20 km southwest of the INEEL,
toward the Mud Lake basin (Shaded Relief Map).
North of this ridge is a closed topographic basin that
encompasses the mouth of the Big Lost River Valley near Arco and
then slopes gently to the north, occupying a substantial portion
of the INEEL (Nace et al. 1972). Prior to agricultural development in the region, three major
perennial streams drained into this basin.
The Big Lost River flows through the basin, entering the
southwest corner of the INEEL and meandering some 48 km before
reaching the “sinks” of the contiguous Big Lost River and
Birch Creek playas. Former channels are common topographic features along the
alluvial plain, and the present channel becomes braided
to the north as it approaches the sinks area.
|
|
A cataclysmic Pleistocene
glacial flood sent an estimated 60,000 m3 s-1
(2 million cfs) of water down the Big Lost River and carried
boulders, probably on ice rafts, from Copper Basin to Box Canyon
near the INEEL’s western border (Rathburn 1993).
That torrential discharge ranks as the third most powerful
flood known, exceeded only by the Lake Missoula and Lake
Bonneville floods (ibid.). Rathburn (1993) estimated that water velocity in Box Canyon
at peak discharge reached 12 m s-1 (27 mph). The flood, thought to have occurred about 20,000 years before
present (B.P.), left distinctive fluvial deposits along the length
of the river (Hackett and Smith 1992) and created scabland
topography, boulder bars, and cataracts comparable to those of the
Missoula and Bonneville events (Rathburn 1993).
The other
perennial streams that flowed into the closed basin are Birch
Creek and the Little Lost River.
Birch Creek arises from springs between the Lemhi and
Bitterroot Ranges and, prior to its total diversion for irrigation
and power production, ran into the Birch Creek playa.
The Little Lost River terminated in a separate playa just
north of the INEEL boundary near Howe. (Continued
on next page)
|
