Abstract

The Lemhi River, a northwest-trending river in east-central Idaho, apparently flowed to the southeast until late Pliocene or early Pleistocene time, and was then reversed by the rise of the Gilmore Summit. The river and its parallel--and probably also reversed--sister streams the Pahsimeroi River and Warm Spring Creek, are now tributaries to the Salmon River, but the Salmon could not have existed in anything resembling its present form until after the reversal. The Big Hole River, in adjacent southwestern Montana, may also have drained to south at one time, which suggests that fossil placer deposits of gold might be found in the Big Hole drainage area by analysis of late Cenozoic tilting and faulting and reconstruction of late Pliocene and early Pleistocene drainage systems.

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