Abstract

The drainage history of the Bonneville Basin from Miocene to present, has been interpreted from fossil and modern aquatic molluscs. In Middle to Late Miocene times the northwestern part of the basin drained to the west, and the eastern part perhaps to the Colorado River. Latest Miocene (Hemphillian) was an interval of erosion in extreme southeastern Idaho and adjacent Utah, with drainage likely to the south. A large Pliocene lake with rich fauna occupied Cache Valley, Idaho-Utah, and received drainage from the eastern Snake River Plain through Marsh Creek and Gentile Valleys. Drainage of those two valleys was reversed northward at about the end of the Tertiary; they became tributary to a newly integrated Snake River with course much like that of the present stream. Bear River was gradually pieced together from separate streams, and became tributary to the Bonneville Basin through a complex history of lava damming, erosion, and possibly tectonism. Upper Bear River, now adding about 12% of Bonneville Basin inflow, became a permanent tributary about 30,000 years ago, and perhaps caused the rise and spillover of Lake Bonneville. Later aridity has been more extreme than at any time in the Pleistocene. Local isolation and evolution of molluscs are shown by the localized faunas of Pleistocene Lake Thatcher and Bear Lake, and by the sharp difference between modern faunas of the Sevier Lake and Great Salt Lake Basins.