THE EVOLUTION OF THOUGHT ON THE EVOLUTION OF LOCO [THE LOWER COLORADO RIVER]: ALL AGREE THAT WATER WAS INVOLVED

The keys to the origin of the lower Colorado River (LOCO) lie in one of the world's hottest deserts as scattered, commonly small outcrops of limestone, mud, sand, and gravel that comprise the early Pliocene / late Miocene Bouse Formation. Bouse Formation deposits (BFDs) are conspicuous and enigmatic, standing in stark contrast to their substrata at every outcrop. Fittingly, they indicate a stark and abrupt environmental change at those locations—the sudden appearance of deep standing water in a low desert. Early observers recognized the enigma of BFDs and invoked rapid marine inundation to explain it. The ‘marine model’ (MM) was quickly and firmly seated. It prevailed for decades in spite of its implications of a particularly complex chain of geologic events required to link the nascent northern Gulf of California and Grand Canyon: subsidence; marine transgression; headward erosion; drainage capture; rapid uplift; forced marine regression; and, finally, thick river aggradation. Partly in response to the complexities of the MM, Spencer and Patchett (1997) advanced a simpler ‘spilling-lake model’ (SLM) based on a geochemical argument linking carbonate BFDs to LOCO water, not seawater. Eliot Blackwelder first proposed the basic idea of the SLM in the 1930s on the basis of geomorphology; but it was ignored. In contrast to the relatively long-lived acceptance of the MM, the renewed SLM quickly generated collective interest and controversy. Within 3 years of the rebirth of the geochemical SLM, our geologic mapping efforts near Laughlin, NV, and Bullhead City, AZ, discovered a key exposure of BFDs behind a casino. This ‘golden section’ contains stratigraphic and sedimentologic evidence that is strongly consistent with the SLM and particularly hard to reconcile with the MM. Subsequent work in this and other reaches of the LOCO have discovered key dateable horizons that compress the time frame of LOCO integration from no river to full-blown river in less than 1 My. Our ongoing, mapping-focused geologic work in the LOCO corridor continues to contribute to renewed interest and productive scientific debate about the geologic causes, consequences, and evidence for modes of large-scale river integration. The river of science flows on and, in this case, with far greater gusto than the modern LOCO.