HOW RAPIDLY MIGHT THE LOWER COLORADO RIVER HAVE INTEGRATED TO THE GULF OF CALIFORNIA?

The spatial distribution, geochemistry, and geochronology of Bouse Formation (BF) deposits along a ~400 km reach between Lake Mead and the Gulf of California (GoC) accord with rapid, downstream-directed lake fill-and-spill integration of the lower Colorado River (CR) in the early Pliocene. Evidence for lake spillover includes: (1) similar Sr ratios from BF basal carbonate and modern CR values; (2) deposits recording a divide spillover; (3) DZ chronologies showing CR origin of most BF siliciclastic deposits; (4) variations in C and O ratios from fine laminae in BF carbonate deposits consistent with annual varves; (5) stepwise decreases in highest BF carbonate deposits between basins [~560 m above sea level (m) in Cottonwood-Mohave (CM) basin, ~440 m in Chemehuevi (Ch) basin, ~350 m in Blythe basin (BB)]; (6) N-S decreasing elevations of the highest BF siliciclastic (delta) in each basin; and (7) deposits linked to rapidly receding lake levels, post-lacustrine incision, and through-flowing river deposits in CM, Ch, and BB. Recent dating has constrained the integration process more tightly: Ar/Ar dating and magneto-strat constrain arrival of CR water in CM basin to <5.24 Ma; inclusion of a 4.9 Ma tuff in BF carbonate high in the southern valley; Ar/Ar dates on 5 detrital sanidine grains and magneto-strat indicate CR sediment arrived in the GoC between 4.8-4.63 Ma.

Quantitative modeling based on the fill-and-spill concept, average historical annual CR flow volume, sediment flux estimates from pre-dam CR measurements and reservoir resurveys, and reasonable regional evaporation rates was used to roughly estimate the time required to feed sediment through to the GoC. In this model, CM and Ch basins would have filled with water in <0.5 kyr and spilled into BB. CM basin filled with CR siliciclastic sediment for 2.5-9 kyr as its southern spillover was lowered by fluvial erosion, while CR water began to fill the much larger BB. After filling of CM basin, CR sediment fed into a delta in Ch and northern BB. In next 10-30 kyr, the Ch-BB delta top rose and its front advanced southward as BB filled to overflowing at ~350 masl. The delta then filled central and southern BB for 3-10 kyr as the spillover lowered to ~160 masl. This ended the BB lake and allowed unfettered delivery CR sediment to the GoC, with the entire integration process taking <50 kyr.