HR: 17:20h
AN: H44A-05    [Abstracts]
TI: Holocene Cyclical Switching of Colorado River Water Alternatively to the Sea of Cortez or to the Salton Sink
AU: * Howard, K A
EM: khoward@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 973, Menlo Park, CA 94025, United States
AU: Stock, G M
EM: greg_stock@nps.gov
AF: Yosemite National Park, P.O. Box 700, El Portal, CA 95318, United States
AU: Rockwell, T K
EM: trockwell@geology.sdsu.edu
AF: Department of Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, United States
AU: Schafer, J
EM: jschafer@asmaffiliates.com
AF: ASM Affiliates, Inc., 2034 Corte del Nogal , Carlsbad, CA 92011, United States
AU: Webb, R H
EM: rhwebb@usgs.gov
AF: U.S. Geological Survey, 520 N. Park Avenue, Tucson, AZ 85719, United States
AB: The former giant lake (ancient Lake Cahuilla) that intermittently filled the Salton Sink with a volume half that of Lake Erie has profound implications for the hydrologic and ecologic history of the Colorado River delta. Because the delta dams and isolates the sink from the Sea of Cortez (Gulf of California), the delta cone has a rare geometry that drains distributaries toward two unconnected termini: sea level on the south side and a fluctuating level in the Salton Sink on the north side. This level fluctuated in the Holocene between 85 m below modern sea level when the Salton Sink was dry and 12 m above sea level when occupied by successive incarnations of full Lake Cahuilla. Geologic and archaeologic records indicate that over the last 1300 years the Salton Sink cycled several times between dry and recurrently holding this 97 m-deep lake. At about 12 m above sea level the lake spilled southward across the delta to the Sea of Cortez most recently in the late 1600s or early 1700s. A simple model based on delta gradient can explain cyclical switching of the river from one side of the delta to the other. When Lake Cahuilla was dry or low, any northward flows off the delta would encounter greater gravitational potential and a steeper gradient compared to the south side. The floods of 1905-1906 dramatically demonstrated that flows down the steep north flank of the delta could cause channel entrenchment, in this case headward retreat of a waterfall 9 m high as fast as 30 cm/min. Except for human intervention, this rapid downcutting would have led to complete capture of Colorado River water until Lake Cahuilla filled. Similar entrenchment and capture events must have recurred many times during the Holocene and also earlier times. We infer that when Lake Cahuilla rose to its spillover level, the feeding distributaries silted in and lowered their grade enough to provide an impetus for the river to switch back to paths down the south side of the delta to the Sea of Cortez. Shut off from inflow, evaporation of 1.8 m/yr would dry Lake Cahuilla in a few decades, again lowering the base level below sea level and setting the stage for another cycle of northward diversion, downcutting, lake filling, and spillover. Fluctuating Lake Cahuilla must have profoundly impacted the ecology of the delta. Bones of razorback sucker and bonytail at shoreline archaeologic sites imply that when full, the lake served as a huge Holocene temporary fish incubator for these two now-endangered species endemic to the Colorado River. In contrast, the high evaporation rate of Lake Cahuilla and nearly 2 decades of average Colorado River discharge needed to fill the lake to 12 m above sea level would have deprived the south half of the delta of river water for long periods.
DE: 0744 Rivers (0483, 1856)
DE: 1817 Extreme events
DE: 1825 Geomorphology: fluvial (1625)
SC: Hydrology [H]
MN: 2007 Joint Assembly