The origin of the Lower Colorado River has been a controversial topic within the geological community. The latest hypothesis was put forth by Philip Pearthree of the Arizona Geological Survey, Kyle House (now with USGS), and Michael Perkins (Univ. of Utah). Their paper, “Stratigraphic evidence for the role of lake spillover in the inception of the lower Colorado River in southern Nevada and western Arizona,” Geological Society of America Special Paper 439, has just been awarded GSA’s prestigious Kirk Bryan Award for Research Excellence for 2013. (The paper is pay-walled at GSA but you can download the full paper here, 26.8 Mb.)
To set the stage for the research in this paper, here is some background taken from my post Origin of the Grand Canyon. The Colorado Plateau initially tilted to the northeast and rivers, including the ancestral Colorado River, flowed in that direction into Utah and Colorado. Beginning about 18 million years ago, crustal stretching formed the Basin and Range province west and south of the plateau. Also around this time, plate tectonic adjustment began to tilt the Plateau toward the southwest. Sometime around 10 million years ago, plate tectonic movement began to open the Gulf of California and a river at its north end began to cut northward. At about the same time, the northeastward flowing rivers of the Colorado Plateau reached the southern escarpment of the plateau and began to flow south forming lakes along what is now the course of the Colorado River. Actual cutting of the Grand Canyon probably began about 5.5 million years ago.
Now we take up the story as told by House, Pearthree, and Perkins in their paper. Their story is based upon detailed stratigraphic mapping of the Bouse Formation and associated sediments in the Lower Colorado River valley, and upon tephrochronology, the precise dating of two layers of volcanic ash interspersed in the sediments.
In a nutshell, these authors propose that as the ancestral Colorado River cut through the Colorado Plateau and began to flow south, the water filled a series of basins from north to south, one at a time. When the northernmost basin was filled, it breached the natural dam on its south side, and filled the next basin to the south. This process was repeated until the river reached the Gulf of California. The paper goes into great stratigraphic detail in support of this hypothesis which is illustrated in the graphic below.
An earlier competing hypothesis proposes that an estuarine river cut northward from the Gulf of California, to form the Lower Colorado River. This hypothesis was initially supported by the presence of limited marine fossils in the lowest member of the sediments along the river. House, Pearthree, and Perkins counter this by proposing that:
“The lowermost Bouse lake would have encompassed an immense area extending from Parker Valley to the Chocolate Mountains and westward into a series of low-lying basins in the Mohave Desert. Recent hydrologic modeling suggests that it may have taken tens of thousands of years to fill this extensive lake to overflowing due to likely high rates of evaporation; consequently, the lake water could have become quite saline prior to spilling into the Yuma area. This might have allowed salt-water fauna to survive in the lake, but the mechanism for transportation of marine fauna into such a lake, if it existed, is disputed.”
In addition, they found that “strontium isotope ratios in Bouse carbonates throughout the extent of the deposit are more similar to modern Colorado River water than to seawater.” Also, the northward cutting river hypothesis requires over 1,800 feet of uplift along the river after the lake sediments were deposited. Case closed – for now.
The paper is well-illustrated with maps, cross-sections, and photographs.