The Mojave River and Associated Lakes
|Layers of alluvium exposed in Afton Canyon reveal that changes in the landscape have occurred over time. The different colored layers of sediments exposed in the cliffs reflect changing environmental conditions and stream source areas as sediments filled a basin. Afton Canyon formed when a great Pleistocene-age lake that filled the Manix Lake Basin overflowed through a low divide, creating a new path for the Mojave River. Through portions of Afton Canyon the Mojave River is a perennial stream fed in dry periods by groundwater. In the image above the Mojave River is hidden in the thick tamarask and other brush in the foreground. The intermittent stream draining a small side canyon produced the fan-like sediment apron where it enters the Mojave River floodplain. Note the elevated stream terrace on both sides of the side canyon.|
Debate remains whether earlier in the Quaternary the greater Mojave River drainage system (including Lake Manley and the Owens River drainage) filled to overflowing capacity and spilled into the Bristol Lake basin (south of the Mojave National Preserve), or possibly to the Colorado River (Enzel et al., 2003, Cox et al., 2003; Anderson & Wells, 2003). No water from the Mojave River system flows into the Bristol Lake basin at present or in the recent past. And in general, groundwater drainage systems follow patterns in surficial flow, but whether groundwater originating from the Mojave River basin ever makes it to the Colorado River is not resolved.
The synchronicity of lake formation with cooler and wetter climatic conditions in the Mojave region's past has been the target of abundant research from many angles and for many decades. Developing an understanding of climatic cycles (wet to dry) is fundamental to resolving changing landscape conditions (including plant cover, soil development, erosion, alluvial fan processes, playas and lakes, etc.); and understanding the relationships between these different variable elements. For instance, Soda Lake is now essentially a dry lake basin (a playa), of which only temporary flooding occurs on roughly a decade basis. Currently, the Soda Lake basin is fed by groundwater flow and episodic stream discharge from the surrounding mountain ranges and high-flow discharge from the Mojave River. Enzel et al. (2003) suggests that the average annual flow of the lower reaches of the Mojave River would have to be nearly a magnitude larger than the river's average current annual flow of 9,500,000 cubic meters. This 10-times volume would be necessary, along with reduced evaporative conditions, to maintain a lake consistent with the last glacial maximum. This means that rainfall amounts would probably need to be several times greater than current regional averages, and seasonal cooling and humidity would need to be significantly greater and last longer to overcompensate for evaporation effects. Currently, evaporative potential in the basin regions exceeds stream and groundwater flow input, and hence, today no lakes exist and desert conditions prevail.
The linkage of climatic changes with surface processes is only partly understood. There is undetermined lag between when cool and wet conditions start and when lakes can form and fill to their capacity. Also, the timing between the start of cool and wet conditions and the when ecological communities adapt and spread across the region may not not be synchronous. The reverse is true for changes from wet to dry conditions. Regional lake deposits show drying occurred about 9,000 years ago as sedimentary records transition from lake deposits, to intermediate marshland conditions, and to dry and desiccated playa conditions. The change from wet to dry conditions had a major impact on the ecology of the Mojave region (the vegetative cover and the animal species the landscape supported, including humans). Plant communities that were probably prevalent throughout the lower landscape regions are now restricted to the highest and wettest mountain areas, and probably many other species existed in the region that were adapted to a completely different seasonal pattern, but do not exist today. Likewise, aquatic or semi-aquatic species (fish, amphibians, turtles, arthropods, etc.) adapted to lake environments are restricted to spring-fed ponds or ephemeral spring fed streams, such as in portions of the Mojave River in Afton Canyon. The distribution of these species across the Mojave region (including in the Colorado River) may require a greater regional drainage system in prehistoric times.
For more information about ecology of the Mojave Desert ecology and climate history, see the USGS Biological Resources Discipline website: http://biology.usgs.gov/s+t/SNT/noframe/gb150.htm.
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USGS Western Region Geology and Geophysics Science Center