Closed and semiclosed basins along the Mojave River in southern California were occupied by pluvial lakes during the latest Pleistocene. The chronologies of Harper Lake, Lake Manix (Coyote Lake and Troy Lake playas and the Afton basin), and Lake Mojave (Soda Lake and Silver Lake playas) are summarized here from available data. We evaluate the chronologies, compare them with each other, and then use them to determine coexistence of lakes within the Mojave River hydrological system. The average annual flow in the lower reaches of the Mojave River that is needed to form and maintain a lake in one of these basins is at least an order of magnitude larger than the present-day average discharge of 9.5 x106 m3. The discharge could have increased by (a) more frequent storms and floods, and/or (b) reduced loss by transmission of flood water along the river length. This reduction in transmission losses could have been caused by longer river reaches either covered by lakes or characterized by base flow that, in turn, was formed by water table near or at the surface. The increase in flood discharge is caused by an increased storm frequency in the headwater of the river. The discharge increase needed to support individual lakes is multiplied when the total lake area of coexisting lakes fed by the Mojave River is considered. It demands an even larger increase of the number of storms in the headwaters than the number needed to support an individual lake. This indicates a large increase in atmospheric moisture transported to this relatively low latitude along the coast of western North America. The coexistence of lakes during the last glacial maximum and the highstands of other lakes in similar latitudes in the southwestern United States indicate that the storm tracks were frequently directed at 32°-34° N latitude at that time. The chain of lake basins along the Mojave River is an example of a hydrological system that is integrated through time into one large arid river basin. This was possible mainly because of the elevated headwaters located at the San Bernardino Mountains where orographic effects cause heavy storms and large floods. These large floods fill the depositional basins along the river with water and sediments and allow them to overflow downstream. Afton Canyon was formed by such an overflow from the Manix basin to the Lake Mojave basin. The incision of the 150-meter-deep canyon was previously proposed to be rapid and geomorphically catastrophic. Here we propose that a timetransgressive incision lasting over a few thousand of years is more plausible explanation for the formation of this canyon; geologically it is still a rapid event. In addition, we discuss how the findings along the Mojave River reflect upon two long-term hypotheses of (a) mega-Lake Manly that supposedly filled a large area in the Mojave Desert sometimes in the middle Pleistocene, and (b) Lake Manix and/or Lake Mojave overflow to Bristol Lake and to the Colorado River.
|Original language||American English|
|Number of pages||17|
|Journal||Special Paper of the Geological Society of America|
|State||Published - 2003|