Lake Lahontan History


Introduction

Lake Lahontan is a pluvial lake that formed within the western portion of the Great Basin, occupying the majority of northwestern Nevada during the middle to late Pleistocene. A pluvial lake is one that has had considerable fluctuations in water levels primarily due to climatic changes and fluctuations in precipitation and evaporation rates. Lake Lahontan formed at a time when regional climatic conditions were much different than they are today. Precipitation in the Great Basin region was at a much higher level than it is today, and evaporation rates were very much decreased, allowing for the formation of a large inland body of water within the enclosed basin complex. Presently, Lake Lahontan exists only as several small remnant lakes that have managed to survive due to perennial tributary streams.                                                                                [FrontPage Image Map Component]                                          

   

Geography

Lake Lahontan was the second largest pluvial lake in the northern hemisphere, covering approximately 21,000 kmē during its period of high water level. The basin which encompassed Lake Lahontan covers the majority of the northwestern portion of the Great Basin, and extends between approximately 117° 30’, and 119° 30’ west longitude, and 38° 30’, and 41°30’ north latitude. The drainage basin covers approximately 115,000 kmē and extends from the northwest corner of Nevada to the northeast portion of California, and north into Oregon.    

 

 

Great Basin Formation

    Lake Lahontan’s history begins approximately 12 to 15 Ma with the final stages of the Great Basin’s formation.  During this period, the upper plates began a series of faults that were upward steepening and that surfaced as normal faults.   This period of detachment faulting continued on into the early Pliocene (approx. 5 Ma) causing the Great Basin to expand by almost 50 percent.

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It wasn’t until the Pliocene to early Pleistocene that the west edge of the basin was completely enclosed by the Sierra Nevada Range.  Previous to this time period, water could flow out of the Great Basin through channels in the young Sierra Nevada.  During the Pliocene and Early Pleistocene, the Sierra Nevada were tilted westward by a series of heavy uplift.   This formation created a completely enclosed basin with very few inflows or outflows.

Pluvial Lake Formation

    Pluvial lakes are, by definition, lakes that have had considerable fluctuations in water levels primarily due to climatic changes and fluctuations in precipitation. The Pleistocene is the only time period known to have had enough fluctuation in climatic patterns and precipitation levels to produce pluvial lakes of any magnitude. Pluvial lakes are formed within closed basin formations that have no through-drainage outlets into surrounding fluvial or coastal systems. Because of this restriction, ancient pluvial lakes of North America, as well as their modern day remnants, are mainly found in the arid and semi-arid regions mostly within the Great Basin itself.

    The closed-system restriction is probably the most important factor for the formation of a pluvial lake, the second being a high precipitation to evaporation ratio (more precipitation than evaporation). A closed system is interpreted as one which is almost entirely shut off from surrounding environments, therefore making it nearly self-contained. The Lahontan basin area isn’t completely cut off from outside precipitation systems since precipitation from the west does make its way over the Sierra Nevada from time to time. However, these small amounts of precipitation have an overall very miniscule effect on the inner workings of the system itself. Lake Lahontan formed during a time period of heightened precipitation and decreased evapotranspiration. Precipitation in the Lake Lahontan region today is thought to be about one tenth of what it was in the Pleistocene, and evapotranspiration is now much greater, therefore creating an inverse relationship between the two time periods.

   

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