Internal drainage of the Carrizo Plain and the formation of Soda Lake began during
Pliocene-
Pleistocene time when
tectonic deformation associated with the
San Andreas Fault defeated a
stream that once drained the valley. An originally fresh to brackish water lake probably persisted through much of the Pleistocene during which coastal California was wetter and cooler than now. Diminished
Holocene precipitation and a higher
evaporation rate led to shrinkage of the ancestral lake and associated increased
salinity which set the stage for clay dune formation. About 30% of the closed or
endorheic drainage basin is to the north of the lake and 70% is south of it. The Soda Lake complex consists of two larger basins, and at least 130 smaller pans. The large L-shaped northern basin is about in area. The remaining basins, including both those that feed the northern basin and those that have no surface outlets, comprise an area of about . Water levels in the basins rise and fall seasonally. Rainfall on the Carrizo Plain is only about in an average year but is higher on the surrounding mountains. Following exceptionally wet winters, typically
El Niño years, the large northern and southern basins do not evaporate completely, although the water retreats toward the center of the basin leaving a
salt crust up to thick. Most of the large and small pans are fringed by
clay dunes. It is one of the largest alkali
wetlands remaining in natural condition in
California. It supports
fairy and
brine shrimp, as well as
migratory and nesting
birds. A
boardwalk was built along the shore to prevent damage to the
saltbush habitat that borders the lake. ==Climate==