Geoscience Reference
In-Depth Information
(a)
(b)
(c)
Figure 20.16 Irrigation in the San Joaquin Valley.
(a) Water used in spray irrigation is derived from the Central Valley aquifer. (b) Irriga-
tion canals such as this one divert water from the streams to specific farm areas. (c) Row irrigation of lettuce. Water from irrigation canals
is delivered to a key part of your next salad in this way.
the clay becomes perpetually more saline, which, in turn, causes
the salinization of soils in a manner entirely consistent with what
the Sumerians experienced thousands of years ago.
In an effort to mitigate the impact of increased soil salinity,
the Bureau of Reclamation devised a way in the 1960s to drain
away the shallow groundwater to stabilize the depth of the water
table below the root zone of plants and to facilitate leaching of sur-
face salts. This diversion was accomplished by the construction
of the San Luis Drain, which flowed 134 km from the southern
end of the valley to Kesterson Reservoir (Figure 20.18), a man-
made impoundment containing 12 evaporation ponds within the
Kesterson National Wildlife Refuge. The drainage system was
completed in 1971 and appeared to work well until it was discov-
ered in the early 1980s that water in the impoundment contained
elevated levels of selenium, which is toxic to wildlife in high con-
centrations. Although selenium occurs naturally in the area soils,
especially the eastern side of the valley near outcrops of marine
rocks within the Coast Ranges, it became concentrated in the
groundwater because percolating irrigation water flushed the se-
lenium from the surface and it ultimately drained into Kesterson
Reservoir. The impact of this selenium enrichment in the ground-
water became apparent when many overwintering migratory
birds and newborn chicks at the reservoir developed deformities
or became so sick that they drowned. In addition, ranchers near
the drain and reservoir reported numerous livestock deformities
Figure 20.17 Soil salinization in the San Joaquin Valley.
This
photo nicely illustrates the effects of salinization in the San Joa-
quin Valley. Notice how the salty patch (large white area) is very
poorly vegetated.
