Geoscience Reference
In-Depth Information
the annual salmon runs, depending on yearly variability in salmon popula-
tions. h ese harvests did not seriously af ect the salmon over the thousands
of years that humans have lived along the Pacii c Coast. Yet now, over the
past century, the salmon along the West Coast have almost disappeared.
Causes of Salmon Declines
Today, salmon are at the center of some of the most contentious environ-
mental battles in the West, all swirling around water and land development.
h e steady decline of the salmon along the West Coast began in the mid-
nineteenth century, when pioneers settled the region and began altering the
landscape. In California, hydraulic gold mining was the i rst major assault
on migrating salmon. Streams were choked with sediment that was washed
out of the Sierra Nevada foothills by enormous hoses. Moreover, commercial
i shing of salmon began in rivers in the 1860s and in the ocean in the 1890s.
But it was the twentieth-century era of dam building, the hydraulic era, that
had the greatest impact on salmon populations, reducing them to a small
fraction of their former levels. Other factors af ecting the salmon—such
as logging and cattle grazing in the watersheds, pollution from abandoned
mines, gravel mining in streams, overi shing in the ocean, and periods of
natural climate variability with unusually warm ocean conditions—have also
contributed to their plight. But all of these factors combined are thought
to have played only a minor role compared with the severe impacts of dam
building on West Coast rivers.
Dams physically block salmon migration, preventing the i sh from reach-
ing their spawning habitat, and this is thought to be the primary cause of
salmon decline. In California, juvenile salmon that manage to reach the
Sacramento-San Joaquin Delta and the San Francisco Bay ot en fail to reach
the ocean because many are entrained in the huge battery of pumps in the
southern delta that transport water to the California Aqueduct and down
to the San Joaquin Valley and Southern California (see i gure 30). h e small
juveniles have evolved to follow the currents, and these pumps have reversed
the currents in the delta to l ow south, away from the estuary and the Pacii c
Ocean. Millions of salmon i ngerlings are lost each year, sucked into the
pumps and killed, decimating future salmon populations. In addition, the
delta pumps draw a greater proportion of river water during droughts, when
farmers have a greater demand for water, leading to even higher i sh mortality
during an already stressful time.
