if the constant attack on coastlines by storms, waves, and currents
were not doing damage enough, the “works of man” have
greatly exacerbated coastal erosion, in both the short and the
long term. And often the most damaging activities take place far
from the ocean.
Our preliminary research, surprisingly, tells
us that the primary cause of sediment removal from beaches is not
dams but sand and gravel mining. Dams are the second major cause.
Sand mining along the beaches of California and Oregon began in
the late 1800s. Although it had been outlawed by 1991, substantial
legal mining still goes on in coastal watersheds and even a stretch
of coastal dunes near Monterey. Overall in northern California,
from the Russian River to the Oregon border, some 11 million tons
of sand and gravel are removed each year. That figure pales in
comparison with southern California, where an annual average of
55.8 million tons is extracted, mostly around Los Angeles and San
Diego, though wherever there is major construction of roads and
buildings, there is usually a mine nearby.
All this extraction affects the shoreline. Even
off-stream gravel mining is detrimental to beaches. Floodplain mining
creates large pits next to dynamically changing river channels. If
these pits are flooded during peak flows, they act as sinks for sediment.
The normal transportation of sediment helps to dissipate a river
system’s energy. When sand and gravel are removed from within
the channel, “hungry water,”
deprived of its natural sediment content, creates a disequilibrium
that can cause erosion of gravel bars downstream.
The second major cause of sediment removal is damming.
More than 500 dams impound rivers and streams in California, affecting
38 percent of the coastal watershed (some 16,000 square miles). The
dams reduce the average annual sand and gravel flow by 2.8 million
cubic meters (enough to fill 1,000 Olympic-sized swimming pools)
or 25 percent of normal, according to Cope Willis and Gary Griggs
in the Journal of Geology (June 2004).
There are also a few minor culprits: “debris
built to trap sediments during floods; seawalls or coastal armoring,
which can have major local impacts; dredged navigation channels,
where material is taken offshore out of the nearby littoral system,
as happens in Humboldt Bay and the Columbia River estuary; and subsidence,
both natural and human-induced.