In many parts of the coast of Southern California beaches have become smaller in

ID: 291651 • Letter: I

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In many parts of the coast of Southern California beaches have become smaller in recent years. This has resulted, at least part, from the damming of mountain streams for purposes of flood control. Explain why this might cause the beaches to become smaller

In many parts of the coast of Southern California beaches have become smaller in recent years. This has resulted, at least part, from the damming of mountain streams for purposes of flood control. Explain why this might cause the beaches to become smaller

In many parts of the coast of Southern California beaches have become smaller in recent years. This has resulted, at least part, from the damming of mountain streams for purposes of flood control. Explain why this might cause the beaches to become smaller

Explanation / Answer

In addition to impounding water, dams block sand and sediment that would otherwise flow down rivers and streams to the sea. All that sand and sediment silt up reservoirs and leave less room for water. Several California reservoirs, including those behind Matilija, Rindge Dam above Malibu and Searsville Dam in the Bay Area are nearly completely full of sediment.

The dam-building craze of the 20th century is now having a profound impact on the Southern California oceanfront as it's under assault from rising seas.Historically, streams and rivers provided the California coast at least 75 percent of its sand, according to oceanographer Gary Griggs of the University of California, Santa Cruz.About a quarter of that sand has been blocked by California's more than 500 dams on coastal streams and rivers. All of California's dams ,both coastal and inland have trapped at least 2.7 billion cubic yards of sediment in their reservoirs in total.

The effect is strongest in Southern California, where there are more coastal dams, and also more beach recreation and tourism supporting a $44 billion ocean economy. Griggs found that 50 percent of the sand that should be transported to beaches by Southern California streams gets trapped behind dams — nearly 200 million cubic yards since the 19th century. This is reason the beaches to become smaller.

Recent research from the U.S. Geological Survey confirms dams may be having an irreversible impact. According to modeling released in March, up to two-thirds of Southern California beaches may become completed eroded by 2100 unless greater action is taken.

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In many parts of the coast of Southern California beaches have become smaller in

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