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12.15: The Dam Problem

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    The Dam Problem

    Dams have been constructed on most of the small rivers and streams throughout upland regions of San Diego County. The intentions of dam construction were to store water (reservoirs) and to reduce flood damage in low-lying communities. The problem is that dams have largely shut off the supply of sand from rivers and streams to the shore. One of the largest dams is for Lake Hodges on the San Dieguito River near Escondido, California (Figure 12.53). Construction of highway and railroad bridges, dikes, and causeways also restrict the flow of sediment-bearing water, preventing the migration of sediment to the coast. As a result, less sand is finding its way to the shore, resulting in narrower beaches. Without the protection of well-developed beaches, erosion of the sea cliffs are progressively endangering homes and infrastructure along the coast.

    Lake Hodges Dam
    Figure 12.53. Lake Hodges Dam.

    Dam construction: The easy way to kill a coastal community.

    Dams on rivers trap sediments that would otherwise find their way to ocean beaches. A classic example was the construction of a dam on Matilija Creek in Ventura County. The dam is currently being demolished in order to return the sediment flow to sensitive habitats along the river downstream, but also to return a sediment supply to the Ventura County coastline (Figure 12.54). Many other dams constructed in the 19th and 20th century are being removed for the same reasons.

    Coastal Dynamics—The Unending Saga

    Sea-level rise due to global warming is a highly political topic of our times. The world's Scientific Community has been studying the changes happening around the world for many decades. Real-time observations show that the atmospheric temperatures are steadily rising along with the concentrations of greenhouse gases in the atmosphere. What is perhaps most alarming is that the rate of sea-level rise is accelerating as the ice caps melt and the oceans expand from increased warmth (there are many NASA and NOAA websites on these matters). However, the threat doesn't seem to fit within the interest span of politicians and corporations keen on making high profits from the extraction of coal, oil, and gas. Sea-level rise will likely continue unabated until either we either consume all the economically available carbon-based resources, or human populations collaborate to change the fate we, collectively, are facing.

    Ventura River Delta
    Figure 12.54. The Ventura River (left) supplies massive amounts of sediments to the coast during infrequent floods, then persistent coastal erosion processes take over the action. Construction of the Matilija shut of much of the sediment supply to the coast. The dam is now being removed.

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