In 1938, the San Gabriel Mountains (Los Angeles, USA) experienced heavy amounts of rain. This then led to mass movement, such as mudslides, and flash flooding, which flushed through downtown LA, damaging buildings, homes and vehicles. After these events, a flood committee was formed, and networks of hard defences were set up along the long profile of the San Gabriel River. The aim: decrease the hydrograph, and reduce the risk of flooding. In the upper course of the San Gabriel River, the San Gabriel Dam was built. This is a filtering dam, and uses sluice gates to prevent large boulders from progressing down river.
By doing this, the amount of sediment in the river decreases, which reduces the possibility of flooding from high sediment yield, and in the case of a flood, there will be no harmfully large sediment. Problems, however, are that the velocity of the river increased as there is less load to carry, which makes the erosion process scarce, as attrition and corrasion cannot take place. Also the riverbed and slip-off slopes of the river are very weak and can erode easily from hydraulic action. Then there are open dam systems, like Santa Fe and Maddock, where the river is stored and filtered of its sediment. This shares the same benefits of the sluice gates, however, it means no natural levees can form on the riverbanks. Further downstream, there are large control dams and artificial diversion-zone river basins, like the Wittier Narrows.
When the river begins to flood, the control dams close, and form a barrier. This completely prevents the river from flowing downstream. The water is then detoured into the artificial basins, like the Wittier Narrows where it is stored until the river returns to its normal level. Then the control dams gradually re-release the stored water into the stream flow. This is effective as it prevents the flooding entering the downtown area.
It also allows for things like hydroelectric power and water supply to be easily obtainable, without affecting the main river flow itself for too long. This problem is that this can lead to the starvation of habitats living alongside the river, as there is no water flow. The San Gabriel River successfully protects Los Angeles from flooding, however, at a cost. Only 2% of the initial river at the source reaches the mouth. The lower course of the river is a channelized river through the centre of L. A.
and is only a small stream, with little velocity. The consequences are that the beach suffers from displacement and mass erosion, which can lead to mass movement on the coastal front. Bibliography: BBC Science's Guide to Californian Habitats.