Los Angeles Basin Analysis

4.1 Geology of the Los Angeles River The Los Angeles River as we know it now is a product of human intervention and the several geologic changes in the Cenozoic Era, which extends from 66 million years ago to the present. The River is directly tied to changes in the Los Angeles Basin over millions of years, therefore understanding changes in the basin is part of understanding how the river came to exist as we know it now. Geologically speaking, the Los Angeles Basin is a relatively young geographic lowland that formed due to geomorphic changes in the last 16 million years. In fact, during the Tortonian age, more than 10 million years ago, the Los Angeles Basin was inundated with water. These waters started to recede as mountain ranges

Following the floods of the 1880s, authorities failed to act because the cycle of dry years and mild winters was present. Over the next two decades, Los Angeles’ population reached 500,000, in great part due to the presence of railroads facilitating moving to the area. However, El Niño of 1913-14 proved to be a very damaging and costly event. This flood covered thousands of acres south of Los Angeles. This was the flood that convinced authorities that something had to be done as the damage to infrastructure and homes exceeded $20 million (around $400 million in current figures). The risk to human life was becoming more evident as the population grew and changes made to the land exacerbated the risk of flooding. The public and city administrators pushed for flood prevention, and by 1921, the direction of the River had been moved away from the harbor. In addition, three large dams were built in the foothills to help regulate runoff. Only one of the three, Devils Gate Dam, which was constructed on Arroyo Seco, was built in the River system. Over 3700 check dams (small dams) were built in over 60 mountain canyons. This was the era of big dams in the United States and several more were built in those

The River was channelized for most of its length, partially or completely, with the exception of the Sepulveda flood control basin. In fact, only three areas of the river were left with an earthen-bottom: The Sepulveda Flood Control Basin, Glendale Narrows, and the River estuary in Long Beach. This channelization was an ecological catastrophe. The trees that survived the floods were taken out and along with them habitats were destroyed. Channelization also caused native fish, birds, and mammals to disappear or stay away from the middle and lower river banks. The test to see if channelization worked came in the form of El Niño of 1969. Just before the projects were complete in 1970, the Los Angeles River basin witnessed nine days of nonstop rain. The mouth of the River at Long Beach recorded 102,000 cubic feet per second in spite of flood regulation. The areas that were channelized did not suffer any damages, however, the areas that had been developed closer to foothills were heavily damaged due to mud floods. As long as population and development keep growing, damage will continue to occur during wet years that are affected by El