It has been a long time since the Los Angeles River was anything like natural. After devastating floods hit the region in 1938, locals decided to turn the river into little more than a huge drainage ditch. In an earlier time, it might have been considered more prudent to develop elsewhere, but by 1938 developers were bound and determined to bend nature to their will. And bend they did. What was once a natural course, providing wildlife habitat and wetlands, is now just a huge, slab-sided, concrete channel.
More recently, voices have been raised calling for a return to a more natural river environment. However, some scientists now are voicing concerns that any efforts which reduce the pipeline effect of the concrete channel could have dire implications regarding future flooding. It seems that LA has become a mainlining junkie for its concrete fix. The BBC has more: http://www.bbc.com/news/science-environment-30456672