The Upper Los Angeles River was a-roaring in Sunday's rainstorm. Above is the view downstream from Lindley Avenue in Reseda. Below is the way that stretch of the river looks most of the time.
When the river is running high and fast, as it was Sunday, you can see why Army Corps of Engineers and the city widened and paved over what was a lazy stream most of the year. In March 1938 most of the San Fernando Valley and a good bit of the rest of Los Angeles flooded when storms hit back to back to back. The official response was to channelize the river in concrete most of the way to the ocean and to construct the Sepulveda Dam and a retention basin behind it.
The Sepulveda Basin was closed to traffic on Sunday to allow water to back up behind the dam, as it is supposed to work. Even so, a mile or so downstream the river channel was speeding along at Hazeltine Avenue in Sherman Oaks. The photo is looking upstream under the Ventura Freeway. The video is the other way.
While I was out getting soaked, I also took a look at the starting point of the Los Angeles River. The river begins where Calabasas Creek, coming in from the left with runoff from the Santa Monica Mountains, meets the runoff from Bell Creek in the Simi Hills. The confluence creating the old Rio Porciuncula is behind the Canoga Park High School athletic field.
From there, the Los Angeles River runs 51 miles to the Pacific in Long Beach and in that short span loses as much elevation as the great Mississippi River does. This looks like a lot of water, but the river picks up bigger loads downstream when Tujunga Wash and the Arroyo Seco join the Los Angeles. Along the way, numerous washes and creeks that still drain the local mountains feed into the river, along with the storm drains you see all over the city.