They make it sound as if she fell straight down 20 feet.. The wall is on an incline.. Parts of the L.A. river, particularly around that part of town, do have vertical walls.
Hollywood always shoots the part of the river that runs through downtown L.A., so that's all most folks are familiar with. That section does have a sloped embankment.
So many movies have utilized the paved LA river in them. Escape from NY is the one that springs to mind first (the early outdoor scens with Lee Van Cleef and the resolution scene at the very end), but then there is Terminator 2, and To Live and Die in L.A., etc.
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