My best guess is SM may have attempted to walk from the GD store directly toward her house by attempting to cross the stretch of creek on the side of S. 5th Street where the GD store and her house are located... she may have slipped in the rushing creek, as it was deep enough from thunderstorm rain for creek rush to have carried her under the bridge to the discovery location.
This is a video of water flowing in the DG/Comet store parking area on Sept 9, 2016:
[video]https://video-iad3-1.xx.fbcdn.net/v/t42.1790-2/14314550_766260756810934_210051037_n.mp4?efg=eyJ2Z W5jb2RlX3RhZyI6InN2ZV9zZCJ9&oh=7b7441d5f7e1a35e9d2fef9e1588c9a6&oe=57D7A35F[/video]
This is the Clarion Mall parking lot on Sept 9, 2016... note the water level is up to vehicle doors:
It could be that the creek had risen to street level at the bridge and SM may have attempted to avoid the water rushing in the road.