Wild speculation?
She needed a wee after sitting on a cold bench. She's left the phone on the bench where it will get a signal and gone off into the bushes by the river for a 30 second tinkle. She's taken a tumble and gone into the river, and unfortunately for he she's got her trousers round her knees so her ability to either stop herself, or climb out, is severely restricted.
If she was lucky, she'll have hit her head on the way down and reduced her understanding of what was happening.
Cold shock and violently moving water will have done the rest.