She was taken to the hospital after being submerged in a river.

after being submerged in a river 等于：

after submerging herself in a river

一个及物动词，其宾语是反身代词时，主动语态和被动语态表达一样的意思。又如：

He seated himself on the chair. 等于：

He was seated on the chair. 

She dressed herself in a black coat. 等于：

She was dressed in a black coat.