As a result of relentless rain the river had broken its banks and overflowed violently.

overflowed 是动词 overflow 的过去分词形式。overflow 由前缀 over-（表示"超过"）和动词 flow（流动）组成，字面意思是"流动超过"，即"泛滥、溢出"。 在句中 overflowed 表示河流水位超过了河岸的容纳能力，导致河水向外泛滥。