这是张培基的一段汉译英：

一条浩浩荡荡的长江大河，有时流到很宽阔的境界，平原无际，一泻千里。 

A mighty long river sometimes flows through a broad section with plains lying boundless on either side, its waters rolling on nonstop for thousands upon thousands of miles.

请问老师，做定语的with复合结构的宾补是lying boundless还是on either side？its water....是独立主格吗?