with which to do，来源于定语从句，省略了从句主语和一个情态动词，算是定语从句的简化/变种形式。还原成完整的从句形式，应该是这样的:
......they have created strong long living bridges with which they can cross streams......
这就是一个普通的“介词+which”引导的定语从句。
定语从句的这种用法现在已经很少见到了，至少在目前的中学教材中没出现过。然而这种类似的变形在名词性从句中还是非常普遍的:
I don't know what I should do(what to do).(宾语从句简化)
When we can have dinner (=When to have dinner) is still unknown.(主语从句简化)
My question is how I can get my money(how to get money).(表语从句简化)
I had no idea how I could handle the situation (how to handle the situation) .(同位语从句简化)

缩减版的定从，替代先行词bridges

定语从句

这种不是很好的英语，就不用太细究了。只能拿来吸收知识，不能拿来学英语。

应该带着前面的secondary roots

因为created living bridges和cross streams and rivers是同一批人，with which略去后整个句子意思基本没变。which代指living bridges，with which在这个句子里表“状态”，个人觉得这个句子挺好的呀

有点意思