Although I should mention that by 1996, the water in one of the rivers flowing through the basin was clean enough that it was the site of the Olympic white water kayaking competition.

【答】原句抄写似乎有误，见红色部分。把is 改为in。

本句还有一个问题，就是：如果that 后面的部分是mention 的宾语从句，那么，句子就不完整，只有让步从句，没有主句。如果that 是指示代词作宾语的话，那么，断句就有问题。应该在that 之后加逗号停顿，而by 1996 后面可以不需要逗号。这样就是：

Although I should mention that：让步从句

by 1996 the water in one of the rivers flowing through the basin was clean enough that it was the site of the Olympic white water kayaking competition：主句

关于你的两个问题：

（1）enough 之后可以接that 从句表示“结果”，即that 从句是结果状语从句。

（2）although 和though 引导的让步从句，位置灵活，在主句前后均可。

下面举一些enough that...的例句：

She is old enough that she can get married.

她已达到可结婚的年龄。

His gross income was enough that he had to file a tax return.

他的总收入高到了他不得提出纳税申请。

The power of this machine was good enough that it became a business tool.

这部机器的力量是够好的它变成一个生意工具。