用强调句表示原有的意思,时态是不能随意改变的,也就是说,原来不强调的句子是现在时态就用 is,原来的句子是过去时态则用 was.
由上可以推断,你所提到的两个句子都是过去时,因此只能依旧保持原来的时态不变：
1.Mrs.Williams didn't know anything about it until I told her.
可以有以下两种改法：
—— It was until I told her that Mrs.Williams didn't know anything about it.(用 it is ...that 句型强调)
—— Not until did I told her Mrs.Williams knew anything about it.（用倒装句型强调）
2.We could not cross the river because the water had risen.
—— It was because the water had risen that we could not cross the river