I remember reading about this method years and years ago in a old survival book. Next time I was out in the bush, I gave this a very concerted effort and found that it was vastly overrated...There are much better and more reliable methods of collecting water.

In the case of a flood...which you usually know is coming days before, the best water collection method is to simply prepare ahead of time by storing the water on high ground before it is too late. Of course that nicely stored water on high ground might as well be on the moon if you do not evac and get stuck in the flood..

As others have mentioned, a quality water filtration system is a good investment.