Flood water will ruin the clean water. This will cause the water to be undrinkable. Flood water will get into homes and ruin roads. Even though it may seem like it is doing a good thing by adding more water, the water is not filtered and clean for drinking.
Answer: 2 H2 + O2 → 2 H2O, you get the same number of moles of water as H2, as long as you have 1 mole of O2. So, with 3 moles of H2, as long as you have 1.5 moles of O2, you will get 3 moles of H2O. In my opinion.