How does clearing of forests by burning down trees causes water pollution?

Answer 1

There is the immediate effect of changing the pH of water runoff. Rainwater interacts with wood ash to produce lye a high pH material.

In the long term trees and the organic soil below them improves the retention of water in the forest. Without this sponge effect water runs more quickly through the soil leaching out nutrients in to the water causing a degree of eutrophication. In addition the unprotected soil erodes burdening the water with suspended solids.

Answer 2

When clearing the forest by burning down trees cause water pollution because the smoke will form acid rain and go into the water where we drink.