Water quality is a good predictor of human health. When the water is good, life is good and when the water is bad, you get sick people and sick environment.
Intact forests play an important role to ensure that ensures there is fresh, clean water for both wildlife and human beings. But deforestation disrupts the water cycle.
The removal of trees results in the groundwater tables getting depleted because the trees lose their function of helping the soil absorb flowing water. The land then becomes unproductive as soil properties responsible for supplying soil nutrients are leached from rain falling freely on the soil. The large quantity of sediments washed away from deforested areas end up in streams and river cause high turbidity and siltation, causing negative impacts on fisheries further downstream as far as the reefs.
The surest way to ensure a good water supply is to keep forests intact.