The riverbeds can also expand to a level where flooding occurs during heavy rains. Siltation and filling reduces their usefulness for flood protection, power generation, it also decreases fish populations and alters river flows. Trees are often considered natural sponges which regulate the earth's water supply by holding water. Forest waste matter, such as decomposing leaves or humus, slows down surface evaporation from the soil. Rain first falls on leaves and plants rather than directly hitting the soil, decreasing the rate of water and soil erosion.
Deforestation leads to a decrease in the amount of water absorbed by the soil.
Less water seeps down into the soil to recharge the groundwater. This leads to an absence of water available during the dry season and freshwater springs can dry up. Trees also hold water with their vast root systems, by creating air pockets deep in the soil. These pockets hold water during heavy rains. Forests help regulate the water in the air through a process called transpiration .