Trees are designed by nature to hold and clean water. Thank a tree for filtering ground water and keeping streams healthy, thus reducing the dependency on water treatment facilities. 

After a period of heavy rainfall or flash flooding, trees will absorb a large part of the rainfall, which prevents water from ending up in storm drains, less runoff and soil erosion. Eventually trees will release water back into the earth and atmosphere over time rather than all at once.

Source: https://onetreeplanted.org/blogs/stories/trees-clean-water