A tree can absorb up to 1000 gallons of water a day, holding it so that the stream doesn't have to. This is a way to slow flooding. Trees keep water cool enough to support fish and other wildlife. If our creeks can't support wildlife it ultimately can't support us. This is a way to improve stream productivity.
Trees filter many agricultural, household and industrial pollutants.
Without such a filter the stream is much more susceptible to contamination. This is a way to slow down pollution. Because trees slow down erosion they also slow down nutrient loss that occurs in agricultural fields due to erosion. If we can slow down the loss of our watershed's most valuable commodity, fertile topsoil, then perhaps there will be less need for expensive fertilizers. This is a way farmers can help save money.