Why is stormwater runoff a problem?
Let's take a quick look at the hydrologic cycle. When forests are present, rainwater stays on site and soaks into the soil, eventually percolating down to the lower layers of rock. This helps stave off drought, and in some areas provides the only drinking water available, through wells. As forest cover is removed, more and more runoff occurs, with the greatest amount of runoff coming from impervious surfaces.
Stormwater runoff impacts both water quality and water quantity problems. This is someone's backyard and they are losing more of it with each storm.
Runoff increases both erosion and flooding. This is a neighborhood creek that you can usually just step across. Of course, we would expect it to have more water during a rain event, but upstream development has caused it to be a torrent of water, eroding its banks.
Runoff also decreases the water that would soak into the ground, aiding groundwater replenishment.
Here is the same creek, usually ankle deep, but immediately after a rain, it is over 6 foot deep. It is so dangerous we couldn't allow grad students to measure its flow on their own. Also, it's in a neighborhood, on a greenway, so now it a liability. Much of this water could be slowly soaking into the ground, instead the urban forest has been bypassed with underground pipe that send water from stormsewers and parking lots directly into the creek.
Not only does it increase erosion and sedimentation, but it increases many pollutants, such as Nutrients, Sediment, Pathogens like e.Coli, Organic Materials, Toxic Contaminants, and Debris. In addition, it increases the temperature of streams receiving the runoff, causing problems and sometimes death of organisms, including trout.
All that trash was washed downstream from heavy runoff. The creek on the bottom now requires the greenway path to be stabilized and you can see all the sediment. This mountain trout stream on the bottom has a stormwater pipe leading right into it.
That trout doesn't just make that one happy woman, it is the example of economic stability in a mountain community, where the guide next to her makes a living, and the nearby hotel and restaurants supply jobs to locals so this woman can go on vacation and catch a whopper.
Forests Reduce Runoff
Forests reduce the amount of runoff because they absorb water, allowing it to infiltrate and NOT runoff.
In addition they absorb pollutants AND they don't add additional pollutants to the water.
Less runoff decreases both water quantity and water quality problems.