Stormwater runoff decreases available groundwater. When rain lands on impervious surfaces, it is diverted to the nearest stream (where it is hurried away) rather than soaking into the ground to become groundwater.

Decreased groundwater results in streams and tributaries having lower flows (or no flow) in dry weather—reducing habitat for aquatic creatures or raising the water temperature beyond what they can handle.

Decreased groundwater also challenges the moisture requirements of natural communities that flourish on floodplains and other low areas. Insufficient water for a plant's needs lowers that plant's stress tolerance, making it more susceptible to disease and less able to survive an infestation of insect pests.

Decreased groundwater can even pave the way for plants whose roots don't like to stay wet to settle in the floodplain. This makes for lots of drowned plants later when flooding occurs.