Floods and droughts are natural events determined by the amount of rain that falls in a particular area at a particular time. Any time when rainfall is much higher or lower than average, floods and droughts become a concern.

The damage done by floods is obvious. High waters block roads and damage property. The rapidly moving waters wash away spawning areas and eggs, uproot aquatic plants and temporarily cloud water with sediment from eroded soil. However, floods can also wash away sediment, making stream bottoms better suited for future spawning beds or create new deep pools where fish thrive. Floodwaters also deposit fertile sediments that nourish streamside plant communities.

While they may not seem as dramatic as floods, droughts often affect larger areas and have greater impacts. A drought causes water tables and water levels of streams, rivers and creeks to fall. As lake levels drop and wetlands dry up or shrink, plants, animals and birds can suffer from lack of habitat. The dry conditions can also increase the danger of forest fires that can ignite and spread quickly during dry conditions.

Urban development increases the probability of floods and droughts and intensifies the negative effects of these two extremes. Roads and other paved surfaces increase runoff, making streams, creeks and rivers rise faster and higher than they otherwise would, increasing the likelihood of dangerous flash flooding. The same surfaces prevent water from soaking into the ground. In times of little rainfall, droughts are more severe because less water has been stored in the ground.