A stormwater system can capture, detain, slow, and infiltrate runoff/stormwater into the soil. The are many ways cities can deal with stormwater. Typical stormwater systems include detention and retention ponds, as well as raingardens, green roofs, permeable pavements, and cisterns.
Raingardens are, dollar for dollar, the most cost-effective way to handle stormwater in a natural way. They are not always right for every space. You can disconnect your downspouts, and instead direct roof water into a raingarden. See 12000 Raingardens for information about building a raingarden in your yard.
You can calculate how much stormwater comes off your roof and/or driveway. For example, One inch of rain on a 1000 square foot roof generates 700 gallons of water. You can collect this rainwater, by using cisterns. Building for a relatively rare (100 year storm) rain event, may require a lot of infrastructure.
Retention ponds or “wet ponds” are ponds constructed to treat and store stormwater runoff. Retention ponds are permanent pools of standing water and eventually empty into a receiving water.
How long does it take (how many steps are there) for water to go from sky to sound in a natural system versus an artificial system? Note that lots of layers means that water moves slowly. Water moving slowly takes less potential sediment and contaminants with it. Water is not as good a solvent with less energy. Also, it moves through living things such as trees and other plants, which take more up water for evaporation & evapotranspiration.
In a “natural” system, containing plants, pollution is also broken down by the plants as the water is taken up. Plants can be considered a “living engine” that help to break down pollution. However, the speed with which water moves through our landscape does much environmental damage. Slowing down the rate of water runoff also ensures a more steady, all-seasons flow, which is critical to maintaining healthy habitat for salmon.
Ten percent of total surfaces being impervious is considered a tipping point (by example of Chesapeake Bay; this is not always applicable) for watershed pollution.
Following link is a description of the current Sammamish stormwater system. The city is in the process of updating this system.