Life Depends on Water how streams work

How Streams Work

Natural Stream Ecology	Altered Stream Ecology
		Urban Stream Dynamics
			Channelized Streams
			Dams

	Urban Stream Dynamics
	As land is developed, impervious surfaces increase and decreases the chance for water to absorb into the ground. Cities install storm sewer systems that quickly channel runoff from roads and other impervious surfaces. Runoff gathers speed once it enters the storm sewer system. When it leaves the storm drain or sewer system and empties into a stream, large volumes of quickly flowing runoff erode streambanks, damage streamside vegetation, and widen stream channels.

This results in lower water depths during non-storm periods, higher than normal water levels during wet weather periods, increased sediment loads, and higher water temperatures (see urban temperature graphic). Native fish and other aquatic life cannot survive in urban streams severely impacted by urban runoff.

		In streams, these physical changes occur in developing or urbanizing areas as a result of increased imperviousnes.
		• As headwater streams are buried or culverted, the stream network is simplified leaving less in stream area to absorb, filter and process storm water, nutrients, and floods. • Increased runoff volumes and velocities (more water into streams faster). With the greater amount of water in the streams getting there quicker, storm water causes more stream channel erosion.

• Changes in stream shape occur as channels erode and widen leaving streams shallower as water spreads out across wider channels. These changes in stream shape affect human infrastructure such as bridges, roads and buildings that are expensive to fix.

• Building on a floodplain decreases the floodplain's ability to weaken and slow flood waters. More floodwater moves downstream at faster rates increasing flooding downstream.

• High volume of water flushes out of urban streams quicker. Since it also reaches streams quicker through storm drain systems, it reduces the ability of storm water to absorb into the soil. This results in less groundwater available to recharge streams during dry season

• As streams are channelized (straightened) and eroded in urban areas, habitat loss occurs. Riparian zones are separated from each other leaving fewer habitats for animals that are dependent on these areas.

In addition, scouring of stream bottoms with increased volume of flow in urban areas lessens substrate variability and habitat for aquatic life. Sedimentation also decreases habitat variability as interstitial spaces of rocks on stream bottoms are filled in with sediment.


		Water quality changes also occur in urban streams.
		• Increased polluted runoff reaches streams. As storm water travels across impervious surfaces, it picks up pollutants (sediment, pesticides, fertilizers, nutrients, bacteria, oil, grease, toxins and trace metals) from the landscape from driveways, roads, roofs, parking lots and lawns. • Warmer stream temperatures result from storm water traveling across hot parking lots, roads, patios and sidewalks in the summer depositing warmer water than usual in streams, rivers and lakes. In addition, as riparian habitat is lost along urban streams, more stream channels are exposed to sunlight further warming the water. Warmer water holds less dissolved oxygen and results in less aquatic life in streams. • More algae grows in urban streams because of higher temperatures and increased nutrients from pollution. Dying and decaying algae further reduces the oxygen in the water which makes it difficult for other animals to live in the water.



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