Effects of urban runoff[link]

A 2008 report by the United States National Research Council (textbox below) identified urban runoff as a leading source of water quality problems.

...further declines in water quality remain likely if the land-use changes that typify more diffuse sources of pollution are not addressed... These include land-disturbing agricultural, silvicultural, urban, industrial, and construction activities from which hard-to-monitor pollutants emerge during wet-weather events. Pollution from these landscapes has been almost universally acknowledged as the most pressing challenge to the restoration of waterbodies and aquatic ecosystems nationwide.
  --National Research Council, Urban Stormwater Management in the United States ^[5]

Weasel Brook in Passaic, New Jersey has been channelized with concrete walls to control localized flooding.

The runoff also increases temperatures in streams, harming fish and other organisms. (A sudden burst of runoff from a rainstorm can cause a fish-killing shock of hot water.) Also, road salt used to melt snow on sidewalks and roadways can contaminate streams and groundwater aquifers.^[6]

One of the most pronounced effects of urban runoff is on watercourses that historically contained little or no water during dry weather periods (often called ephemeral streams). When an area around such a stream is urbanized, the resultant runoff creates an unnatural year-round streamflow that hurts the vegetation, wildlife and stream bed of the waterway. Containing little or no sediment relative to the historic ratio of sediment to water, urban runoff rushes down the stream channel, ruining natural features such as meanders and sandbars, and creates severe erosion—increasing sediment loads at the mouth while severely incising the stream bed upstream. As an example, on many Southern California beaches at the mouth of a waterway, urban runoff carries trash, pollutants, excessive silt, and other wastes, and can pose moderate to severe health hazards.

Because of fertilizer and organic waste that urban runoff often carries, eutrophication often occurs in waterways affected by this type of runoff. After heavy rains, organic matter in the waterway is relatively high compared with natural levels, spurring growth of algae blooms that soon use up most of the oxygen. Once the naturally occurring oxygen in the water is depleted, the algae blooms die, and in their decomposing process, cause further eutrophication. Algae blooms mostly occur in areas with still water, such as stream pools and the pools behind dams, weirs, and some drop structures. Eutrophication usually comes with deadly consequences for fish and other aquatic organisms.

Excessive stream bank erosion may cause flooding and property damage. For many years governments have often responded to urban stream erosion problems by modifying the streams through construction of hardened embankments and similar control structures using concrete and masonry materials. Use of these hard materials destroys habitat for fish and other animals.^[7] Such a project may stabilize the immediate area where flood damage occurred, but often it simply shifts the problem to an upstream or downstream segment of the stream.^[8] See River engineering.

Prevention and mitigation of urban runoff[link]

A percolation trench infiltrates stormwater though permeable soils into the groundwater aquifer. An oil-grit separator is designed to capture settleable solids, oil and grease, debris and floatables in runoff from roads and parking lots

Effective control of urban runoff involves reducing the velocity and flow of stormwater, as well as reducing pollutant discharges. A variety of stormwater management practices and systems may be used to reduce the effects of urban runoff. Some of these techniques (called best management practices (BMPs)) in the US, focus on water quantity control, while others focus on improving water quality, and some perform both functions.^[9]