Urbanization brings services to many people at one time, and raises the standard of living for residents of urban areas with jobs, cultural attractions, educational openings, sightseeing, and business. However, there are also downsides to urbanization too, especially the effects it has on the environment. As one of earth’s valued assets, freshwater ecosystems, especially groundwater, have the potential to be heavily impacted by urbanization. Urbanization comes with an increase in surfaces such as parking lots, streets, driveways and buildings. As a result of the installation of surfaces, runoff water does not have as much of a chance to seep underground, and so it runs off into the streams, and with it, pollution. Increased water in the streams leads to increased stream flow, which in turn leads to increased pollution. As a result, erosion is more pronounced and streams contain more silt and sediment. This pollutes the fresh-water environment even more for many of its inhabitants, which need cool, clear and running water to succeed. Runoff also has the potential to carry hazardous substances into streams and rivers such as motor oil, antifreeze, pesticides, chemicals, and litter. The combined amount of pollution from sediment and man-made chemicals leads to a dramatic decline of the freshwater ecosystem.
Streams are fed by runoff from rainfall and snow melt moving as overland or surface flow. Floods occur when large volumes of runoff flow quickly into streams and rivers. The peak of a flood is influenced by many factors, including the power and length of storms and snow melt, the topography and geology of stream basins, vegetation, and the conditions prior to storm and snow melt events. Land use and other human activities also influence the floods by adjusting how rainfall and snow melt are stored and run off the surface and into streams. In undeveloped areas such as forests and grasslands, rainfall and snow melt are collected and stored in plant life, in soil, or in surface depressions. When this capacity is filled, runoff flows slowly through soil as subsurface flow. In contrast, urban areas, where much of the land surface is covered by roads and buildings, have less capacity to store rainfall and snow melt. Construction of roads and buildings often involves removing vegetation, soil, and depressions from the land surface.
The leaky soil is replaced by resistant surfaces such as roads, roofs, parking lots, and sidewalks that store little water, reduce access of water into the ground, and speed up runoff to ditches and streams. Even in suburban areas, where lawns and other leaky landscaping may be common, rainfall and snow melt can saturate thin soils and produce overland flow, which runs off quickly. Dense networks of ditches in cities reduce the distance that runoff must travel overland or through subsurface flow paths to reach streams and rivers. Once water enters a drainage network, it flows faster than either overland or subsurface flow.
With less storage capacity for water in urban basins and more rapid runoff, urban streams rise more quickly during storms and have higher peak discharge rates than do rural streams. In addition, the total volume of water discharged during a flood tends to be larger for urban streams than for rural streams.