96 Natural systems and greenspaces
blockage of fish migration; and severely degraded aquatic ecosystems and fish
populations. Yet urban rivers can be the people’s central focus and joy.
Waterpollutants cause physical changes (e.g., covering fish-spawning gravel
beds with fine sediment, or adding sun-heated roadside ditchwater to a cool
stream); chemical changes (e.g., adding nitrogen and phosphorus from fertil-
izer runoff, organic matter from sewage effluent, or toxic substances from an
industry); and biological changes (e.g., an explosion of blue-green algae, loss
of fish due to loss of oxygen, or disappearance of mussel populations due to
sediment-laden muddy water). Urban regions are especially characterized by four
types of water pollution: (1)agricultural runofffrom the urban-region ring adds
nitrogen and phosphorus from fertilizers plus sediment from surface erosion;
(2)stormwater runofffrom roads, buildings, and other surfaces carries hydro-
carbons and nitrogen oxides from vehicles, heavy metals from many sources,
and an array of atmospheric pollutants that have settled on surfaces, only to
be rain-washed into stormwater pipes and water bodies (it also picks up heat
and dissolved chemicals from the surface materials themselves); (3)septic and
sewage eff luent,the former from dispersed residential locations and the latter
from sewage treatment facilities, adds organic matter, nitrogen, and phospho-
rus towaterbodies; (4)industrial wastesare extremely diverse and sometimes
little known, but include inorganic materials such as heavy metals, plus numer-
ous organic substances associated with the manufacture of plastics, paints, and
other products common in markets, hardware stores, and automotive operations.
Sediment runoff from cropfield erosion and construction sites produces
muddy water, smoothes the bottom (a loss of important microhabitats), and
clogs up fish gills and other filter-feeding animals. Toxic substances kill aquatic
organisms of many sorts. Organic matter from sewage and some industrial pol-
lution causes an exponential growth of bacteria decomposing it, thus creating
anaerobic conditions, which kill fish and other aquatic organisms. An excess of
nitrogen or phosphorus normally produceseutrophication,anutrient-enrichment-
caused explosion of algae near the surface, and sometimes anaerobic conditions
lower down as bacteria decompose dead algal cells falling to the bottom. Con-
tinuous, relatively clean, water along a river system is important for passage of
stream-to-sea-to-streammigratory fish,such as salmon and eels, a special chal-
lenge where the river passes a city.
Urban-region streams are usually heavily impacted by the removal of woody
riparian vegetationcovering the floodplain, channel straightening (channeliza-
tion), adding rocks or concrete along streamsides, and directing the stream into
alargeunderground pipe.Waterquantityor hydrology, rather than water quality,
is the main issue for flooding and low flows. Impermeable surface cover in the
drainage basin produces rapid and high water flows (Arnold and Gibbons1996,