RIVERS AND STREAMS - Water Quality Engineering in Natural Systems - page 77

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RIVERS AND STREAMS

4.1 INTRODUCTION commonly occurs in the spring, and lowest flow rates in

the low rainfall season. This seasonality of low rates is
very important from a water-quality perspective, since
low low rates and high temperatures are typically asso-
ciated with the lowest assimilative capacity of streams,
and must generally be given prime consideration from
a water-quality control perspective. Such critical condi-
tions typically occur in the summer. Streams that always
contain flowing water, regardless of rainfall and snow-
melt conditions, are called perennial streams , and the
flow in these streams that is not derived from surface
runoff in called baseflow. and is typically contributed by
groundwater inflow, although, particularly in large
urban areas, it is not unusual for wastewater discharges
to be the primary contributor to baseflow. Streams that
can go dry during periods of no rainfall are called
ephemeral streams , and such streams are usually found
in regions with arid climates.
The most common contaminants in rivers and streams
are pathogens, suspended solids (which cause siltation),
oxygen-depleting substances, and nutrients. Pathogen
contamination of surface waters can cause human-health
problems, ranging from simple skin rash to acute gastro-
enteritis. Common sources of pathogens include the dis-
charge of inadequately treated municipal wastewater,
agricultural and urban runoff, and wildlife fecal material.
Siltation refers to the accumulation of small soil particles
(silt) on the bottom of a stream, causing suffocation of
fish eggs and destruction of aquatic insect habitats, and
damaging the food web that supports fish and other
wildlife. Siltation can occur as a result of agriculture,
urban runoff, construction, and forest-harvesting opera-

Rivers and streams have long been the primary sources
of drinking water to support human populations, and
the water quality of rivers and streams has been studied
more extensively and longer than any other bodies of
water. Streams are defined as natural drainage channels
that collect surface water runoff and groundwater inflow
from the surrounding area. Hydrologists classify the
hierarchy of streams according to the number of tribu-
taries upstream using a classification system originally
proposed by Horton (1945) and later refined by Strahler
(1957). First-order streams are the initial, smallest tribu-
taries; the stream located just below where two first-
order streams combine is a second-order stream; the
stream located below the confluence of two second-
order streams is a third-order stream; and so on. In most
cases, streams are called rivers when they become
seventh-order streams or higher (DeBarry, 2004). Alter-
native definitions of a river vary from a fifth- to a ninth-
order stream (Ramaswami et al., 2005). The Amazon
River, which is the largest river in the world, is a 12th-
order stream. In the present chapter, rivers and streams
will be referred to collectively as streams.
All streams have associated watersheds (or catch-
ments ) where any rainfall within the watershed can
potentially contribute surface runoff and associated pol-
lutants to the stream. The outflow end of the stream is
sometimes called the pour point of the watershed, or
simply the outlet of the watershed . Flow rates in streams
are typically seasonal, with the highest flows occurring
in the high rainfall and high snowmelt season, which