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Stream System
The Physical Environment
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Fluvial Systems
Fluvial Systems
The Stream System
The fundamental unit of study for fluvial processes is the drainage basin or watershed. A
drainage basin is a portion of the Earth's surface that contains a main stream and its tributaries and
is bounded by a drainage divide. The drainage divide represents the boundary between adjacent
drainage basins and determines into which basin precipitation flows. There is no upper or lower limit
to the size of a drainage basin. We can delimit the drainage basin of the Mississippi River as most of
the area east of the Rocky Mountains. The main stream is the Mississippi River and its tributaries
are rivers like the Missouri, Arkansas, and Wisconsin to name just a few. Or, at a more local
scale we can delimit the Wolf River basin found in central Wisconsin.
Figure 18. 1 The Drainage Basin
Streams within the drainage basin are either perennial or intermittent in flow. Perennial streams
flow all year. The base flow of these streams is provided by groundwater seepage into the channel.
Tributary streams are small streams that enter into the main stream. Tributary streams,
especially the smaller ones around the periphery of the basin, are intermittent. Intermittent
streams
only flow during wetter times of the year. Much of their flow is provided by surface
runoff and when the water table is higher as a result of moist conditions. The upland between
tributaries is called an interfluve.
Figure 18.2 Mississippi River Basin
Courtesy NOAA (Source)
The famous geographer John Wesley Powell recognized the value of the watershed concept in
environmental planning. In 1878 Powell published his Report on the Lands of the Arid Region, which
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Stream System
propose a strategy for settling the western United States without fighting over limited water
resources. All Things Considered's Howard Berkes reports "Powell's knowledge of the region
convinced him that water, or the lack of it, would be a major and ongoing problem in America's
westward expansion."
audio icon Powell proposed to organize settlements around water and
watersheds because overuse or pollution would impact everyone in the watershed.
Figure 18.3 The Victoria Nile, near Murchison Falls
in northern Uganda. (Courtesy K. Dunn, FAO_17389)
Some streams are classified as exotic streams. Exotic
streams are those that originate in a humid region but
flow through an arid region. Such is the case of the Nile
and Colorado Rivers. The headwaters of the Nile River is
in the wet Ethiopian Highlands, and travels through the
eastern portion of the Sahara Desert on its way to the
Mediterranean Sea. Along its route, the river loses
substantial amounts of water to evaporation. The same is
true for the Colorado River. With its headwaters in the
Rocky Mountains, it flows south and west towards the
southwest desert of the United States. During its journey it loses nearly half its flow to evaporation.
The Colorado used to flow to the ocean, astonishingly it no longer does. The reason is that the
remaining water has been diverted for agriculture and municipal water use.
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For Citation: Ritter, Michael E. The Physical Environment: an Introduction to Physical Geography.
Date visited. https://www.earthonlinemedia.com/ebooks/tpe_3e/title_page.html
Michael Ritter (tpeauthor@mac.com)
Last revised 6/5/12
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