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Guide
to Reading 
Water is so common on Earth we rarely pause to appreciate
how rare it is in our solar system and how unique it makes our planet.
Water has literally shaped Earth’s surface and enabled life
to exist here.
The chapter begins by presenting a concept you’ve
probably encountered before; water cycles through all of Earth’s
“spheres”—atmosphere, hydrosphere, lithosphere,
and biosphere. That’s a lot of territory to cover, so this
chapter focuses on water only while it flows on land, not while it’s
in the oceans (that’s Chapter 15) or under the land (that’s
Chapter 16), or in the atmosphere (that’s Chapter 17).
For very common events, geologists use lots of very
common terms, such as surface water, runoff, sheetwash, infiltration,
and downcutting. They use a mix of common and specialized vocabulary
to discuss the origin of surface water (precipitation) and to describe
every part and aspect of a stream and its system: bed, mouth, headwaters,
stream gradient, tributaries, distributaries, trunk stream, drainage
network, meanders, braided stream, reach of a stream, drainage basins,
drainage divides, incised meanders, wetted perimeters, and drainage
networks (dendritic, radial, rectangular, and trellis).
Once stream parts and patterns are defined, the
author points out that characteristics of a stream are usually related
to its geologic surroundings. Folds, fractures, and faults serve
as structural controls of network patterns, except in special cases
where the timing of events allows streams to ignore such controls
and become superposed and antecedent streams. Streams with beds above
the water table are ephemeral streams; those with beds below the
water table are permanent streams. Not only are streams affected
by their geologic environment, streams in turn affect their geologic
environment. The nature and magnitude of the effect are influenced
by the amount of water (discharge) and character of flow (turbulence).
The work that streams do can be organized into three
categories:
- Transporting materials: You read about types of sediment load
(dissolved, suspended, and bed load), the size of the sediment particle
(competence), and the quantity of load (capacity).
- Eroding the land by scouring, abrasion, breaking, lifting, and
dissolution: Erosion can produce varied and spectacular results,
such as the goosenecks of the San Juan River in Utah.
- Depositing materials (generically called alluvium or fluvial
deposits): Materials can be deposited as bars in a stream’s
channel, as natural levees along its banks, as alluvial fans where
it quickly loses gradient on land, and as deltas where it quickly
loses gradient in quiet waters.
Like the Earth itself, a stream evolves (changes
over time). It experiences youth, maturity, and old age, and its
features change as the stages progress. Its longitudinal profile
develops a gentler gradient as it downcuts lower and gets closer
to local base levels and to its ultimate base level (an ocean). It
may achieve a condition known as graded (no net erosion or deposition),
create and eventually lose waterfalls and rapids, create and rework
valleys or canyons, and develop meanders on a floodplain. Meanders
are themselves a complex story involving cut banks, point bars, neck
cutoffs, oxbow lakes, backswamps, and yazoo streams.
You shouldn’t be surprised to find, toward
the end of the chapter, the usual reminder, with examples, that plate
tectonics is the ultimate reason for many geologic happenings.
The chapter ends with a discussion of human interaction
with rivers. Early civilizations needed to be close to rivers because
they provided so many essentials of life, like food, drinking water,
and transportation. Today rivers provide us these same things, plus
electric power and recreation sites. But rivers don’t always
behave as we want them to; sometimes they get out of control and
kill us. You read about several great historic floods (Johnstown,
Pennsylvania; Big Thompson River, Colorado; Bangladesh; the Mississippi
River).
Can humans ever totally control stream flow and prevent
flooding? Probably not. A stream seems ultimately to win every battle
to confine it or to direct its waters against its will. But society
hasn’t given up the fight. Scientists analyze potentially dangerous
situations, determine recurrence intervals of various size floods,
and recommend the construction of appropriate dams, reservoirs, levees,
and concrete flood walls to protect humans and their property.
Society has come to realize that while sometimes
rivers harm us, our growing human population is doing increasing
amounts of damage to rivers. The mere building of a city (urbanization)
can increase the likelihood of flooding of an area. The quality and
quantity of a water supply anywhere can change. We currently have
usable surface water, but this condition is not assured forever.
In summary, there are a lot of science facts in this
chapter, plus the message that life as we know it is intimately connected
with the water flowing across Earth’s surface. We shouldn’t
take this water system for granted, and we should be careful with
the changes we’re making to it.
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Key Terms
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