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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, and lithosphere. 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 18) or under the land (that’s Chapter 19), or in the atmosphere (that’s Chapter 20).
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 (meteoric water) and to describe every part and aspect of a stream and it 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 (thalweg, turbulent, and laminar flows).
The work that streams do can be organized into three categories:
- Transporting materials: You read about types of sediment load (dissolved, suspended, saltation, 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 badlands of South Dakota and 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) and its features change as time passes. 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 even kill us. You read about several great historic floods (Johnstown, Pennsylvania; Big Thompson River, Colorado; Yellow and Yangtze Rivers, China; Bangladesh; and the Mississippi River) and a humongous prehistoric flood 11,000 years ago (the Great Missoula Flood) that created the channeled scablands of Washington State.
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 and annual probabilities 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. Humans have carelessly polluted streams, and this has resulted in severe damage to the physical and biological environment. Our increasing population has caused us to possibly go overboard on dam and reservoir construction and has resulted in serious overuse of stream waters (as in the Aral Sea region of Central Asia and along the Colorado River in the southwestern United States). The quality and quantity of a water supply anywhere can change. Mars once had surface water, but it no longer does. 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. |
