Chapter 10 Streams - 2009 Allan Ludman and Stephen Marshak...

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1 CHAPTER 10 LANDSCAPES FORMED BY STREAMS PURPOSE • To learn how streams erode and deposit material • To become familiar with landforms formed by stream erosion and deposition • To interpret active and ancient stream processes from landscape features. MATERIALS NEEDED • Thin string • Ruler with divisions in mm or tenths of an inch • Graph paper for constructing topographic profiles • Colored pencils • A magnifying glass or hand lens to help read close-spaced contour lines 10.1 INTRODUCTION Water flowing in a channel is called a stream whether it is as large as the Amazon River or as small as the smallest creek, run, rill, or brook. Streams are highly effective agents of erosion and may move more material after one storm in an arid region than wind does all year. This chapter explores why not all streams behave the same way and how streams can produce very different landscapes. 10.2 How do streams work? All streams operate according to a few simple principles regardless of their size: • Water in streams flows downhill because of gravity. • Streams normally flow in a well-defined channel , except during floods when the water overflows the channel and spill out across the surrounding land. • The motion of water gives a stream kinetic energy, enabling it to do the geologic work of erosion and deposition. The amount of energy depends on the amount of water and its velocity (remember, Kinetic energy = ½ mv 2 ), so big, fast-flowing streams will erode more than small, slow-flowing streams. • Its kinetic energy allows streams to transport sediment, from the finest mud-sized to small boulders. These particles slide or roll on the bed of the stream, bounce along, or are carried in suspension within the water. • The flow of water erodes unconsolidated sediment from the walls and bed of the channel and streams use this sediment to abrade solid rock. • Streams deposit sediment when they lose kinetic energy by slowing down (or evaporating). The heaviest particles are deposited first, then the smaller grains as the energy wanes. © 2009 Allan Ludman and Stephen Marshak W.W. Norton & Company
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