River Profiles
Date: 2015-10-07; view: 403.
River Profiles reflect the varying development of drainage systems. During early development, changes are reflected by direction and shape of stream courses. Ultimately, erosion produces an equilibrium between the slope and volume of the river and its erosional and depositional power. This results in a graded profile, or profile of equilibrium. Final equilibrium is never reached because of seasonal and geologic changes. The downward limit of erosion by a river is the base level, below which the river cannot downcut appreciably because it has reached the level of the body of water into which it flows. Sea level is the ultimate base level for all rivers. Larger rivers and lakes into which rivers flow constitute local base levels.
Stages in the cycle of river erosion were once labeled as “youth,” “maturity,” and “old age.” Although these stages describe certain characteristics, they imply no particular age in years, only changing phases in development. Rivers often show a condition of old age near their mouths, but are mature or youthful in their higher reaches. For that reason, the terms are rather misleading, and are now rarely used.
Exercise 8. Separate the following text “River Profiles”into words and sentences and then read it and translate it into Russian:
Riverprofilesareinfluencedbygeologyandstructureoftheirdrainageareaandtendtoassumeagradedconcaveprofileastheyapproachequilibrium.
Exercise 9. Read the following texts and decide what of the titles below correspond to each of them:
Text A. It is common in upstream portions of rivers — shows high gradients, waterfalls, rapids, steep-sided valleys, irregular courses, few tributaries, and erosion. The Gunnison River in Colorado is an example.
Text B. It is steep, with high relief, but fewer irregularities. Erosion and transportation of underlying rocks gives wider valleys and smoother topography. Tributaries are well-established. The White River of southwestern Missouri shows these features.
Text C. It reflects deposition of transported load. The river flows sluggishly over a wide, flat flood plain, with meandering pattern, ox-bow lakes, and sand bars. This represents a baseline equilibrium between erosion by the river and deposition in the sea or lake into which it flows. The lower Mississippi and Amazon are examples.
Longitudinal profile
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