Rivers, Revised Edition

Laurie Burnham

Description

From Earth's two longest rivers, which flow through African deserts and Amazon jungles, to Siberia's great Yenisei-Angara river system, which drains into the Arctic Ocean, this appealing eBook vividly brings the world's great waterways into focus. Students will discover how these rivers came to exist, their place in history, what makes each unusual, and current environmental challenges. 

Sujets

• Science de la Terre
• Science
• Nature
• Volume
• Water
• Global
• Cartes communes au poker
• Climate
• Geology
• Environment
• River
• Sciences et techniques
• Young Adult Nonfiction
• Earth Sicences

Informations

• Publié par: Infobase Publishing
• Date de parution: 01 juin 2019
• Nombre de lectures: 0
• EAN13: 9781438182551
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Rivers, Revised Edition
Copyright © 2019 by Laurie Burnham
All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For more information, contact:
Chelsea House An imprint of Infobase 132 West 31st Street New York NY 10001
ISBN 978-1-4381-8255-1
You can find Chelsea House on the World Wide Web at http://www.infobase.com
Contents Chapters Rivers Nile River Amazon River Yangtze River Mississippi River Huang Ho River Yenisey River Ob River Congo River Amur River Mackenzie River Support Materials Glossary Index
Chapters
Rivers

Rivers have adorned the Earth's surface for billions of years, ever changing and ever moving. They change course as continents drift apart and come together, as mountains rise and fall, as canyons and bends form. They move as if alive, with the power and unpredictable behavior of a living organism. They travel, carry, destroy, build up, swell, and shrink.
Rivers carve their way through majestic mountains, roil through gorges, meander through farmland, and spew great volumes into the ocean. No two are alike. Some pound and thunder downstream, churning through rapids and over waterfalls; others lap sleepily at their banks, rolling ever so gently downhill to the sea. Some do both. And some are so wide and long that they are visible from outer space, appearing to orbiting satellites as silvery ribbons crisscrossing the continents.
Rivers have shaped not only the Earth but also human history. They are cultural icons, responsible for the rise of many great civilizations. For thousands of years, they have served as critical avenues for commerce and migration. Those with powerful waters are dammed to provide hydroelectric power. Those that move slowly give rise to green and lush land, which in turn produces agricultural riches for human consumption. But rivers are not only benefactors of humanity; they can also be cruel masters. Throughout history, they have unpredictably jumped their banks and drowned civilizations within their reach.
How Rivers Form
For all their power, rivers have surprisingly modest origins. No matter how wide and forceful they are when they reach the sea, they inevitably start small, usually in mountainous regions. Many are so small, in fact, that their flow is barely discernible. Some begin as little more than puddles in areas where groundwater oozes from waterlogged soil. Others bubble meekly from underground springs that well up through the Earth's surface. Still others enter existence as falling rain. Gravity plays an especially critical role in these early stages, pulling the surface water—which may lack its own momentum—downhill and funneling it into channels of ever-increasing volume.
The birth and evolution of a river system takes place over thousands of years, so no one has directly observed the event. Hydrologists rely instead on numerous methods to reconstruct a river's history. They do fieldwork, mostly in remote mountainous areas. They also examine historical maps, study aerial photographs, use global positioning systems, and create laboratory simulations.
Understanding How a Channel Is Created
Carving out a channel, that is, the bed where a natural stream of water runs, is the first step in forming a river. To make such a channel requires momentum. In other words, water must flow across the land with enough force to not only erode the surface but also to transport the eroded particles elsewhere. Such momentum depends on two factors: sufficient water and sufficient slope.
Though seemingly inconsequential, rain plays a pivotal role in the birth of a river. Field hydrologists (specialists who study water in nature) know from observation that raindrops can strike muddy soil with enough force to create miniature gullies or indentations. If the land is either nonporous or too waterlogged to absorb the rain, water collects in these gullies, which are known technically as rills. With the help of a small incline and groundwater seepage, water flowing through the rills creates a current strong enough to carve a small channel. Rills themselves are not permanent features on the ground, but are formed anew when it rains. But the water from these rills may combine with seepage to form a small channel. Over time—particularly in areas prone to torrential rains—the continual movement of water further deepens and widens the channel. The process then becomes self-reinforcing: The more defined the channel, the more likely that surface water will be captured and the greater the force of erosion, which in turn further deepens the channel, and so on until the channel is permanent.
How River Systems Form
Few rivers begin and end as single channels. Particularly in mountainous regions, tiny streams form at numerous locations, then come together to form larger channels. As their water cascades downward, one trickle joins another and another, resulting in a steady increase in the volume of water heading toward sea. A flow that was barely perceptible uphill may soon become a brook large enough for small fish. Brooks combine to form streams, which in turn empty—along with countless other streams—into ever-larger rivers that eventually discharge their watery load into the ocean.
Virtually all rivers grow larger as they flow downhill, fed by runoff and smaller streams called tributaries. The upshot is a complex river system, which consists of all the tributary systems (and there can be hundreds, sometimes thousands of branches) that successively converge to form the main trunk of a river. Each stream in a river system has its own drainage basin, or watershed, the area from which it receives runoff and groundwater.
Sometimes the combining streams are markedly unequal in size, sometimes they are almost the same, but a river system's main trunk is always the channel that carries the greatest volume of water. Not surprisingly, the world's biggest trunks bear the names listed in this volume: the Nile, the Amazon, the Congo, the Mississippi, and so on.
Mapping a Waterway
Mapping a river's main trunk becomes progressively harder the further upstream one travels, as each tributary and channel becomes smaller and less differentiated. Identifying the exact origin of a given river is often impossible. To begin with, one cannot precisely pinpoint the source of a body of water that begins as seepage or rainfall over a broad area. In addition, many rivers empty into and exit from lakes, blurring the line between where one stream begins and another ends. A typical lake receives input from many small streams but discharges into a single exit stream. The source of the Mackenzie River, for example, is Canada's Slave Lake, which in turn is fed by other rivers, including the Hay and Slave, both of which are considered tributaries of the Mackenzie. The Amazon discharges many miles into the Atlantic Ocean, a distance that varies seasonally. So where does the Mackenzie technically begin? Where does the Amazon end? The answers are somewhat arbitrary.
A River's Key Parameters
Determining greatness in a river is also somewhat arbitrary. Rivers are typically described by three parameters: length, volume, and the speed of their currents. But volume and speed shift dramatically by season, by climatic event, and by year. Accurate measurements of a river's length are almost impossible to obtain. No two sources agree on the exact mileage from beginning to end of any river in this volume. Even sophisticated satellite images rarely provide accurate, i.e., consistent, measurements.
Why? One reason is that rivers are dynamic entities, forever changing and altering their course. A bend may become more exaggerated from one year to the next; a channel may grow deeper and straighter. A second reason is that many rivers begin subtly—sometimes as little more than bogs—and thus their origins are impossible to pinpoint.
A third reason is that no standard methodology for determining a river's length exists. Rivers have traditionally been measured by running map wheels over aerial photographs, but no rule dictates whether the wheel must be run along the outside or inside of a bend, or even follow the channel's midsection. Over thousands of miles, these variables become significant. In addition, computer-assisted measurements lack standards for counting pixel lengths and so cannot guarantee accuracy either.
A fourth reason is that rivers are rarely single channels. Virtually all of them have tributaries, raising questions about which branch is the primary one, that is, the one leading to the true source. One need only look at a map of the Amazon to grasp the complexities involved. In some cases, a major tributary, such as the Missouri, which feeds into the Mississippi, is a formidable river in its own right. While most cartographers (mapmakers) include the Missouri in their length calculations, which puts the Mississippi at number four or five of the world's longest rivers, some choose to omit it, which drops the Mississippi to number 14. Thus few experts agree on, beyond the top two or three—those indisputable giants such as the Nile and the Amazon—which rivers belong on the lists of the world's longest, or even greatest, rivers. To give you an idea of the problem, estimates of length of the Amazon range from 3,700 to 4,850 miles (5,955 to 7,805 km).
As Rivers Age
Rivers do not age in a traditional chronological sense, but in a geological sense, as they move toward sea level. Young rivers are typically found in mountainous areas, where the slope is relatively steep and the narrow stream channels are filled with rocks and boulders. Here water rushes over rocks and forms waterfalls, especially in the spring when melting ice and snow add to the flow. Tumbling exuberantly downhill, these fast-flowing wat