Rivers have long played an important role in the cultures and economies of the world’s greatest civilizations—from the Mesopotamian societies that took root between the Tigris and Euphrates rivers, to the dramatic rise of New York City at the mouth of the Hudson River. Pioneers searching for viable land on which to settle have typically chosen areas close to major rivers. Rivers offer a convenient source of food, fresh water for agriculture, and a means of transportation. For well-established settlements, rivers are vital for the development of economic and industrial endeavours. This thorough overview of rivers and streams will remove any doubt in the mind of the reader why so many of the world’s largest cities are strategically situated on the banks of major rivers.
Yet, the same civilizations that depend on rivers also risk harming them. Modern societies have altered the way rivers flow using dams to create hydroelectric power and canals to aid transportation and agriculture. Others have created fisheries and factories to aid their economies. These developments benefit the local economies, but they also damage natural habitats and the wildlife that depend on the environmental services of rivers for their survival. Scientists have spent many years studying rivers to help reduce the risks posed by human activity.
Several criteria are used to compare the world’s greatest rivers, including the length of the main stem, the size of the drainage area, and the mean discharge of the river’s flow. However, it is difficult to say exactly which river is the “greatest.” For example, most people know that the Nile River in Africa is the longest river in the world, but many don’t realize that the Amazon River in Brazil has the greatest drainage area and accounts for one-fifth of all the ocean runoff from world rivers.
Many factors affect the distribution and flow of Earth’s rivers, including terrain, climate, precipitation, glacier melt, water tables, and drainage basins. The manner in which these factors shape a river’s seasonal cycle defines the river’s regime. For example, a river may be slow and shallow during the winter, but swift and deep during spring snowmelt.
The drainage patterns of rivers vary greatly based on the geological makeup of the land. The shape and direction of a river depends on the type of land over which it flows. River formation occurs fastest on weak rocks. Tributaries form along faults. Streams that form fastest often become the main rivers in a network. Movement in Earth’s crust can create raises or folds that change a river’s course.
American hydraulic engineer Robert E. Horton developed the concept of stream order to better analyze and understand river networks. A river that forms the source of other rivers is known as a first-order river. When two first-order rivers join, the result is a second-order river. Two second-order rivers come together to form a third-order river, and so on. This concept forms the basis for the morphometry—or measurement—of drainage systems. Morphometry is, in turn, used to study the evolution and the hydraulic geometry of drainage systems.
Hydraulic geometry is the study of variations in river channel characteristics, which include water-surface width, depth, velocity, sediment load, downstream slope, and channel friction. Data regarding these characteristics are collected and analyzed in relation to the river discharge, or the volume rate of water flow at any given location. River channels patterns are sorted into several categories. The most common types include straight, meandering, and braided channels.
A sudden change in elevation of a river channel, which causes the water flow to drop vertically, or nearly so, generates waterfalls. Waterfalls that are less steep are called cascades. The taller a waterfall is and the swifter its current, the stronger its erosive power. In many cases the site of the waterfall moves gradually upstream as the current erodes the ground beneath it. Erosion may also cause a tall waterfall to flatten out over time. The power of falling water can also erode the riverbed at the bottom of the waterfall, creating deep bowls called plunge pools.
Although large, well-known waterfalls—such as Niagara Falls on the border of New York State and Canada, and Angel Falls in Venezuela—may seem eternal, they are ephemeral features when compared to the length of geologic time. Niagara Falls, for example, formed as recently as 11,700 years ago as the last glaciers retreated north. In time, the falls will weaken into mere rapids.
Waterfalls primarily form in three types of terrain: along the margins of high plateaus; along fall lines (a geological boundary between erosion resistant rocks and rocks that erode more easily); and in mountains, particularly those shaped by glaciers in the recent past. Some waterfalls are created by the discordance of their river profile. This means that they are the result of Earth’s plates shifting and creating new landforms, or from glaciation. Others are formed by differential erosion, which occurs where weak and strong rocks are positioned next to each other. Still other waterfalls form as a result of human-constructed barriers and dams.
Perhaps the most important aspect of river study in correlation to human civilizations is the rapid variation in water-surface level of river channels over time. This includes an understanding of peak discharge or peak flow, better known as flooding. Peak flow is primarily influenced by precipitation and snowmelt. For small rivers and streams, peak flow lasts for a relatively short time. For larger rivers, peak flow can last for days, causing flooding and the related problems for communities and businesses situated on the floodplain.
Forecasting floods can be difficult for many reasons. The chance of a flood occurring depends on the amount of precipitation, the rate of evaporation, the makeup of the terrain, and storm characteristics such as length and severity. Nonetheless, a long-term study of peak discharge for a given area can increase the chances of predicting a flood. Such predictions help determine how to protect communities from future floods and whether an area is a safe place to develop new communities.
Sediments are created from rock in a process called weathering. Over many years, solid rock cracks under geological pressures and forces. Water and roots work their way into these cracks, loosening them. The freezing and thawing of water in the cracks creates blocks of rock, which are further broken down by physical and chemical processes. Smaller pieces of rock tumble into streams and are slowly eroded by the stream’s current. Soils and sediments are washed out of the ground by runoff, flow into streams, and are carried away.
The river’s sediment yield, which is measured by weight or volume, is the amount of sediment carried away from a drainage basin, that is, the area that feeds water into a given stream or river. The eroded deposits carried by a river are called its sediment load, and sediment load is classified into three categories. Bed load is made up of rocks and sand that rest on the bottom of the river. These materials are drawn along slowly by the current. The suspended load of a river—the largest sediment load component—is made up of clays and silts floating in the water. Last, the dissolved load is comprised of chemicals and minerals mixed in with the water.
Eventually, sediments are deposited downriver. Some are carried all the way to an ocean or lake where they settle and form clay. Sediments may form fanlike landforms called deltas at the end of a river. Deltas are areas rich in minerals. Historically, these landforms—such as the Nile River delta in Egypt—have served as productive farmland. The deposition of sediments can be affected by human activities such as dam building and irrigation. Dams create reservoirs that become mammoth receptacles for sediments, and irrigation systems can move sediments out of the rivers and back onto land.
Rivers are more than just conduits for water and sediments. Over many years, they transform the geological landscape around them. Rivers cut through soil and rock, eroding and sculpting the land. They carry sediment to new areas where it collects and forms new geological structures. As such, rivers contribute greatly to landscape evolution.
Several present-day gorges and canyons were formed by recent tectonic activity. The Grand Canyon, for example, was formed by the uplift of the Colorado Plateau. The uplift of the Allegheny Plateau in the eastern United States has resulted in smaller but similar gorges and canyons that occur from New York to West Virginia. These landforms often become conduits for streams and rivers, upon which they enter the early stages of valley development.
A river running through a valley may be the one that originally formed the valley many years ago, or the initial river may have been diverted into another valley. The running water of a stream or river continually washes sediment downstream, carving out a bed. Early in a river’s existence, waterfalls are common. But as time passes, rivers gradually smooth out the valley floor. Most valleys begin as narrow landforms that grow wider until they reach their baselevel, or the lowest point at which its water can flow.
Rivers that flow through valleys are surrounded by relatively flat land called a floodplain. A valley bottom is often one large floodplain, and the surface of a floodplain is built on layers of alluvium—sediments deposited by the river during floods. Flooding delivers a rich supply of sediment-born nutrients to the soil, and thus the areas around rivers with wide floodplains are often used for farming.
A river that becomes narrower over time can create multiple floodplains at different elevations. These landforms are called terraces. A terrace has two distinct parts—a flat surface (the old floodplain), and a scarp, or steep slope connecting the new floodplain to the former floodplain. Terraces are formed when a river abruptly becomes narrower. The new river and its narrower flood-plain cut a channel into the valley floor, creating “steps.”
Valleys, canyons and floodplains often occur in the upper and middle courses of rivers and streams. However, different features—such as alluvial fans, deltas, and estuaries—tend to form at the ends of rivers and streams, where rivers and streams empty into larger water bodies.
An alluvial fan is a cone-shaped sediment deposit in a wide area not contained by a river valley, or at the mouth of a river. Alluvial fans are especially common in mountainous areas where snowmelt creates temporary streams. These streams erode the soil and rock and carry the sediment down to a valley basin. When the narrow stream reaches an open slope or plain, the sediments spread out in a fan share, giving the landform its name. There are two types of alluvial fans. Dry fans form as a result of ephemeral flow, such as in the mountain streams already mentioned. Wet fans are created by continually flowing streams.
As previously mentioned, deltas are fanlike plains of deposited sediment that forms at a river’s mouth. These landforms create new shoreline over time as the sediment accumulates. Many deltas are cone-shaped and thus resemble alluvial fans. A cone-shaped delta is created when many smaller streams break off of the main river trunk as the river approaches the sea. Deltas can be found all over the world, particularly at the ends of major rivers with a substantial sediment yield. Deltas have three parts. The upper delta plain begins where the river leaves the area where its water is confined by valley walls. Here the river breaks into numerous channels that radiate away from the river mouth. Marshes, swamps, and freshwater lakes may exist between these channels. The lower delta plain begins at the highest tidal level. This zone features both marine and fluvial (river) activity. The subaqueous delta plain is the part of the delta that remains completely below sea level.
An estuary is a coastal body of water that is partially surrounded by land. Estuaries form where freshwater from rivers and streams mixes with seawater. The size and extent of an estuary depends on several factors, particularly the distance to which tidal waters reach inland. Modern estuaries, such as the prominent embayments of Chesapeake Bay in North America, were formed at the end of the last ice age as sea levels rose. Estuaries have always been important locations for developing societies. Many cultures used them as centres for shipping, trade, and commerce. Today, because such unique species reside in estuaries, many are protected by law.
In the pages that follow, the reader will travel across the globe in search of the world’s greatest rivers. From the mighty Mississippi River of North America to the Yangtze of China, Earth’s rivers are both natural treasures and important tools for the industries and economies of modern societies. They also serve as valuable geological laboratories for scientists, teachers, and students.