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▲Figure GN 14.1 Map of the Mississippi River basin. 409

GEOSYSTEMSnow

The Disappearing Delta Before modern engineering of the chan- nel, the Mississippi River carried over 400 million metric tons of sedi- ment annually to its mouth. River deposits built from this sediment now underlie most of coastal Louisiana. Today, the flow carries less than half its previous sediment load. This decline, combined with land subsidence and sea-level rise, means that the delta region is shrinking in size each year.

The tremendous weight of sediment deposition at the Mississip- pi’s mouth has caused the entire delta region to lower as sediments become compacted, a process that is worsened by human activities such as oil and gas extraction. In the past, additions of sediment bal- anced this subsidence, allowing the delta to build. With the onset of human activities such as upstream dam construction, the delta is now subsiding without sediment replenishment.

Compounding the problem is the maze of excavated canals through the delta for shipping and oil and gas exploration. As the land surface sinks, these canals allow seawater to flow inland, changing the salinity of inland waters. Freshwater wetlands whose roots help stabilize the land surface during floods are now declining. This makes the delta more vulnerable to flooding from hurricane storm surge, another factor hastening the delta’s demise.

Finally, sea-level rise threatens coastal land and wetlands, most of which are less than 1 m (3.2 ft) above sea level. With continued local sea-level rise, lands not protected by levee embankments and other structures that prevent flooding will con- tinue to submerge.

In this chapter, we examine the natural pro- cesses by which rivers erode, transport, and de- posit sediment, forming landforms such as deltas.

1. Why are engineers trying to keep the Mississippi River in its present channel?

2. What three factors are causing the Mississippi delta to disappear?

Changes on the Mississippi River Delta

T he immense Mississippi River basin drains 41% of the continental United States (Figure GN 14.1). From its head-

waters in Lake Itasca, Minnesota, the Missis- sippi’s main stem flows southward, collecting water and sediment over hundreds of miles. As the river nears the Gulf of Mexico, the flow energy diminishes and the river depos- its its sediment load. This area of deposition forms the delta, the low-lying plain at the river’s end.

Like most rivers, the Mississippi continu- ously changes its channel, seeking the short- est and most efficient course to the ocean. In southern Louisiana, the Mississippi’s chan- nel has—over thousands of years—shifted course across an area encompassing thou- sands of square miles. Throughout this time span, floods caused the river to abandon pre- vious channels and carve new ones. The Mis- sissippi River attained its present position about 500 years ago and began building the delta we see today (Figure GN 14.2).

Engineering the River Channel Since about 1950, engineers have worked to keep the Mississippi River in its present channel, a feat accomplished by dams, floodgates, and artificial levees (earthen embankments designed to prevent channel overflow). The U.S. Army Corps of Engineers built the Old River Control Structure in 1963 to block the Mississippi River from shifting westward toward the Atchafalaya River, which takes a steeper, shorter route to the Gulf of Mexico. Such a shift would cause the river to bypass two major U.S. ports, Baton Rouge and New Orleans, with negative eco- nomic consequences. Despite such measures, the Atchafalaya delta is growing even as the rest of the Mississippi’s delta disappears.

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▲Figure GN 14.2 Mississippi River landscape, southern Louisiana. Inset photo shows the Old River Control Auxilliary Structure. NASA/USGS; Inset photo by Bobbé Christopherson.

Mobile Field Trip

https://goo.gl/bpcQAU

Mississippi River Delta

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410 Geosystems

Earth’s rivers and waterways form vast arterial net-works that drain the continents. Even though this volume is only 0.003% of all freshwater, the work per- formed by this energetic flow makes it an important natural agent of landmass denudation. Rivers shape the landscape by removing the products of weathering, mass movement, and erosion and transporting them downstream.

Remember from Chapter 8 that hydrology is the sci- ence of water at and below Earth’s surface. Processes that are related expressly to streams and rivers are termed fluvial (from the Latin fluvius, meaning “river”). The terms river and stream share some overlap in usage. Spe- cifically, the term river is applied to the trunk or main stream of the network of tributaries forming a river sys- tem. Stream is a more general term for water flowing in a channel and is not necessarily related to size. Fluvial systems, like all natural systems, have characteristic pro- cesses and produce recognizable landforms.

The ongoing interaction between erosion, transpor- tation, and deposition in a river system produces fluvial landscapes. Erosion in fluvial systems is the process by which water dislodges, dissolves, or removes weath- ered surface material. This material is then transported to new locations, where it is laid down in the process of deposition. Running water is an important erosional force; in fact, in desert landscapes it is the most signifi- cant agent of erosion even though precipitation events are infrequent. We discuss fluvial processes in arid land- scapes in Chapter 15.

Rivers also serve society in many ways. They provide us with essential water supplies; dilute, and transport wastes; provide critical cooling water for industry; and form critical transportation networks. Throughout his- tory, civilizations have settled along rivers to farm the fer- tile soils formed by river deposits. These areas continue to be important sites of human activity and settlement, plac- ing lives and property at risk during floods (Figure 14.1).

Drainage Basins Streams, which come together to form river systems, lie within drainage basins, the portions of landscape from which they receive their water. Every stream has its own drainage basin, or watershed, ranging in size from tiny to vast. A major drainage basin system is made up of many smaller drainage basins, each of which gathers and delivers its runoff and sediment to a larger basin, even- tually concentrating the volume into the main stream. Figure 14.2 illustrates the drainage basin of the Amazon River, from headwaters to the river’s mouth (where the river meets the ocean). The Amazon carries millions of tons of sediment through the drainage basin, which is as large as the Australian continent.

Drainage Divides In any drainage basin, water initially moves downslope as overland flow, which takes two forms: It can move as

A flooding river carries not only water but also sediment and debris. When a river overflows its banks into human develop- ments, the flow can pick up vehicles and knock houses off their foundations. As the floodwaters recede, debris such as trees come

to rest and sediment is deposited over most surfaces, including the interiors of houses. In June 2016, flooding in West Virginia caused extensive damage, 23 fatalities, and left residents cleaning up a land- scape of mud.

everydaygeosystems What kind of damage occurs during a river flood?

◀Figure 14.1 The aftermath of flooding along the Elk River, Clendenin, West Virginia, in June 2016. [Ty Wright/Getty Images.]

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Chapter 14 River Systems 411

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georeport 14.1 Locating the source of the Amazon Over the past several centuries, scientists and explorers have designated at least six different sources as the true beginning of the Ama-

zon River. In the 1970s, southwest Peru’s Apurímac River was deemed the longest tributary stream, and in 2000, Lake Ticlla Cocha on the slopes of Mount Mismi was named as the Apurimac's source. Then in 2014, a team of kayakers used GPS tracking data and satellite images to determine that the Mantaro River, also in southwest Peru, is the longest upstream extension of the Amazon River. However, the new claim remains under debate.

sheetflow, a thin film spread over the ground surface, and it can concentrate in rills, small-scale grooves in the land- scape made by the downslope move- ment of water. Rills may develop into deeper gullies and then into stream channels leading to the valley floor.

The high ground that separates one valley from another and directs sheetflow is called an interfluve (Figure 14.3). Ridges act as drainage divides that define the catchment, or water-receiving, area of every drain- age basin; such ridges are the dividing lines that control into which basin the surface runoff drains.

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412 Geosystems

A special class of drainage divides, continental divides, separate drainage basins that empty into dif- ferent bodies of water surrounding a continent (Figure 14.4). For North America, these bodies are the Pacific Ocean, the Gulf of Mexico, the Atlantic Ocean, Hudson Bay, and the Arctic Ocean. These divides form water- resource regions and provide a spatial framework for water-management planning. In North America, the con- tinental divide separating the Pacific and Gulf/Atlantic basins runs the length of the Rocky Mountains, reaching its highest point in Colorado at the summit of Gray’s Peak at 4352 m (14,278 ft) elevation (Figure 14.5).

As discussed in Geosystems Now, the great Mississippi–Missouri–Ohio River system drains 41% of the continental United States. Within this basin, rain- fall in northern Pennsylvania feeds hundreds of small streams that flow into the Allegheny River. At the same time, rainfall in western Pennsylvania feeds hundreds of streams that flow into the Monongahela River. The two

rivers then join at Pittsburgh to form the Ohio River. The Ohio connects with the Mis- sissippi River, which eventually flows to the Gulf of Mexico. Each contributing tributary, large or small, adds its discharge and sedi- ment load to the larger river. In our example, sediment weathered and eroded in Pennsyl- vania is transported thousands of kilometers and accumulates on the floor of the Gulf of Mexico, where it forms the Mississippi River delta.

Internal Drainage The ultimate outlet for most drainage ba- sins is the ocean. In some regions, however, stream drainage does not reach the ocean. Instead, the water leaves the drainage basin by means of evaporation or subsurface gravi- tational flow. Such basins are described as having internal drainage. Regions of inter- nal drainage occur in Asia, Africa, Australia, Mexico, and the western United States in Nevada and Utah (discussed in Chapter 15). An example within this region is the Hum- boldt River, which flows westward across Nevada and eventually disappears into the Humboldt Sink as a result of evaporation and seepage losses to groundwater. The area surrounding Utah’s Great Salt Lake, out- let for many streams draining the Wasatch Mountains, also exemplifies internal drain- age, since its only outlet is evaporation. In- ternal drainage is also a characteristic of the Dead Sea region in the Middle East and the region around the Aral Sea and Caspian Sea in Asia (Figure 14.6).

Drainage Basins as Open Systems Drainage basins are open systems. Inputs include pre- cipitation and the minerals and rocks of the regional geology. Energy and materials are redistributed as the stream constantly adjusts to its landscape. System out- puts of water and sediment disperse through the mouth of the stream or river into a lake, another stream or river, or the ocean.

Change that occurs in any portion of a drainage basin can affect the entire system. For example, the building of a dam not only affects the immediate stream envi- ronment around the structure, but can also change the movement of water and sediment for hundreds of miles downstream. Natural processes such as floods can also push river systems to thresholds, where banks collapse or channels change course. Throughout changing condi- tions, a river system constantly strives for equilibrium among the interacting variables of discharge, chan- nel steepness, channel shape, and sediment load, all of which are discussed in the chapter ahead.

(a) Loveland Pass, Colorado, lies along the continental divide between the Pacific and Gulf/Atlantic drainage basins.

(b) A backpacker approaches the continental divide at Cutbank Pass, Glacier National Park, Montana.

▲Figure 14.4 The U.S. Continental Divide, Colorado and Montana. [(a) Erika Nusser/Alamy. (b) Design Pics Inc./Alamy.]

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◀Figure 14.5 Drainage basins and continental divides, North America. Continental divides (red lines) separate the major drainage basins that empty through the United States into the Pacific Ocean, Atlantic Ocean, and Gulf of Mexico, and to the north, through Canada into Hudson Bay and the Arctic Ocean. Subdividing these major drainage basins are major river basins. [After U.S. Geological Survey; The National Atlas of Canada, 1985, “Energy, Mines, and Resources Canada”; and Environment Canada, Currents of Change— Inquiry on Federal Water Policy—Final Report 1986.]

◀Figure 14.6 Utah’s Great Salt Lake, part of an interior drainage system. [Delphotos/ Alamy.]

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414 Geosystems

number and length of channels in a given area reflect the landscape’s regional geology and topography. For exam- ple, landscapes with underlying materials that are easily erodible will have a higher drainage density than land- scapes of more resistant rock.

The drainage pattern is the arrangement of channels in an area. Distinctive patterns can develop based on a combination of factors, including

• regional topography and slope inclination, • variations in rock resistance, • climate and hydrology, and • structural controls imposed by the underlying rocks.

Consequently, the drainage pattern of any land area on Earth is a remarkable visual summary of every geologic and climatic characteristic of that region.

A familiar pattern is dendritic drainage (Figure 14.7a), a treelike pattern (from the Greek word dendron, meaning “tree”) similar to that of many natural systems, such as capillaries in the human circulatory system or the veins in tree leaves. Energy expenditure in the mov- ing of water and sediment through this drainage system is efficient because the total length of the branches is mini- mized. In landscapes with steep slopes, parallel drainage may occur (Figure 14.7b). In some landscapes, drainage patterns alter their characteristics abruptly in response to slope steepness or rock structure (Figure 14.7c).

Other drainage patterns are closely tied to geo- logic structure. Around a volcanic mountain or uplifted dome, a radial drainage pattern results when streams flow off a central large peak. New Zealand’s Mount Rua- pehu, an active volcano on the North Island, shows such a radial drainage pattern (Figure 14.8). In a faulted and

(a) Note the drainage channels flowing off the central peak of Mount Ruapehu, which last erupted in 2007.

(b) Radial drainage pattern.

◀Figure 14.8 Radial drainage on Mount Ruapehu, North Island, New Zealand. This false-color image of the composite vocano shows vegetation as red, the crater lake as light blue, and rocks as brown. [NASA.]

Drainage Patterns A primary feature of any drainage basin is its drainage density, determined by dividing the total length of all stream channels in the basin by the area of the basin. The

(a) Dendritic drainage pattern.

(c) Dendritic and parallel drainage in response to local geology and relief in central Montana.

(b) Parallel drainage pattern.

Drainage divide

▲Figure 14.7 Dendritic and parallel drainage patterns. [Bobbé Christopherson.]

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Chapter 14 River Systems 415

(a) A rectangular stream pattern develops in areas with jointed bedrock.

(b) A trellis stream pattern develops in areas where the geologic structure is a mix of weak and resistant bedrock (such as in folded landscapes).

Ridges of resistant rock

Valleys cut in less-resistant rock

▲Figure 14.9 Drainage patterns controlled by geologic structure: rectangular and trellis.

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Water gap

in the eastern United States and in the folded land- scapes of south-central Utah. Some landscapes display a deranged pattern with no clear geometry and no true stream valley. Examples include the glaciated shield re- gions of Canada, northern Europe, and some parts of the U.S. upper Midwest.

Occasionally, drainage patterns occur that seem to be in conflict with the landscape through which they flow. For example, a stream may initially develop a channel in horizontal strata deposited on top of up- lifted, folded structures. As the stream erodes into the older folded rock layers, it keeps the original course, downcutting into the rock in a pattern contrary to the structure of the older layers. Such a stream is a super- posed stream, in which a preexisting channel pattern has been imposed upon older underlying rock struc- tures (Figure 14.10). For example, Wills Creek, presently cutting a water gap through Haystack Mountain at Cum- berland, Maryland, is a superposed stream. A water gap is a notch or opening cut by a river through a mountain range and is often an indication that the river is older than the landscape.

▲Figure 14.10 The Susquehanna River in Pennsylvania, a superposed stream. The Susquehanna River established its course on relatively uniform rock strata that covered more complex geologic structure below. Over time, as the landscape eroded, the river “superposed” its course onto the older structure by cutting through the resistant strata. [Landsat-7, NASA.]

WoRkitOut 14.1 Stream Drainage Patterns

Choose among dendritic, parallel, radial, rectangular, trellis, and deranged drainage patterns to answer the following questions.

1. Which drainage pattern often occurs in a landscape with a central mountain peak?

2. Which pattern is prominent in the Amazon River basin in Figure 14.2?

3. Which pattern often occurs in landscapes of jointed bedrock?

4. Which pattern occurs in landscapes of folded rock, such as in southern Utah?

5. Which pattern might be found in the Canadian Shield land- scape shown in Figure 12.2?

jointed landscape, a rectangular pattern (Figure 14.9a) directs stream courses in patterns of right-angle turns. In dipping or folded topography, the trellis drainage pattern develops, influenced by folded rock structures that vary in resistance to erosion (Figure 14.9b). Paral- lel structures direct the principal streams, while smaller dendritic tributary streams are at work on nearby slopes, joining the main streams at right angles, as in a plant trellis. Such drainage is seen in the nearly par- allel mountain folds of the Ridge and Valley Province

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