Drainage Density and Drainage · PDF file467 Every stream has a base level, the elevation below which it cannot flow. Sea level is the ultimate base level for virtually all stream

467

Every stream has a base level, the elevation below which it cannot flow. Sea level is the ultimate base level for virtually all stream action. Streams with exterior drainage reach ultimate base level; the low point of flow for a stream with interior drainage is referred to as a regional base level. In some drainage basins, a very resistant rock layer located somewhere upstream from the river mouth can act as temporary base level, temporarily controlling the lowest elevation of the flow upstream from it until the stream is finally able to cut down through it ( ● Fig. 17.7).

Drainage Density and Drainage PatternsEach organized system of stream tributaries exhibits spatial char-acteristics that provide important information about the nature of the drainage basin. The extent of channelization can be repre-sented by measuring drainage density (D

d ), where D

d = L/A

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the total length of all channels (L) divided by the area of the drainage basin (A

d ). Drainage density indicates how dissected the

landscape is by channels, thus it reflects both the tendency of the drainage basin to generate surface runoff and the erodibility of the surface materials ( ● Fig. 17.8). Regions with high drainage densities will have limited infiltration, promote considerable run-off, and have at least moderately erodible surface materials.

Lessresistrock

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● FIGURE 17.7The lowest point to which a stream can flow is its base level. Stream water travels downslope by the force of gravity until it can flow no lower due to factors of topography, climate, or both. Most humid region streams have sufficient flow to make it all the way to an ocean basin. Thus, sea level represents ultimate base level for all of Earth’s streams. In arid climates, many streams lose so much water by evaporation to the atmosphere and infiltration into the channel bed that they cannot flow to the sea. The lowest point they can reach is instead a regional base level, a topographic basin on the continent. A temporary base level is formed when a rock unit lying in the pathway of a stream is significantly more resistant than the rock upstream from it. The stream will not be able to cut into the less resistant rock any faster than it can cut into the resistant rock of the temporary base level.

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● FIGURE 17.8 Drainage density—the length of channels per unit area—varies accord-ing to several environmental factors. For example, everything else being equal, highly erodible and impermeable rocks tend to have higher drain-age density than areas dominated by resistant or permeable rocks. Slope and vegetation cover can also affect drainage density.What kind of drainage density would you expect in an area of steep slopes and sparse vegetation cover?

T H E S T R E A M S Y S T E M

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C H A P T E R 1 7 • F L U V I A L P R O C E S S E S A N D L A N D F O R M S468

In addition to the factors noted previously that reduce infil-tration and promote runoff—impermeable sediments, thin soils, steep slopes, sparse vegetation—the ideal climate for high drain-age densities is one that is semiarid. Humid climates encourage extensive vegetation cover, which promotes infiltration through interception and reduces channel formation by holding soils and surface sediment in place. In arid climates, although the vegetation cover is sparse, there is insufficient precipitation to create enough runoff to carve many channels. Semiarid climates have enough precipitation input to produce overland flow, but not enough to support an extensive vegetative cover. The easily eroded Dakota Badlands, which are located in a steppe climate, have an extremely high drainage density of over 125 kilometers per square kilo-meter (125 km of channel per 1 sq km of land), whereas very resistant granite hills in a humid climate may have a drainage density of only 5 kilometers per square kilometer. Another way to understand the concept of drainage density is to think about what would happen on a hillside in a humid climate if the natural vegetation were burned off in a fire. Erosion would rapidly cut gullies, creating more channels than previously existed there. In other words, the drainage density would increase. We could use the quantitative measure of D

d = L/A

d to determine the change

in channelization precisely and to monitor it over time.

When viewed by looking down at Earth’s surface from the air or on maps, the tributaries of various stream systems may form distinct drainage patterns (also called stream patterns). Two primary factors that influence drainage pattern are bedrock struc-ture and surface topography. A dendritic (from Greek: dendros, tree) stream pattern ( ● Fig. 17.9a) is an irregular branching arrangement with tributaries joining larger streams at acute angles (less than 90°). A dendritic stream pattern is the most common type, in part be-cause water flow in this spatial arrangement is highly efficient. Den-dritic patterns form where the underlying rock structure does not strongly control the position of stream channels. Hence, dendritic patterns tend to develop in areas where the rocks have a roughly equal resistance to weathering and erosion and are not intensely jointed. In contrast, a trellis drainage pattern consists of long, parallel streams linked by short, right-angled segments (Fig. 17.9b). Trel-lis drainage is usually evidence of folding where parallel outcrops of erodible rocks form valleys between more resistant ridges, as in the Ridge and Valley region of the Appalachians. A radial pattern develops where streams flow away from a common high point on cone- or dome-shaped geologic structures, such as volcanoes and domal uplifts (Fig. 17.9c). The opposite pattern is centripetal, with the streams converging on a central area as in an arid region basin of interior drainage (Fig. 17.9d). Rectangular patterns occur where

● FIGURE 17.9Drainage patterns often reflect bedrock structure. (a) The dendritic pattern is found where rocks have uni-form resistance to weathering and erosion. (b) A trellis pattern indicates parallel valleys of weak rock between ridges of resistant rock. (c) Multiple channels trending away from the top of a domed upland or volcano form the radial pattern. (d) The centripetal pattern shows multiple channels flowing inward toward the center of a structural lowland or basin. (e) Rectangular patterns indicate linear joint patterns in the bedrock structure. (f) A deranged pattern typically results following the retreat of continental ice sheets; it is characterized by a chaotic arrangement of channels connecting small lakes and marshes.

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(a) (b) (c)

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streams follow sets of intersecting fractures to produce a blocky network of straight channels with right-angle bends (Fig. 17.9e). In some regions that were recently covered by extensive glacier ice, streams flow on low-gradient terrain left by the receding glaciers, wandering between marshes and small lakes in a chaotic pattern called deranged drainage (Fig. 17.9f ).

Many streams follow the “grain” of the topography or the bedrock structure, eroding valleys in weaker rocks and flowing away from divides formed on resistant rocks. Many examples also exist of streams that flow across, that is, transverse to, the structure, cutting a gorge or canyon through mountains or ridges. These transverse streams can be puzzling, giving rise to questions as to how a stream can cut a gorge through a mountain range or how a stream can move from one side of a mountain range to the other. Such streams are probably either antecedent or superimposed.

Antecedent streams existed before the formation of the mountains that they flow across, maintaining their courses by cutting an ever-deepening canyon as gradual mountain building took place across their paths. The Columbia River Gorge through the Cascade Range in Washington and Oregon and many of the great canyons in the Rocky Mountain region, such as Royal Gorge and the Black Canyon of the Gunnison, both in Colo-rado, probably originated as antecedent streams ( ● Fig. 17.10a). Other rivers, including some in the central Appalachians, have

cut gaps through mountains in a very different manner. These streams originated on earlier rock strata, since stripped away by erosion, so that the streams have been superimposed onto the rocks beneath (Fig. 17.10b). This sequence would explain why, in many instances, the rivers flow across folded rock structure, creat-ing water gaps through mountain ridges. Examples include the Cumberland Gap and the gap formed by the Susquehanna River in Pennsylvania, both of which were important travel routes for the first European settlers crossing the Appalachians.

Stream DischargeThe amount of water flowing in a stream depends not only on the impact of recent weather patterns but also on such drainage basin factors as its size, relief, climate, vegetation, rock types, and land-use history. Stream flow varies considerably from time to time and place to place. Most streams experience occasional brief periods when the amount of flow exceeds the ability of the chan-nel to contain it, resulting in the flooding of channel-adjacent land areas.

Just as it was important to develop the technique of stream ordering to indicate quantitatively a channel’s place in the hier-archy of tributaries, it is also crucial to be able to describe quan-

● FIGURE 17.10Transverse streams form valleys and canyons across mountains. (a) An antecedent stream maintains its course by cutting a valley through a mountain as it is uplifted gradually over much time. (b) A superimposed stream has uncovered and excavated ancient structures that were buried beneath the surface. As the stream erodes the landscape downward, it cuts across and through the ancient structures.

Water gap

(a)

(b)

S T R E A M D I S C H A R G E



suspension, with more than 1 million tons of suspended load per year. Compared to the “muddy” Mississippi River, the Huang He transports five times the suspended sediment load with only one fifth the discharge. Streams dominated by bed load tend to occur in arid regions because of the limited weathering rate in arid climates. Limited weathering leaves considerable coarse-grained sediment in the landscape available for transportation by the stream system.

Stream DepositionBecause the capacity and competence of a stream to carry material depend on flow veloc-ity, a decrease in velocity will cause a stream to reduce its load through deposition. Velocity decreases over time when flow subsides—for example, after the impact of a storm—but it also varies from place to place along the stream. Shallow parts of a channel that in cross section lie far from the deepest and fastest flow typi-cally experience low flow velocity and become sites of recurring deposition. The resulting ac-cumulation of sediment, like what forms on the inside of a channel bend, is referred to as a bar. Sediment also collects in locations where velocity falls due to a reduction in stream gra-dient, where the river current meets the stand-ing body of water at its mouth, and on the land adjacent to the stream channel during floods.

Alluvium is the general name given to fluvial deposits, regardless of the type or size of material. Alluvium is recognized by the charac-teristic sorting and/or rounding of sediments that streams perform. A stream sorts particles by size, transporting the sizes that it can and depositing larger ones. As velocity fluctuates due to changes in discharge, channel gradient, and roughness, particle sizes that can be picked up, transported, and deposited vary accordingly ( ● Fig. 17.18). The alluvium deposited by a stream with fluctuating velocity will exhibit al-ternating layers of coarser and finer sediment.

When streams leave the confines of their channels during floods, the channel cross-sectional width is suddenly enlarged so much that the ve-locity of flow must slow down to counterbalance it (Q = wdv). The resulting decrease in stream competence and capacity cause deposition of sed-iment on the flooded land adjacent to the chan-nel. This sedimentation is greatest right next to the channel where aggradation constructs channel-bounding ridges known as natural levees, but some alluvium will be left behind wherever load settled out of the receding flood waters.

Floodplains constitute the often extensive, low-gradient land areas composed of alluvium

that lie adjacent to many stream channels ( ● Fig. 17.19). Flood-plains are aptly named because they are inundated during floods and because they are at least partially composed of vertical accretion deposits, the sediment that settles out of slowing and standing flood-water. Most floodplains also contain lateral accretion deposits. These are generally channel bar deposits that get left behind as a channel gradually shifts its position in a sideways fashion (laterally) across the floodplain ( ● Fig. 17.20).

● FIGURE 17.16Transport of solid load in a stream. Clay and silt particles are carried in suspension. Sand typically travels by suspension and saltation. The largest (heaviest) particles move by traction.What is the difference between traction and saltation?

● FIGURE 17.17Some rivers carry a tremendous load of sediment in suspension and show it in their muddy appearance, as in this aerial view of the Mississippi River in Louisiana. Much of the load carried by the Mississippi River is brought to it by its major tributaries.What are some of the major tributaries that enter the Mississippi River?

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Sand

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Size (mm)

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Fall velocity

SEDIMENTATION

TRANSPORTATION

Clayparticle(0.003 mm)

Sandgrain(0.1 mm)

Pebble(10 mm)

EROSION

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● FIGURE 17.18This graph shows the relationship between stream flow velocity and the ability to erode or transport material of varying sizes (inability to erode or transport particles of a particular size or larger will result in deposition). Note that small pebbles (particles with a diameter of 10 mm, for example) need a high stream flow velocity to be moved because of their size and weight. The fine silts and clays (smaller than 0.05 mm) also need high veloci-ties for erosion because they stick together cohesively. Sand-sized particles (between 0.05 and 2.0 mm) are relatively easily eroded and transported, compared to clays or gravel (particles larger than 2.0 mm).

● FIGURE 17.19During floods, low areas adjacent to the river are inundated with sediment-laden water that flows over the banks to deposit alluvium, mainly silts and clays, on the floodplain. This is the Missouri River floodplain at Jefferson City, Missouri, during the 1993 Midwest flood.What would the river floodwaters leave behind in flooded homes after the water recedes?

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● FIGURE 17.20Sediment deposited in a bar on the inside of a channel bend becomes part of the floodplain alluvium if the stream migrates away leaving the bar deposits behind, as occurred with these lateral accretion deposits. In this photo, remnants of winter ice fill swales between ridges that mark successive crests of the laterally accreted bar deposits.

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Channel PatternsThree principal types of stream channel have traditionally been recognized when considering the form of a given channel seg-ment in map view. Although straight channels may exist for short distances under natural circumstances, especially along fault zones, joints, or steep gradients, most channels with parallel, lin-ear banks are artificial features that were totally or partially con-structed by people.

If a stream has a high proportion of bed load in relation to its discharge, it deposits much of its load as sand and gravel bars in the streambed. These obstructions break the stream into strands that interweave, separate, and rejoin, giving a braided appear-ance to the channel, and indeed such a pattern is called braided ( ● Fig. 17.21). This channel pattern may develop wherever the coarse-sediment input into a stream is extremely high owing to banks of loose sand and gravel or proximity to a nearly unlimited supply of coarse bed load, such as that found downstream from glaciers and also in many desert areas. Braided streams are com-mon on the Great Plains (for example, the Platte River), in the desert Southwest, in Alaska, and in Canada’s Yukon.

The most common channel pattern in humid climates displays broad, sweeping bends in map view. Over time, these sinuous, meandering channels also wander from side to side across their low-gradient floodplains widening the valley by lat-eral erosion on the outside of meander bends and leaving be-hind lateral accretion (bar) deposits on the inside of meander bends ( ● Fig. 17.22). These streams and their floodplains have a higher proportion of fine-grained sediment and thus greater bank cohesion than the typical braided stream.

Land Sculpture by StreamsOne way to understand the variety of landform features result-ing from fluvial processes is to examine the course of an idealized river as it flows from headwatersw in the mountains to its mouth at the ocean. The gradient of this river would diminish continually

● FIGURE 17.21The braided stream channel of the Brahmaputra River in Tibet, viewed from the International Space Station. Stream braiding results from an abundant bed load of coarse sediment that obstructs flow and separates the main stream into numerous strands. Braided channels typically occur when a stream has low discharge compared to the amount of bed load.What does the common occurrence of braided channels just downstream from glaciers tell you about the sediment transported and deposited by moving ice?

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r ● FIGURE 17.22Over time, a meandering (sinuous) stream channel, like the Missouri River shown on this colorized radar image, may swing back and forth across its valley. Where the outside of a meander bend on the floodplain (purple and blue tones) impinges on the edge of higher terrain (orange areas), stream erosion can undercut the valley side wall and, with the assistance of mass wasting, contribute to floodplain widening.

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downstream as it flows from its source toward base level. In nature, exceptions exist to this idealized profile because some streams flow entirely over a low gradient ( ● Fig. 17.23), while other streams, particularly small ones on mountainous coasts, flow down a steep gradient all the way to the standing body of water at their mouth. Rather than having a smoothly decreasing slope from headwaters to mouth, we would expect real streams to have some irregularities in this longitudinal profile, the stream gradient from source to mouth (see again Fig. 17.12).

The following discussion subdivides the ideal stream course into upper, middle, and lower river sections flowing over steep, moderate, and low gradients, respectively. Fluvial erosion processes dominate the steep upper course, whereas deposition predomi-nates in the lower course. The middle course displays important elements of both fluvial erosion and deposition.

Features of the Upper CourseAt the headwaters in the upper course of a river, the stream pri-marily flows in contact with bedrock. Over the steep gradient high above its base level, the stream works to erode vertically downward by hydraulic action and abrasion. Erosion in the upper course cre-ates a steep-sided valley, gorge, or ravine as the stream channel in

the bottom of the valley cuts deeply into the land. Little if any floodplain is present, and the valley walls typically slope directly to the edge of the stream channel. Steep valley sides encourage mass movement of rock material directly into the flowing stream. Valleys of this type, dominated by the downcutting activity of the stream, are often called V-shaped valleys because with their steep slopes they attain the form of the letter V ( ● Fig. 17.24).

The effects of differential erosion can be significant in the upper course where rivers cut through rock layers of varying resistance. Rivers flowing over resistant rock have a steeper gra-dient than where they encounter weaker rock. A steep gradient gives the stream flow more energy, which the stream needs to erode the resistant rock. Rapids and waterfalls may mark the location of resistant materials in a stream’s upper course. Where rocks are particularly resistant to weathering and erosion, val-leys will be narrow, steep-sided gorges or canyons; where rocks are less resistant, valleys tend to be more spacious.

Many streams spill from lake to lake in their upper courses, either over open land (like the Niagara River at Niagara Falls, between Lake Erie and Lake Ontario) or through gorges. In ei-ther case, the lakes will eventually be eliminated if stream erosion lowers their outlets enough or if fluvial sediment deposited at the inflow points fills the lakes.

● FIGURE 17.23Not all streams fit the generalized pattern of characteris-tics for upper, middle, and lower stream segments. The Mississippi River, here a relatively small stream, meanders on a low gradient near its headwaters in Minnesota, far upstream from its mouth.

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● FIGURE 17.24Where the upper course of a stream lies in a mountainous region, its valley typically has a characteristic “V” shape near the headwa-ters. Such a stream flows in a steep-walled valley, with rapids and waterfalls, as shown here in Yellowstone Canyon, Wyoming.How does the gradient of the Yellowstone River compare with that of the stretch of the Mississippi River shown in Figure 17.23?

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L A N D S C U L P T U R E B Y S T R E A M S



Features of the Middle CourseIn the middle section of the ideal longitudinal profile, the stream flows over a moderate gradient and on a moderately smooth channel bed. Here the river valley includes a floodplain, but remain-ing ridges beyond the floodplain still form defi-nite valley walls. The stream lies closer to its base level, flows over a gentler gradient, and thus directs less energy toward vertical erosion than in its up-per course. The stream still has considerable energy, however, due to the downstream increase in flow volume and reduction in bed friction. The river now uses much of its available energy for transport-ing the considerable load that it has accumulated and toward lateral erosion of the channel sides. The stream displays a definite meandering chan-nel pattern with its sinuous bends that wander over time across the valley floor. The stream erodes a cut bank on the outside of meander loops, where the channel is deep and centrifugal force accelerates stream velocity. The cut bank is a steep slope, and slumping may occur there particularly when there is a rapid fall in water level. Slumping on the out-side of meander bends contributes to the effect of lateral erosion by the stream and adds load to the stream. In the low velocity and shallow flow on the inside of the meander bends, the stream deposits a point bar ( ● Fig. 17.25). Erosion on the out-side and deposition on the inside of river meander bends result in the sideways displacement, or lateral migration, of meanders. This helps increase the area of the gently sloping floodplain when cut

banks impact the confining valley walls. Tributaries flowing into a larger stream also aid in widening the valley through which the trunk stream flows. Although flooding of the valley floor is always a potential hazard, the richness of floodplain soils offers an irresist-ible lure for farmers.

Features of the Lower CourseThe minimal gradient and close proximity to base level along the ideal lower river course make downcutting virtually impossible. Stream energy, now derived almost exclusively from the higher discharge rather than the downslope pull of gravity, leads to con-siderable lateral shifting of the river channel. The river mean-ders around helping to create a large depositional plain (see Map Interpretation: Fluvial Landforms). The lower floodplain of a major river is much wider than the width of its meander belt and shows evidence of many changes in course ( ● Fig. 17.26). The stream migrates laterally through its own previously deposited sediment in a channel composed exclusively of alluvium. During floods, these extensive floodplains, or alluvial plains, become inundated with sediment-laden water that contributes vertical accretion deposits to the large natural levees and to the already thick alluvial valley fill of the floodplain in general. Natural levees along the Mississippi River rise up to 5 meters (16 ft) above the rest of the floodplain.

(a)

(b)

Erosion ofcut bank

Point bardeposition

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● FIGURE 17.25Characteristics of a meandering river channel. Note that water flowing in a channel has a tendency to flow downstream in a helical, or “cork-screw,” fashion, which moves water against one side of the channel and then to the opposite side. The up-and-down motion of the water contrib-utes to the processes of erosion, transportation and deposition.


479

A common landform in this deposition-dominated environ-ment provides evidence of the meandering of a river over time. Especially during floods, meander cut-offs occur when a stream seeks a shorter, steeper, and straighter path; breaches through the levees; and leaves a former meander bend isolated from the new channel position. If the cut-off meander remains filled with water, which is common, it forms an oxbow lake ( ● Fig. 17.27).

Sometimes people attempt to control streams by building up levees artificially in order to keep the river in its channel. Dur-ing times of reduced discharge, however, when a river has less energy, deposition occurs in the channel. Thus, in an artificially constrained channel, a river may raise the level of its channel bed. In some instances, as in China’s Huang He and the Yuba River in northern California, deposition has raised the streambed above the surrounding floodplains. Flooding presents a very serious danger in this situation with much of the floodplain lying below the level of the river. Unfortunately, when floodwaters eventually overtop or breach the levees, they can be even more extensive and destructive than they would have been in the natural case.

The presence of levees—both natural and artificial—can pre-vent tributaries in the lower course from joining the main stream. Smaller streams are forced to flow parallel to the main river until a convenient junction is found. These parallel tributaries are called yazoo streams, named after the Yazoo River, which parallels the Mississippi River for more than 160 kilometers (100 mi) until it finally joins the larger river near Vicksburg, Mississippi.

DeltasWhere a stream flows into a standing body of water, such as a lake or the ocean, the flow is no longer confined in a channel. The cur-rent expands in width, causing a reduction in flow velocity and thus a decrease in load competence and capacity. If the stream is

carrying much load, the sediment will begin to settle out, with larger particles deposited first, closer to the river mouth, and smaller par-ticles deposited farther out in the water body. With continued ag-gradation, a distinctive landform, called a delta because the map view shapes of some resemble the Greek letter delta (D), may be constructed ( ● Fig. 17.28a).

Deltas form at the inter-face between fluvial systems and coastal environments of lakes or the ocean and therefore origi-nate in part from fluvial and in part from coastal processes. Del-tas have a subaqueous (under-water) coastal component, called the prodelta, and a fluvial part, the delta plain, that exists at, to slightly above, the lake level or

● FIGURE 17.26This colorized radar image shows part of the Mississippi River floodplain along the Arkansas–Louisiana–Mississippi state lines. Images like this help us assess flood potential and learn much about the geomorphic history of the river and its floodplain. The colors are used to enhance landscape features such as water bodies (dark), field patterns, and for-ested areas (green). Note how the river has changed its channel position many times, leaving oxbow lakes and meander scars on the floodplain.

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FLOODPLAIN

● FIGURE 17.27Features of a large floodplain common in the lower courses of major rivers. Low marshy or swampy parts of the floodplain, generally at the water table, are called backswamps.What is the origin of an oxbow lake?

D E LTA S



sea level. Deltas can form only at those river mouths where the fluvial sediment supply is high, where the underwater topography does not drop too sharply, and where waves, currents, and tides cannot transport away all the sediments delivered by the river. Al-though these circumstances exist at the mouths of many rivers, not all rivers have deltas.

Delta construction is a slow, ongoing process. A river chan-nel that approaches its base level at a large standing body of water typically has a very low gradient. Lacking the ability to incise its channel below base level, the stream may divide into two channels, and may do so multiple times, to convey its water and load to the lake or ocean. These multiple channels flowing out from the main stream are called distributaries, are typical features of the delta plain, and help direct flow and sediment toward the lake or ocean. Natural levees accumulate along the banks of these distributary channels. Continued deposition and delta formation extend the delta plain and create new land far out from the original shoreline. Rich alluvial deposits and the abundance of moisture allow veg-etation to quickly become established on these fertile deposits and further secure the delta’s position. Delta plains, such as those of the Mekong, Indus, and Ganges Rivers, form important agricultural areas that feed the dense populations of many parts of Asia.

Where it flows into the Gulf of Mexico, the Mississippi River has constructed a type of delta called a bird’s-foot delta (Fig. 17.28b). Bird’s-foot deltas form in settings where the influence of the flu-vial system far exceeds the ability of waves, currents, and tides of the standing water body to rework the deltaic sediment into coastal landforms or to transport it away. Natural levee crests along

numerous distributaries remain intact slightly above sea level and extend far out into the receiving water body. Occasional changes in the distributary channel system occur when a major new dis-tributary is cut that siphons flow away from a previous one, caus-ing the center of deposition to switch to a new location far from its previous center. The appearance in map view of the natural le-vees extending toward the present and former depositional centers leaves the delta resembling a bird’s foot.

Different types of deltas are found in other kinds of settings. An arcuate delta, like that of the Nile River, projects to a limited extent into the receiving water body, but the smoother, more reg-ular seaward edge of this kind of delta shows greater reworking of the fluvial deposits by waves and currents than in the case of the bird’s-foot delta. Cuspate deltas, like the São Francisco in Brazil, form where strong coastal processes push the sediments back to-ward the mainland and rework them into beach ridges on either side of the river mouth.

Base-Level Changes and Tectonism

A change in elevation along a stream’s longitudinal profile will cause an increase or decrease in the stream’s gradient, which in turn impacts the amount of geomorphic work the stream is capable of accomplishing. Elevation changes can occur within the drainage basin due to tectonic uplift or depression. Changes of base level for

● FIGURE 17.28Satellite views of two different types of deltas. (a) The Nile River delta at the edge of the Mediterranean Sea is an arcuate delta, displaying the classic triangular arc shape. Waves and currents smooth out irregularities along the seaward edge of the delta. (b) The unusual shape of the Mississippi River delta resembles a bird’s foot. Waves, currents, and tides in the Gulf of Mexico do little to change the visible shape of this type of delta.Why are the shapes of some deltas controlled more by fluvial processes whereas the shapes of others are strongly influenced by coastal processes?

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basins of exterior drainage result principally from climate change. Sea level drops in response to large-scale growth of glaciers and rises with substantial glacier shrinking. Tectonic uplift or a drop in base level give the stream a steeper gradient and increased energy for erosion and transportation. The landscape and its stream are then said to be rejuvenated because the stream uses its renewed energy to incise its channel to the new base level. Waterfalls and rapids may develop as rejuvenated channels are deepened by ero-sion. Tectonic depression of the drainage basin or a rise in sea level reduces the stream’s gradient and energy, enhancing deposition.

If new uplift occurs gradually in an area where stream me-anders have formed, these meanders may become entrenched as the stream deepens its valley ( ● Fig. 17.29). Now, instead of erod-ing the land laterally, with meanders migrating across an alluvial plain, the rejuvenated stream’s primary activity is vertical incision.

It is important to note that virtually all rivers reaching the sea incised their valleys during the Pleistocene in response to the lowering of sea level associated with continental glaciation. The accumulation of water on the land in the form of glacial ice caused sea level to drop as much as 120 meters (400 ft) and low-ered the base level for streams of exterior drainage. Consequently, near their mouths, the streams eroded deep valleys for their chan-nels. Subsequent melting of the glaciers again elevated base level. This base-level rise caused the streams to deposit their sediment loads, filling valleys with sediment, as the streams adjusted their channels to a second new base level. These consecutive changes in base level produced broad, flat, alluvial floodplains above buried valleys cut far below today’s sea level.

While a drop in base level causes downcutting and a rise causes deposition, an upset of stream equilibrium resulting from sizable increases or decreases in discharge or load can have similar effects on the landscape. Research has shown that variations in base level, tectonic movements, and changes in stream equilib-rium can each cause downcutting in stream valleys, so the valley is slightly deepened and remnants of the older, higher valley floor are preserved in “stair-stepped” banks along the walls of the valley. These remnants of previous valley floors are stream terraces. Multiple terraces are a consequence of successive periods of downcutting and deposition ( ● Fig. 17.30). Stream terraces pro-vide a great deal of evidence about the geomorphic history of the river and its surrounding region.

Stream HazardsAlthough there are many benefits to living near streams, settle-ment along a river has its risks, particularly in the form of floods. Variability of stream flow constitutes the greatest problem for life along rivers and is also an impediment to their use. Stream channels usually contain the maximum flows that are estimated to occur once every year or two. The maximum flows that are probable over longer periods of 5, 10, 100, or 1000 years overflow the channel and inundate the surrounding land, sometimes with disastrous re-sults ( ● Fig. 17.31). Similarly, exceptionally low flows may produce crises in water supply and bring river transportation to a halt.

The U.S. Geological Survey maintains more than 6000 gag-ing stations for the measurement of stream discharge in the United States ( ● Fig. 17.32). Many of these gaging stations operate on solar energy, measure stream discharge in an automated fashion,

● FIGURE 17.29A spectacular example of an entrenched meander from the San Juan River in the Colorado Plateau region of southern Utah.

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● FIGURE 17.30(a) Diagram of a stream valley displaying two sets of alluvial stream terraces (labeled 1 and 2) and the present floodplain (labeled 3). The stream terraces are remnants of previous positions of the valley floor, with the higher (1) being the older. Stream incision into and abandon-ment of each of the two former floodplains (1 and 2) may have oc-curred by the stream experiencing rejuvenation due to a lowering of its base level or uplift in the section pictured or upstream from it. Major changes in the equilibrium of streams (from erosion-dominated to deposition-dominated, or vice versa) can also form terraces. (b) River terraces in the Tien Shan Mountains of China.How many terraces can you identify in this photo?

(b)


Drainage Density and Drainage · PDF file467 Every stream has a base level, the elevation below which it cannot flow. Sea level is the ultimate base level for virtually all stream

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