Chapter 14: Streams and Drainage Systems

Introduction:Streams in the Landscape

A stream is a body of water that:– Flows downslope along a clearly defined natural

passageway.– Transports detrital particles and dissolved substances.

The passageway is called the stream’s channel. The load is the sediment and dissolved matter the

stream transports. The quantity of water passing by a point on the

stream bank in a given interval of time is the stream’s discharge.

Human Use of Streams

Large streams are important avenues of transportation.

People choose to live near streams because:– Valley floors are flat and easy to build on. – Soils tend to be deep and fertile.– Water is available.

Stream valleys have drawbacks.– They are threatened by floods.– As cities grow, human and industrial wastes pollute the

water.

Streams as Geological Agents

Streams are vital geologic agents.– Streams carry most of the water that goes from land to

sea (essential part of the hydrologic cycle).– Streams transport billions of tons of sediment to the

oceans each year.

Stream Channels

A stream’s discharge may vary because of changes in precipitation or the melting of winter snow cover.

In response to varying discharge and load, the channel continuously adjusts its shape and orientation.

Two ways to characterize a channel are:– By measuring its cross-sectional shape.– By determining its long profile.

Cross-Sectional Shape

Because the volume of water moving through a channel generally increases downstream, the ratio of channel width to channel depth is likely to change down stream.

Figure 14.2

Long Profile (1)

The stream’s gradient is the difference in elevation over a known distance along its course.

The gradient of a stream decreases downstream. The stream’s long profile is a line drawn along the

surface of a stream from its source to its mouth. – It is a curve that decreases in gradient downstream.

Long Profile (2)

A long profile is not a perfectly smooth curve because irregularities in the gradient can be produced by:– Beds of resistant rock.– Landslides.– Lava flows.– Hydroelectric dams.

Figure 14.3

Dynamics Of Streamflow (1)

The average annual rainfall on the area of the United States is equivalent to a layer of water 76 cm thick. – 45 cm returns to the atmosphere by evaporation and

transpiration.– 1 cm infiltrates the ground,– The remaining 30 cm forms runoff (the portion of

precipitation that flows over the land surface).

Dynamics Of Streamflow (2)

Initially water tends to move down slopes in broad, thin sheets.– This process is called overland flow.

After traveling a short distance overland flow begins to concentrate into well-defined channels called streamflow.

Factors in Streamflow

Several basic factors control the way a stream behaves:– Gradient (expressed in meters per kilometers).– Stream-cross-sectional area (width х average depth,

expressed in square meters [A]).– Average velocity of waterflow (expressed in meters per

second [V]).– Discharge (expressed in cubic meters per second [Q]). – Load (expressed in kilograms per cubic meter).

Dissolved matter generally does not affect stream behavior.

Discharge, Velocity, and Channel Shape

The relationship of discharge, velocity, and channel shape for a stream can be expressed by the equation:

Q = A x V

Discharge Cross-sectional Average

(m3/s) area of stream velocity

(width x average (m/s)

depth) (m2)

Changes Downstream (1)

Traveling down a typical stream from its head to its mouth:– Discharge increases.– Stream cross-sectional area increases.– Velocity increases slightly.– Gradient decreases.

Figure 14.4

Changes Downstream (2)

On steep mountain slopes, discharge is low so the flowing water is shallow.– The stream bed causes much more resistance to the flow

of shallow water. Discharge increases downstream as each tributary (a

stream joining a larger stream) and inflow of groundwater introduce more water.

To accommodate the greater volume of water, velocity increases together with the cross-sectional area of the stream.

Floods (1)

A flood occurs when a stream’s discharge becomes so great that it exceeds the capacity of the channel, therefore causing the stream to overflow its banks.

Geologists view floods as normal and expected events.

As discharge increases the water rises in the channel and erosion scours the bed.

Figure 14.6

Floods (2)

With an increase in velocity, the enlarged channel is now able to accommodate the increased flood discharge and carry a greater load.

As discharge falls, the stream is unable to transport as much sediment.– At the end of the flood it returns to its preflood

dimensions.

Floods (3)

Unusually large discharges associated with floods appear as major peaks on a hydrograph.– A hydrograph is a graph that plots stream discharge

against time. Regardless of the size of the stream basin, as

discharge increases during a flood, so does velocity. This velocity increase has the double effect

ofenabling a stream to carry:– Greater load.– Larger particles.

Figure 14.7

Flood prediction (1)

A flood-frequency curve is produced by plotting the occurrence of past floods of different sizes on a probability graph.

The measure of how often a flood of a given magnitude is likely to occur is called the recurrence interval.

A flood having a recurrence interval of 10 years is called a “10-year flood.”

Flood Prediction (2)

If Earth’s climate changes during the next several decades, present flood-frequency curves may be a little value in predicting future floods.

Figure 14.8

Catastrophic Floods: An Example (1)

Exceptional floods occur very infrequently. During the last glaciation the continental ice sheet

covering western Canada created a huge lake in the vicinity of Missoula, Montana.

The glacier-blocked lake contained between 2000 and 2500 km3 of water when it was filled and remained in existence only as long as the ice dam was stable.

Catastrophic Floods: An Example (2)

When the glacier retreated or its front began to float in the rising lake water, the dam failed and water was released rapidly from the basin, as though a plug had been pulled from a giant bathtub.

The main exit route lay across the channeled Scabland region and down the Columbia River to the sea.

Figure 14.9A

Figure 14.9B

Base Level (1)

As a stream flows downslope, its potential energy decreases and finally falls to zero as it reaches the sea.

The limiting level below which a stream cannot erode the land is called the base level of the stream.

The base level for most streams is global sea level.

Figure 14.10

Base Level (2)

Exceptions are streams that drain into closed interior basins having no outlet to the sea.

Where the floor of a tectonically formed basin lies below sea level (for example, Death Valley, California), the base level coincides with the basin floor.

When a stream flows into a lake, the surface of the lake acts as a local base level.

Natural and Artificial Dams (1)

The courses of many streams are interrupted by lakes that have formed behind natural dams consisting of:– Landslide sediments.– Glacial deposits. – Glacier ice.– Lava flows.

Such a dam acts as a local base level and creates an irregularity in a stream’s long profile.

Natural and Artificial Dams (2)

Large artificial dams also disrupt the normal flow of water in a stream.– They are being constructed in ever-increasing numbers

for:– Water storage. – Flood control.– Hydroelectric power.

Natural and Artificial Dams (3)

Hydroelectric power is recovered from the potential energy of water in streams as they flow downslope to the sea. – Hydroelectric power is a renewable resource.

An artificial dam built across a stream creates a reservoir that traps nearly all the sediment that the stream formerly carried to the ocean.– Reservoir siltation limits the useful life of power dams.

Channel Patterns

Straight channels are rare. – The highest velocity along a straight channel segment

usually is found near the surface in mid-channel.

If a stream channel has many curves the channel pattern is sinuous.

A deposit of sediment (a bar) tends to accumulate where velocity is lower.

In many stream, the channel forms a series of meanders.

Figure 14.11

Figure 14.12

Meanders (1)

Meanders occur most commonly in channels that lie in fine-grained stream sediments and have gentle gradients.

This meandering pattern’s energy is dissipated as uniformly as possible along its course.

Velocity is lowest along the bed and walls of the channel, where there is maximum frictional resistance to flow.

Figure 14.13

Meanders (2)

Over time, meanders migrate slowly down a valley. Along the inner side of each meander loop, where

water is shallow and velocity is low, coarse sediment accumulates to form a point bar.

Wherever the down stream part of a meander that is cutting into sandy sediment encounters less-erodible sediment, such as clay, its migration can be slowed.

Figure 14.14

Meanders (3)

Meanwhile, the next meander upstream, migrating more rapidly, may intersect the slower moving meander. – Meanders are cut off.– The cut off meander is converted into a curved oxbow

lake.

Braided Channels (1)

The geometry of a braided stream resembles the pattern of braided hair as water flows through channels separated by bars or islands.

If a stream is unable to move all the available load, it tends to deposit the coarsest sediment as a bar that locally divides the flow.

A braided pattern tends to form in streams having highly variable discharge and easily erodible banks.

Braided Channels (2)

Streams of meltwater issuing from glaciers generally have a braided pattern because:– The discharge varies both daily and seasonally.– The glacier supplies the stream with large quantities of

sediment.

Figure 14.15

Erosion by Running Water (1)

Erosion by water begins before a distinct stream has formed. It occurs in two ways:– By impact as raindrops hit the ground.– By overland flow during heavy rains, a process known as

sheet erosion.

The effectiveness of raindrops and overland flows in eroding the land is greatly diminished by a protective cover of vegetation.

Erosion by Running Water (2)

The ability of streams to erode is influenced by the way water moves through a stream channel.

If the velocity is very slow the water particles travel in parallel layers, a motion called laminar flow.

With increasing velocity, the movement becomes turbulent flow.

The ability of a stream to pick up particles of sediment from its channel and move them along depends largely on:

– Turbulence.– Velocity of the water.

Figure 14.16

The Stream’s Load

The solid portion of a stream’s load consists of two parts:– The bed load: the coarse particles that move along the

stream bed.– The suspended load: fine particles that are suspended in

the water.

Wherever they are dropped, these solid particles constitute alluvium.

Streams also carry dissolved substances called the dissolved load.– These are chiefly a product of chemical weathering.

Bed Load

The bed load generally constitutes between 5 and 50 percent of the total load of a stream.

Particles move discontinuously by rolling or sliding at a slower velocity than the stream water.

The bed load may move short distances by saltation (series of short intermittent jumps).

Coarse-grained sediment is concentrated where the velocity is high.

Finer-grained sediment is found in zones of progressively lower velocity.

Figure 14.17

Figure 14.18

Placer Deposits (1)

The 1849 discovery of Gold at Sutter’s Mill, California, was a placer deposit.

A placer deposit is a deposit of heavy minerals concentrated mechanically:– Behind rock bars.– In bedrock holes.– Below waterfalls.– Inside of a meander bed.

Placer Deposits (2)

Most placer gold occurs as grains the size of silt particles, the “gold-dust” of miners;

Sometimes, the gold placer is larger and is called a nugget.

Many heavy, durable minerals other than gold also form placers (platinum, copper,tinstone, diamond, ruby, sapphire).

Figure 14.19A

Figure 14.19B

Figure 14.19C

Suspended Load

The muddy character of many streams is due to the presence of fine particles of silt and clay moving in suspension.

Because upward-moving currents within a turbulent stream exceed the velocity at which particles of silt and clay can settle toward the bed under the pull of gravity, such particles tend to remain in suspension longer than they would in non turbulent waters.

They settle and are deposited only where velocity decreases and turbulence ceases, in a lake or in the sea.

Dissolved Load

All stream water contains dissolved chemical substances that constitute part of its load.

The bulk of the dissolved content of most rivers consists of seven ionic species:

– Bicarbonate (HCO31-).

– Calcium (Ca2+).– Sulfate (SO4

2-).– Chloride (CI1-). – Sodium (Na1+).– Magnesium (Mg2+).– Potassium (K1+).– Dissolved silica as Si(OH)4.

Downstream Changes in Particle Size

Sediment normally decreases in coarseness downstream.– In mountainous headwaters, rivers may transport large

boulders. Through times, the coarse bed load is gradually

reduced in size by abrasion. When the stream eventually reaches the sea, its bed

load may consist mainly of sediment no coarser than sand.

Figure 14.21

The Nile: Downstream Changes in Composition (1)

The Nile includes three major tributaries:– White Nile.– Blue Nile.– Atbara.

The White Nile contributes nearly a third of the total discharge, but only 3 percent of the bed load in the Main Nile.

The Nile: Downstream Changes in Composition (2)

The Blue Nile, which drains the highlands of Ethiopia, contributes more than half the discharge and nearly three-quarters of the bed load.

The more northerly Atbara contributes 14 percent of the discharge and a quarter of the bed load.

Figure 14.22

Sediment Yield (1)

Sediment yield is a function of:– Rock type and structure.– Local climate.– Relief and slope.

The greater the precipitation the greater the erosion. In moist regions, plant roots tend to anchor the soil,

thereby curtailing erosion. – In temperate eastern North America and Western Europe,

vegetation cover is more or less continuous and erosion rates are low.

Figure 14.23

Sediment Yield (2)

In drier regions, reduced precipitation limits vegetation, making the land vulnerable to erosion.

Areas receiving abundant precipitation may actually experience less erosion than some relatively dry regions,

Fields measurements suggest that some of the greatest local sediment yields are from desert landscapes.

Some of the highest measured sediment yields are from basins that drain steep mountains along plate boundaries.

Monsoon regions of southeastern Asia receive abundant precipitation that generates high runoff.

Sediment Yield (3)

– In southern Alaska and the southern Andes, large active glaciers contribute to high sediment yields.

– Rocks that are highly jointed or fractured are more susceptible to erosion than massive rocks,.

– The clearing of forests, cultivation of lands, damming of streams, construction of cities, and numerous other human activities also affect erosion rates and sediment yields.

Stream Deposits

When a stream’s turbulence decreases, its transporting power drops and it deposits part of its load.

When a stream rises during a major flood, the water overflows the banks and inundates the adjacent floodplain.

– The boundary between channel and floodplain may be the site of a natural levee (a broad, low ridge of alluvium built along the side of a channel by debris-laden floodwater).

Many stream valleys contain one or more relatively flat alluvial terraces that lie above the floodplain.

– A terrace is a remnant of an abandoned floodplain.

Figure 14.24

Alluvial Fan

When a stream flowing through a steep upland valley emerges suddenly onto a nearly level valley floor or an alluvial plain, it experiences a decrease in slope, a corresponding drop in velocity, and a decrease in its ability to carry sediment.

As a result, the the stream deposits its load in a fan-like shape called an alluvial fan.

Deltas (1)

When a stream enters the standing water of the sea or a lake, its speed drops rapidly, its ability to transport sediment decreases markedly, and it deposits its solid load.

A fan delta typically is built adjacent to a mountain front.

A braid delta is a coarse-grained delta constructed by a braided stream that builds outward into a standing body of water.

Figure 14.28

Deltas (2)

The coarse, thick, steeply sloping part of a depositional layer in a delta is a foreset layer.

As the deposit becomes rapidly thinner and finer, covering the bottom over a wide area, it is called a bottomset layer.

Deltas (3)

Many of the world’s largest streams have built massive deltas at their mouths.– Ganges-Brahmaputra.– Huang He (Yellow River).– Amazon.– Mississippi.

Distributaries are long, finger-like channels that branch from the main channel in a delta.

Drainage Systems (1)

Every stream is surrounded by its drainage basin, the total area that contributes water to the stream

The line that separates adjacent drainage basins is a divide.

The arrangement and dimensions of streams in a drainage basin tend to be orderly.

The smallest segments lack tributaries and are classified as first-order streams.

Figure 14.29

Drainage Systems (2)

Where two first-order streams join, they form a

second-order stream. Third-order streams are formed by the joining of

two second-order streams. Stream capture is the interception and diversion of

one stream by another stream that is extending its basin by erosion in the headward direction.

Figure 14.30

Figure 14.31

Drainage Patterns (1)

Several types of distinctive drainage patterns have

been observed.– Dentritic: branching of channels (‘tree-like’) many

directions. Differences in rock resistance are so slight they are negligible.

– Parallel: parallel or subparallel channels that have formed on sloping surfaces underlain by homogeneous rocks.

– Radial: channels radiate out, like the spokes of a wheel, from a topographically high area, such as a dome or a volcanic cone.

Drainage Patterns (2)

– Rectangular: channel system marked by right-angle bends produced by the presence of joints and fractures in massive rocks.

– Trellised: rectangular arrangement of channels in which principal tributary streams are parallel and very long.

Outcropping edges of folded sedimentary rocks, both weak and resistant, form long, nearly parallel belts.

– Annular: streams follow paths that are segments of circles that ring a dissected dome or basin.

– Centripetal: streams converge toward a central depression, such as a volcanic crater or caldera.

Figure 14.32

Figure 14.33

Figure 14.34

Tampering with the Nile (1)

Ninety eight percent of the Nile’s load is suspended sediment.

Prior to construction of the Aswan Dam, an average of 125 million metric tons of sediment passed downstream each year.– The dam reduced this value to only 2.5 million metric

tons. – Nearly 98 percent of the suspended sediment is now

deposited in the reservoir behind the dam.

Tampering with the Nile (2)

Under natural conditions this sediment was carried downstream by floodwater, where much of it was deposited over the floodplain and delta, thus adding to the rich agricultural soils at a rate of 6 to 15 cm/century.

Because the annual discharge of sediment has now been cut off, the coast has become increasingly vulnerable to erosion.

Figure 14.35