Chapter 13: Running Water

Fig. 13.17a

OBJECTIVES• Identify the role that running water plays in the hydrologic

cycle.

• Compare and contrast the two types of streamflow and explain the factors that affect a stream’s velocity and gradient.

• Describe the work of running water as an agent of erosion and the drainage patterns streams create.

• Identify the three ways a stream transports its sediment load.

OBJECTIVES• Describe the processes and products of stream deposition.

• Discuss the causes and consequences of flooding.

• Demonstrate how streams are closely linked to plate tectonics and how most of the world’s greatest drainage basins owe their origin to the breakup of Pangea.

• Explain that Earth is the only planet in the Solar System with running water on its surface, although there is compelling evidence for the former existence of running water on Mars.

• Running water dominates the continental drainage systems.

• Running water in all its forms are collectively known as streams.

• The way streams flow and work as agents of erosion, transportation, and deposition shape the landscapes around us.

Running Water: An Overview

• Hydrologic Cycle: the motion of water in Earth’s atmosphere and on its surface

• Channels: narrow depressions where surface water runoff is concentrated

• Stream: the flow of running water within these channels

Running Water and the Hydrologic Cycle

Fig. 13.2

Hydrologic Cycle: In the hydrologic cycle, water evaporated from the oceans and precipitated on land is returned to the sea. Numbers of parentheses show the amount of water involved (in cubic kilometers per year).

Fig. 13.1

• Laminar Flow• Parallel streamlines• Smooth flow• No mixing between parts

of stream

• Turbulent Flow• Crisscrossed streamlines• Water is continually

mixed

Streamflow

Fig. 13.3

Streamflow

• Several factors affect a stream’s flow velocity (speed). • Slope, or gradient, of the stream bed• Roughness of the stream bed• Shape of the stream channel

• A stream’s average gradient is the vertical drop between itsheadwaters and the sea dividedby the distance the streamtravels.

Fig. 13.4

• Stream Channels

• Stream Discharge: the total volume of water that passes a given location within a given period of time

Streamflow

Fig. 13.5

• Base Level: the lowest elevation to which a stream can erode its channel

• Determined by the point at which a stream enters an ocean, a lake, or another stream, or meets at waterfall or rapids

Streamflow

Fig. 13.6

(a) The local base level at the top of Niagara Falls is formed by the resistant Niagara Limestone. (b) The average rate of retreat is approximately 1 meter per year (3 feet per year).

Fig 13.7

• Erosion• Removes rock and sediment• Creates stream channels• Can turn stream channels into stream valleys

• Stream Channels• Water carves a clearly defined channel by

• Removal of rock debris• Downcutting by abrasion• Headward erosion

Stream Erosion

Fig. 13.9

• Downcutting by Abrasion• Sand and gravel erode stream channels by abrasion.• Abrasion wears down the bedrock surface of the stream

bed.• Steep-sided canyons form when downcutting is more rapid

than retreat of valley walls through weathering and mass wasting.

• Headward Erosion• Refers to upslope erosion.• Occurs because the head of a stream is the point at which

surface water moving downslope as runoff first becomes focused in a channel.

Stream Erosion

• Stream Valleys• Most common landform on Earth’s surface• Produced by combined effects of

• Downcutting the stream channel by abrasion • Mass wasting of the valley sides

Stream Erosion

V-Shaped Valley: Streams carve valleys into the landscape that are V-shaped in profile. Example: the Grand Canyon of the Yellowstone River in Yellowstone National Park.

Fig. 13.11

• Floodplains• Streams tend to develop sinuous loops (meanders) when

the valley floor gradient is gentle.

• Stream Piracy• Occurs when the stream of one drainage system is diverted

into the headwaters of another.

Stream Erosion

Fig. 13.13

Stream Erosion

Development of a Floodplain: A floodplain develops and the valley floor widens as a result of migration of the stream channel.

Fig. 13.12

• Stream Terraces• Broad, flat benches in steps toward the sides of the valley• Produced when a stream downcuts repeatedly into its own

floodplain

Stream Erosion

Fig. 13.14

Stream Erosion

Fig. 13.15

• Stream Drainage• Streams form branched networks (the drainage system)• Different networks have different drainage patterns

• Drainage Basin: the total land area that contributes water to a stream system

• The main trunk of the stream is fed by • Tributaries: side channels• Distributaries: at its mouth, streams may branch into

smaller channels• Divides: narrow tracts of higher ground separating basins

Stream Erosion

Stream Drainage

Fig 13.18

Course of a stream

Major Drainage Basins of North America: All drainage basins are separated by higher areas of topography (divides). For example, the Continental Divide of the Rocky Mountains separates streams that flow westward into the Pacific Ocean from those that flow eastward into the Atlantic.

Fig. 13.20

• Drainage Patterns• Dendritic• Trellis• Rectangular• Radial

• Topography and bedrock will influence the pattern of drainage

Stream Drainage

Fig 13.21

• The products of erosion are transported downstream in the form of a sediment load by the stream flow.

• A stream’s sediment load has both a solid and a dissolved component.

• Sediment loads are transported in three ways.• Suspended Load: small particle size• Bed Load: large particle size• Dissolved Load: soluble materials

Stream Transport

• Suspended Load• Small particles lifted off the stream bed and held in

suspension by turbulent flow

Stream Transport

Fig. 13.22

• Bed Load• Particles too large or too heavy to be carried in suspension• Movement by traction or saltation along the stream bed

• Saltation: the intermittent downstream skipping motion of grains of sand and gravel small enough to be lifted off the stream bed floor.

• Traction: occurs when boulders and cobbles hug the channel floor and move downstream in an intermittent sliding, dragging, or rolling motion

• Dissolved Load• Portion of load carried in solution• As much as 25% of stream’s total sediment load• Composed of the soluble products of chemical weathering

Stream Transport

• Capacity• The maximum amount of

sediment a stream can carry• Proportional to the amount

of water flowing in stream

• Competence• The maximum size of the

material a given stream can carry

• Determined by the stream’s velocity

Stream Transport

Fig. 13.25

• Deposition occurs whenever the velocity of a stream falls below that needed to keep the sediment load in motion.

Stream Deposition

Stream Velocity and Particle Size:The velocity of the stream and the size of the particle control sediment erosion, transportation, and deposition. To pick up and remove a particle of size A, for example, the stream velocity must be in excess of 20 centimeters per second. The stream can transport this particle as long as the stream velocity is greater than 1 centimeter per second. When the stream flow falls below this velocity, the stream deposits the particle.

Fig. 13.26

• Channel Bars: midstream accumulations of sand and gravel• May become vegetated midstream islands• May be temporary

Stream Deposition

Channel Bar Development: (a) Deposition of a bar in mid-channel diverts the flow of water, widens the stream. (b) After repetitions of this process, the stream develops a braided pattern.

Fig. 13.27

Fig. 13.28

• Floodplain: the area inundated whenever a stream overflows its banks during flooding

• Coarser material builds up to form a natural levee as the flood recedes

Stream Deposition

Fig. 13.32

• As a stream enters the sea, it deposits most of its sediment load at the stream mouth in a triangular-shaped landform known as a delta.

Stream Deposition

• Sediment is distributed across the delta by distributary channels, which deposit the sediment load in a broad alluvial fan.

Figs. 13.33, 13.36b

• Floods occur when there is too much water for the stream channel to carry.

• Artificial levees, channels dug to divert water, and flood-control dams are methods of flood-prevention.

Floods and Flood Prevention

Fig. 13.37

• Plate tectonics influences the location, pattern, size, and longevity of the world’s river systems.

• Drainage basins are often defined by mountain belts generated by plate tectonics.

Running Water and Plate Tectonics

Aulacogens and the Breakup of Pangea: This map highlights the aulacogens (failed rifts) associated with the breakup of Pangea. Today, several major rivers including the Amazon, Mississippi, and Niger, reach the ocean by way of these failed rift valleys. Fig. 13.39

• Earth is unique among the planets in the Solar System in having running water on its surface.

• On Mars, the former existence of running water is recorded in dendritic drainage systems; in terraced, meandering, and braided channels; and in water-worn boulders and cross-bedded conglomerates.

Running Water and Other Worlds

Fig. 13.41

SUMMARY• Running water is the most effective agent of erosion and

deposition.

• A stream’s rate of flow depends on its gradient, on the roughness of the stream bed, and on the shape of its channel.

• Streams erode their channels by removing debris, by abrasion, and by headward erosion.

SUMMARY• Streams deposit eroded material in deltas, bars, banks, and

floodplains.

• Plate tectonics has influenced most of the world’s great drainage basins, many of which owe their origin to the breakup of Pangea.

• Earth is the only planet known to have running water on the surface, but there is evidence for running water on Mars in that planet’s past.