Physical Geology Chapter 10 Rivers

The hydrologic cycle

Driven by solar energy Figure 10.1

Where does the rainwater go?

Evapotranspiration – evaporation plus

transpiration from plants

accounts for most of the precipitation

during the summer

Surface runoff – rainwater flows across

the land surface and into streams

Percolation – rainwater soaks into the

soil and recharges the ground water

Continental environments

fluvial systems: alluvial fans rivers

desert (eolian) lacustrine (lakes)

glacial

Fluvial systems

include:

streams

rivers – braided and meandering

flood plains

alluvial fans

deltas – are considered separately

The Zen of meandering rivers

: You can’t push a rope

Longitudinal profile of a river

Water moving downhill by gravity Figure 10.2

River flow cross sections Figure 10.6

difference flow velocity across sections of the river

also, difference in flow velocity vertically

cut bank on the outside of a meander (bend in the river)

point bar on the inside of the meander

*** Important: friction with the

banks and the bed of the river slows the water; kinetic energy of the water is expended to move sediment particles, which slows down the water ***

Velocity across a stream

For straight sections, fastest water flow in the center

Vertical velocity profile

Fastest flow just below the water surface

Channel shape & roughness Figure 10.8

A smooth, semi-circular channel has the least friction and fastest water flow

A wide, shallow channel has lots of friction with the bottom

A rocky bottom creates lots of turbulence, which slows down the water

Stream drainage patterns Figure 10.5

Influence of local to regional geology

Dendritic Radial Rectangular Trellis

Dendritic drainage

“Tree-like”

fairly uniform rock or soil in drainage basin

Radial drainage

Usually associated with a conical feature such as a volcano

Rectangular drainage

Shows the regional joint or fracture pattern of the bedrock

Trellis drainage

Forms between ridges, possibly of mountains,

or beach ridges

Mississippi River drainage

Generally dendritic but some local geologic controls

Ephemeral streams

Typically in arid or semi-arid regions

Relation of water velocity to grain size Figure 10.7

erosion

transport

deposition

Speeding up stream flow at a constriction

The venturi effect Figure 10.9

Man-made obstacles, such as bridges

Bridge scour during high flow

Note the difference in water height upstream and downstream of bridge

Hydraulic force Figure 10.11

Water flowing across exposed rock may loosen, roll, or lift clasts

Modes of transport Figure 10.14

{ *** understand these processes *** }

Bedload versus suspended load

also, dissolved load

Maximum discharge

Figure 10.10

Note polish and rounding of boulders

Find evidence of the maximum height of water level

High-gradient streams Figure 10.15

Gravel moved during flood, deposited as flood waned (note sand on top)

Gravel bars and floods Figure 10.16

Original channel

Erosion during flood

Deposition of new bars at end of flood

Placer deposits

(would rhyme with “glasser”)

Concentrations of dense minerals, such as gold, found in point bars and winnowed deposits

Fluvial systems – meandering rivers

Rio Solimoes, Brazil

synthetic aperture radar (can see through canopy of trees)

shows lots and lots of meander scrolls

General cross section of a meandering river valley

Figure 10.26

Characteristics of meandering rivers

• low gradient

•

very sinuous (lots of meanders)

•

large suspended load (relatively less bedload)

•

mostly fine-grained sediments

•

fairly constant discharge throughout the year

Point bars

lateral accretion of point bars

along inside of meander

Section through a point bar Figure 10.26

Where is the fastest water velocity?

Scroll plain Rio Apure, Orinoco Basin

“scrolls” left by the migrating point bars

Active erosion of a cut bank Figure 10.21

rapid erosion under a house between January and March

Examples of river features: Fountain Creek, New Mexico

cut bank point bar

flood channel across

inside of point bar

River levees

Levees are created naturally by floods

Commonly enhanced by engineers to add height & protection

Forming an oxbow lake Figures 10.23 & 10.24

A meander loop bends back on itself through time

Final cut through usually occurs during a flood

The old meander is abandoned, and is left isolated as a lake in the flood plain

Fluvial systems

Two primary types:

braided rivers and meandering rivers

but these intergrade, and a single river will change character downstream

Longitudinal bars

Coarse sediments deposited during high flow become a barrier at low flow

Braided river, with multiple channels and bars Figure 10.18

more sediment than water to carry it

Characteristics of braided-river systems

• moderately steep grade

• fairly straight

• many channels, bars, and islands

• coarse-grained sediments

Factors:

• overloaded with coarse sediment

• sporadic, high-discharge events

• non-cohesive banks

(channels migrate laterally instead of incising)

Typical settings

Mountainous reaches of rivers – Spring high discharge

Glacial outwash plains – Copper River, Alaska

Outer edge of alluvial fans

Types of deltas

Deltas form as the river empties into a large body of water – a lake, bay, ocean

The water flow slows dramatically, and the sediment is deposited at the river mouth

River-dominated delta Figure 10.30

South Pass, Mississippi River Delta

Wave-dominated delta

Nile River Delta, Egypt

Tide-dominated delta

Ganges-Brahmaputra River Delta, Bangladesh

Deposits associated with a delta Figure 10.29

Sand locally near the channel, but most of the volume of the delta is mud (silt)