Streams: Streams: Transport to the Transport to the

OceanOcean

Gary D. McMichael/Photo Researecher

The Hydrologic Cycle• Describes water’s movement on the surface• P = RO + ET + I• Driven by solar heat

– ocean water evaporates – wind carries moist air over land

• topography can force moist air to higher altitudes• as air rises, it cools, water condenses to form clouds and

then precipitate as rain or snow (P)

– rain that falls on the land can:• evaporate back into the atmosphere (E)• be taken up by plants, which return water to the

atmosphere– called transpiration (T)

• flow over the ground, enter streams and rivers, and ultimately be returned to the sea; called runoff (RO)

• infiltrates and becomes groundwater (I)

The Hydrologic Cycle

STREAMS

This is the “RO” of

P = RO + I + ET

StreamsStreams

StreamStream : body of water flowing in a channel

The floor of the channel is called the bedbed.

The sides of the are called the banksbanks..

FloodFlood: when bodies of water overflow their banks and water covers the adjacent land called the floodplainfloodplain.

A stream system network.

Drainage basinDrainage basin

Area of land surrounded by

topographic divides in which all the

water is directed to a single point

In Hawaii, divides are steep & basins are small!

Mississippi River Drainage Basin

Drainage Basin of the Colorado River

Some terminology

• VELOCITY (V) - DISTANCE PER UNIT TIME (cm/s, mph)

• DISCHAGRE (Q) Total amount of water that passes a given point in a stream per unit time (m3/s) = width (m) depth (m) average velocity (m/s)

In the U.S., this is expressed as cubic feet per second (cfs): 1 m3/s = 35.9 ft3/s

River at Low Discharge

River at High Discharge

Downstream changes

Longitudinal Stream Profile of the Platt and South Platt Rivers

FloodingFlooding

• Water in the stream is greater than the volume of

the channel.

• Interval between floods depends on the climate of

the region and the size of the channel

• In Hawaii, lots of FLASH FLOODS for mountains

are steep, flood plains are small & there is lots of

CONCRETE!

City Built on a Floodplain

Xie Jiahua/China Features/Sygma

Recurrence intervalRecurrence interval

AAverageverage time time between the occurrences of a given event

The recurrence intervalrecurrence interval of a flood of a given size at a given place depends on:• • climate of the regionclimate of the region• • width of the floodplainwidth of the floodplain• • size of the channelsize of the channel

Annual Flood Frequency Curve

More stream termsMore stream terms

competencecompetence: measure of the largest particles a stream can transport, proportional to v2

capacitycapacity: maximum quantity of sediment carried by stream, proportional to Q and v

Job of StreamsJob of Streams

• Carry away runoffrunoff to lakes and seas

• ErodeErode land (degradation)

• Transport and depositTransport and deposit sedimentary debris

JOB - TO ERODE

• STREAMS CUT, DEEPEN & WIDEN VALLEYS

• BY:

–HYDRAULIC ACTIONHYDRAULIC ACTION - flowing water can pick up load

–ABRASION & IMPACTABRASION & IMPACT - solid load wears down stream bed

–SOLUTIONSOLUTION - dissolves channel & load

Waterfall Retreating Upriver

Donald Nausbaum

Pebbles Caught in

Eddies Form

Potholes

Carr Clifton/Minden Pictures

Stream behaviorStream behavior • Mostly determined by velocityvelocity and

shapeshape of channel.

• These factors combine to allow either laminarlaminar or turbulentturbulent flow.

• Turbulent flow is much more erosive & picks load up.

• Stream velocities may vary from 0.25 to 7 m/s0.25 to 7 m/s.

Laminar flowLaminar flow

• Smooth sheetsheet--like flow at a low velocity

• Usually confined to edges and top of stream

Laminar flow

Turbulent flowTurbulent flow

• Irregular swirlingswirling flow

• Occurs at most rates of stream flow

• Keeps particles in suspensionsuspension

Turbulent flow

Laminar to turbulent transition

ONERA

Laminar flow Turbulent flow

JOB - TO TRANSPORT

• Function of stream V & Q• Load is the amount of material carried

by a stream in all forms:

–Dissolved load (CATIONS)–Suspended load (fine grains)–Bed load (coarse material)

Sediment Transport

Saltation

Grain Size and Flow Velocity

Lower Velocities Form Ripples

ripple

Higher Velocities Form Dunes

ripples

dune dune

Giant ripples in the Channeled Scablands

JOB - TO DEPOSIT

• As V drops, stream begins to drop its load, coarsest & heaviest first

• Forms are streams, deltas, streams, deltas, alluvial fans & flood plainsalluvial fans & flood plains

Two important stream typesTwo important stream types

1. 1. Meandering StreamsMeandering Streams

• Gentle gradients, fine-grained alluviumalluvium

• Minimizes resistance to flow and dissipates energy as uniformly as possible (equilibrium)

• Features: point bars, oxbow lake, point bars, oxbow lake, migrating meandersmigrating meanders

Meandering River Over Time

Meandering River

Point Bar

Peter Kresan

Meandering stream,Phnom Penh, Cambodia

Oxbow Lake

Incised Meanders, Utah

Tom Bean

Two important stream typesTwo important stream types

2.2. Braided StreamsBraided Streams

• Sediment supply greater than amount stream can support.

•At any one moment the active channels may account for only a small proportion of the area of the channel system, but essentially all is used over one season.

•Common in glacial, deserts, and mountain regions.

Braided River

Tom Bean

Brahmaputra River:

a braided river

Courtesy NASA

Typical Large Marine Delta

Alluvial Fans

Michael Collier

Parts of a River SystemParts of a River System

Formation of Natural Levees

BASE LEVELBASE LEVEL

Definition is:

LOWEST LEVEL TO WHICH A STREAM CAN CUT

Think of it as:

Elevation at which a stream enters a large body of water such as a lake or ocean

Effects of Building a DamOriginal Profile Graded to Regional Base Level

Effects of Building a DamDam Forms New Local Base Level

Effects of Building a DamDeposition Upstream

and Erosion Downstream

Formation of River TerracesWhen Base Level Changes

GEOMORPHIC CYCLE

• STAGES OF LANDSCAPE DEVELOPMENT• HILLS & VALLEYS CUT INTO SIDES OF

VOLCANOES• STAGES ARE:

– YOUTH– SUBMATURE– MATURE– OLD AGE

GEOMORPHIC

CYCLE

INITIAL STAGES OF STREAM EROSION IN HAWAII

• Runoff is difficult to establish for rock is very porous & slope is steep.

• Surface has to be sealed by chemical weathering or ash before it happens.

• Happens during Capping Stage• Streams tend to follow lava flow

margins or former lava rivers.• It begins at the coast & moves upwards.

Development of an experimental drainage network - headward erosion

Typical Drainage Networks

Amphitheater-Headed Valleys

• In Hawai‘i, erosion of the volcanoes by stream erosion and mass wasting produces these distinctive features

• Caused by high rainfall, steep radial drainage, and alternating layers strong and weak rocks dipping seaward

Stream Piracy orStream Capture

planezes

Ha‘iku Valley

Giant “SOIL AVALANCH”

Formation of the Nu‘uanu Pali

Graded streamGraded stream

Stream in which neither erosionerosion nor depositiondeposition

is occurring, due to an equilibrium of

slopeslope, velocityvelocity, and dischargedischarge.

THE END