454 lecture 5

DRAINAGE BASINS

A drainage basin or watershed is defined from a downstream

point, working upstream, to include all of the hillslope &

channel areas which drain to that point

Each basin is surrounded & defined by a drainage divide

(high point from which water flows away)

Channel initiation

Type of downslope movement of precipitation depends on

1) infiltration capacity of surface, which is controlled by

soil texture & structure antecedent moisture

vegetation other surface conditions

2) intensity and duration of precipitation

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water moving downslope as runoff moves rapidly & has erosive

ability:

precipitation

soil

profile

zone of

percolation

water tableground water

Hortonian overland flowsaturation overland flow

throughflow

groundwater flow

runoff

channels begin to develop when the erosive force, F

(function of slope angle, water depth) of the overland flow

exceeds the resistance, R (function of vegetation, nature of

surface) of the surface being eroded

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Stages of overland flow

• rainbeat – impact cratering

• thread flow – integration of raindrops, flow around surface grains

• sheet flow – integration of thread flow, water level rises above

roughness elements

• rill flow – sheet flow concentrates into small, parallel streamlets

of water, which grow by micropiracy as master rills

concentrate water & entrench to small channels

• small channels – tributaries, grow by headward erosion or

sapping

Sapping involves throughflow; subsurface water moves along

percolines – zones of greater soil depth & moisture content,

or in pipes – horizons or surfaces of limited permeability with

enlarged pores

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sheetflow erosion around plant roots

rill erosion, Peru

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Sapping occurs in the saturated zone, where percolines meet

the surface as seeps or springs, undermining overlying material

& causing the collapse of valley head walls

Sapping networks are controlled by the direction of groundwater

rather than surface flow (eg. joints), & produce ampitheater-

headed valleys like those on the Colorado Plateau, Hawaii, and

Mars (??)

Piping also affects network development – occurs in the

unsaturated zone, where differences in permeability create

hydraulic head & differential erosion

Network evolution

• direct observation on recently exposed sites

• flume studies

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Cienega Creek,

Arizona

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Pawnee Buttes,Colorado

Mississippi

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Owl Canyon, Colorado

Alton, Utah

454 lecture 5Box canyon, Dinosaur Ntnl Monument, CO

Dead Horse Point State Park, Utah

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The basic steps in network growth are

• initiation of channels

• elongation through headward growth

• elaboration/branching as tributaries are added

the order & relative importance of these steps depends on

such external factors as slope

Ergodic hypothesis: space substitutes for time

Random-walk model: recognizes element of randomness (each

choice has equal probability of occurrence) in the

development of networks

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age of till surface (103 yrs)

time (hours)

dra

ina

ge

de

nsity

dra

ina

ge

de

nsity

field

observation

flume

experiment

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Network Analysis

• the study of network patterns is necessary to understand the

controls on their formation

• the early approaches to network analysis were largely

qualitative, focusing on the importance of time & geo structure

• at the largest scale, the most commonly used classification at

present divides drainage patterns on the basis of their plan-

view shapes

• other classifications focus on other aspects of the drainage

net, determining quantitative, but dimensionless, numbers

from ratios – facilitates comparison between basins

• such ratios represent linear, areal & relief morphometric

components of the basin

linear – stream number

areal – drainage density

relief – relief ratio

hypsometric integral

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Rapidly expanding drainage network, nw Australia

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Stream numbering: quantify magnitude of basin, or

relation/number of tributaries to trunk (5th order basin, for eg.)

3

22

1

1

1

1

11

1 11

1

1

1

1

1

1

11

1

1

1

1

11

222

222

3

3

3

578

Horton (1945)

Strahler (1952)

Shreve (1967)

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Drainage density: average length of streams per unit area

controlled by interactions among geology

climate

age of basin

dra

ina

ge

de

nsity

arid temperate tropical

shale

sandstone

variation of drainage density with climate &

lithology

drainage density increases as resistance or surface

permeability decreases

drainage density links the form attributes of the basin to the

underlying processes

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Relief ratio: relief morphometry deals with the vertical dimensions

of a drainage basin

relief ratio is the maximum basin relief divided by the longest

horizontal distance of the basin parallel to the main stream

Hypsometric integral: distribution of mass above a datum (see text)

The morphology of a drainage basin can be viewed as the

connection between the way water moves into the basin, &

the way it moves out

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Water input is one of the controls on morphology (types &

distribution of channels), and morphology is one of the controls

on hydrology (flow characteristics)

climatic regime

water

basin morphometry

water

hydrology

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Stream discharge: volume of water passing a given channel

cross section during a specified time interval

Q = w d v (in ft3/s or m3/s)

= A v

Velocity is not evenly distributed –

• varies at cross section

• varies downstream (generally increases)

Discharge measured at gaging stations by US Geological Survey

& other agencies; measured hourly, daily, at peak flows, etc

Discharge is usually estimated from a rating curve

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discharge

% of time flow equalled or exceeded

specified discharge

dis

charg

e

ga

ge

or

sta

ge

he

igh

t

big floods

droughts

average flows

Rating curve

Flow duration

curve

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First gage in the US,Embudo, Rio Grande,New Mexico

Gaging weir, Rocky Mountain NP, CO

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Colorado River at Lees Ferry, AZ

Weir gage, Fool Creek, FraserExperimental Forest, CO

Boat-mounted current meter

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Geomorphologists study the magnitude & frequency of flows in

order to determine the effect of various flows on channel &

basin morphology – use flow duration curves & flood-frequency

analysis

R = (n + 1)/ m

R = recurrence interval (yrs)

n = number of discharge values in sample

m = rank of flow

.

..

.

...

..

.

recurrence interval

dis

ch

arg

e “100-year flood”

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dis

ch

arg

e

time

runoff

peak

base flow

increasing

basin size

Flood hydrograph

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Sediment Yield

Basin morphology also controls the movement of sediment into

the channels & out of the basin

Sediment yield: the amount of sediment leaving the basin

Sediment yield does not necessarily equal erosion rate because

of sediment storage on slopes (colluvial) & in channels (alluvial)

Controls on sediment yield are

• climate – mainly precipitation (Langbein-Schumm curve)

• vegetation – screens & binds regolith

• basin size – small = high sediment yield due to steep slopes & channels

• elevation & relief

• rock type/erosional resistance – eg. clastic sedimentary rocks have

higher sediment yield than crystalline rocks

• land use

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Langbein-Schumm

curve

se

dim

en

t yie

ldse

dim

en

t yie

ld

1800 1860 1900 1960

forest cropping woods &

grazing

urban

construction

aggradation

stable aggradation scour-stable scour

Land use effective precipitation (cm)

0 1000 2000

seasonal

tropics

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May 1990

June 1990

September 1990