Ecological responses of streams to urbanization: A review of

results from the U.S. Geological Survey's urban streams studies

North Carolina Water Science Center

Thomas F. Cuffney

North Carolina Water Science Center

Raleigh, North Carolina

Boston

Birmingham

Salt Lake City

Raleigh

Atlanta

Seattle

Portland

Sacramento Denver

Dallas-Ft. Worth

Milwaukee-Green Bay

National Water-Quality Assessment (NAWQA) Program 11 Urban Stream Studies

Raleigh Urban Study AreaPiedmont

Objectives:1. Define biological, physical, and chemical

responses to urbanization across conterminous US.

2. Identify the primary environmental factors associated with these responses.

3. Compare how responses and driving factors change across the US.

-- What measures best depict urban effects (monitoring)?

– What factors can be changed to mitigate urban effects (planning and remediation)?

– Can urban effects be managed using national criteria or are regional or local criteria required (management and legislation)?

Representing urban intensity: urban intensity index (UII)

Index based on land-cover, population, infrastructure and

socioeconomic factors correlated with changes in population density

Urban variables positively correlated with population density

East Central West

Variable Atlanta Raleigh Dallas Milwaukee Denver Portland

Household density X X X X X X

Density of housing units X X X X X X

Road density X X X X X X

Urban lands in basin (%) X X X X X X

Impervious surface (% basin) X X X X X X

Popl'n in urban areas (%) X X X X X X

Urban lands in riparian (%) X X X X X X

Impervious surface (% riparian) X X X X X X

Housing - utility gas (%) X X X X X

Housing - renter occupied (%) X X

East Central West

Variable Atlanta Raleigh Dallas Milwaukee Denver Portland

Popl'n in rural area (%) X X X X X X

Heat with LP gas (%) X X X X X X

Heat with wood (%) X X X X X X

Forest (% basin) X X X

Shrubland (% basin) X X X X

Grassland (% basin) X X X X

Ag/pasture (% basin) X X X

Forest (% riparian) X X X

Shrubland (% riparian) X X X

Ag/pasture (% riparian) X X X

Urban variables negatively correlated with population density

Urban intensity index (UII)

Raleigh Example:

Census variables: 2000 population density Household density National Land-cover Data: % of basin in developed lands % of stream buffers in developed lands Infrastructure: road density

% Impervious and Urban Intensity index (UII) -- Atlanta

y = 0.0014x2 + 0.1865x + 0.0125

R2 = 0.93

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100

Urban intensity (UII)

% im

per

vio

us

Urban intensity (UII) at 10% impervious surface

0

10

20

30

40

50

60

Atl

anta

Bir

min

gh

am

Bo

sto

n

Ral

eig

h

Dal

las

Den

ver

Milw

auke

e

Po

rtla

nd

Sal

t L

ake

Urb

an in

ten

sity

(U

II)

0.95

0.88

0.950.78

0.92

0.92 0.88 0.95

0.83

Biological responses to urbanization

Fish, Invertebrates, and Algae

Expected response

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70 80 90 100

Urban intensity (UII, % impervious)

Res

po

nse

var

iab

le

Exhaustion

Resistance

Maximum rate of change

(≈ 10 % impervious surface)

Bio

log

ical

co

nd

itio

n

Good

Poor

Urban intensity (UII)

Atlanta urban model (Piedmont)

y = -0.0169x + 1.6651R2 = 0.81

0.0

0.5

1.0

1.5

2.0

2.5

0 20 40 60 80 100

Urban intensity index (UII)

Ord

inat

ion

sco

re (

axis

1)

Typical response (Inverts Atlanta)

10% impervious surface

40% of total change

Inve

rteb

rate

res

po

nse

Boston Fish: Exhaustion

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 10 20 30 40 50 60 70 80 90 100

Urban intensity (UII)

Axi

s 1

site

sco

reF

ish

res

po

nse

Urban intensity (UII)

Invertebrates responses to urbanization (Y = a + b*UII) across US

Region City b R2 PEast

Atlanta -0.017 0.70 < 0.001Birmingham -0.017 0.63 < 0.001Boston -0.017 0.82 < 0.001Raleigh -0.016 0.73 < 0.001

CentralDallas/Fort Worth -0.007 0.16 0.023

  Milwaukee -0.009 0.20 0.007

WestDenver -0.007 0.22 0.008Salt Lake City -0.015 0.47 < 0.001

  Portland -0.016 0.57 < 0.001

Invertebrates responses to urbanization (Y = a + b*UII) across US

Region City b R2 PEast

Atlanta -0.017 0.70 < 0.001Birmingham -0.017 0.63 < 0.001Boston -0.017 0.82 < 0.001Raleigh -0.016 0.73 < 0.001

CentralDallas/Fort Worth -0.007 0.16 0.023Denver -0.007 0.22 0.008

  Milwaukee -0.009 0.20 0.007

WestSalt Lake City -0.015 0.47 < 0.001

  Portland -0.016 0.57 < 0.001

0

20

40

60

80

100

Atla

nta

Bir

min

gh

am

Bo

sto

n

Ra

leig

h

Da

llas

De

nve

r

Milw

au

kee

Po

rtla

nd

Sa

lt L

ake

% B

asin

are

a

Land cover at “background” sites: UII < 20

Forest + shrublandsAgriculture + grasslands

EAST CENTRAL WEST

% B

asin

are

a

Agriculture

Irrigation?Grazing?

Summary of biological responses to urbanization (UII)

• Significant relation between urban intensity and biological degradation.

– Invertebrates are the strongest and most consistent indicators.

– Algae and fish responses are more variable.

– Relations were strongest when urbanization involved conversion of forest or shrub lands.

– Relations were weakest when urbanization involved conversion of agricultural or grass lands.

• Little evidence for resistance to urbanization (no initial threshold).

Changes in water chemistry associated with urbanization and

biological responses

Chemical trends with urbanization

• Conductivity

• Pesticides:– Number detected

– Total concentration

– Pesticide index

• Nutrients:Chemistry Invertebrates

Chemical trends with urbanization (continued)

• Polycyclic aromatic hydrocarbons (PAH):

– Number detected

– Total concentration

• Very few instances where human or aquatic life standards were exceeded.

• Relations with urbanization were also affected by presence of agriculture and grasslands (Central: Dallas, Denver, Milwaukee).

Chemistry Invertebrates

Hydrologic changes associated with urbanization and biological

responses

Relations with

Hydrologic change

Invertebrate condition

Increasing urbanization

Flow variability

Duration of high flows

Key Findings from Urban Studies

1. Degrades biological communities:a. Invertebrates are “best” biological indicator.

b. No level of urbanization without an effect.

2. Increases chemical contamination:a. Number and conc. of pesticides.

b. Number and conc. of PAH’s.

3. Modifies hydrologya. Increases flashiness.

b. Decreases duration of peak flows in many urban areas, but not all.

Key Findings (continued)4. Factors associated with urban degradation that may

be useful for mitigation:a. Decrease effects of road density in basin.

b. Decrease effects housing density in basin.

c. Restore forest and shrub lands (basins and buffers).

d. Restore connection between precipitation, ground water, and surface water (restore normal hydrology).

5. Responses vary geographically: a. East = West in biological (invertebrate) responses.

b. = Central (agriculture + grasslands).

c. Extent of “background” degradation affects how biology and chemistry respond to urbanization.

d. Regional approaches to management and legislation may be required.

/