Synergistic impacts of small dams on South African rivers file• Low annual rainfall (497 mm on average vs. global average of 860 mm) • >600 large dams • >500,000 dams (incl

Synergistic impacts of small dams onSynergistic impacts of small dams onSouth African riversSouth African rivers

SukhmaniSukhmani Mantel, Denis Mantel, Denis Hughes, Hughes, NikiteNikite MullerMuller

Institute for Water Research Institute for Water Research Rhodes UniversityRhodes University

World Commission on Dams (2000)World Commission on Dams (2000)ImportanceImportance …… 3030--40% of40% of irrigated landirrigated landworldwide now relies on dams and that dams worldwide now relies on dams and that dams generate 19% of world generate 19% of world electricityelectricity

ImpactsImpacts ……loss of aquatic loss of aquatic biodiversitybiodiversity, of , of fisheriesfisheries, and of , and of ecosystem servicesecosystem services of of floodplains, wetlands, floodplains, wetlands, …… notnot possible to mitigate possible to mitigate many of the impacts of reservoir creation many of the impacts of reservoir creation

SolutionsSolutions …… environmental flow requirementsenvironmental flow requirementsto reduce impacts of changed to reduce impacts of changed streamflowstreamflow regimesregimes

OptionsOptions for meeting energy, water and food needs include for meeting energy, water and food needs include demanddemand--side managementside management, , supply efficiencysupply efficiency, and , and new new supply optionssupply options

Impacts of damsImpacts of dams

Nilsson et al. (2005) Fragmentation and flow regulation of the world’s large river systems. Science 308: 405-8.

Limpopo and Orange Rivers

South Africa Water ResourcesSouth Africa Water Resources

• Low annual rainfall (497 mm on average vs. global average of 860 mm)

• >600 large dams• >500,000 dams (incl. farm dams) with

total storage capacity of >50% of MAR(mean annual runoff)

• Synergistic impacts of small dams not well understood

Source: DWA (1986) Management of the Water Resources of the Republic of South Africa. Department of Water Affairs, South Africa.

River Health River Health ProgrammeProgramme(Nat’l Biomonitoring Programme)

ASPT Average Score per Taxon - invertebrate river health indicator

Sub-region classification based on gradient / substrate

Water Chemistry

Stream GaugesStream GaugesDischarge (Q90, Q75, Q50, Q10, Mean Daily Flow)

National DatabasesNational Databases

Large Dam (>3x10Large Dam (>3x1066 mm33))Impact AssessmentImpact Assessment

Dam storage capacity vs. MAR Scale0-5% 0>5-10% 1>10-15% 2>15-20% 3>20-25% 4>25% 5

Small Dam Impact Assessment Small Dam Impact Assessment

Number of dams / √Catch. area Scale0-5 0>5-15 1>15-25 2>25-35 3>35-45 4>45 5

Water Management AreasWater Management Areas

Western Western CapeCape

MpumalangaMpumalanga

LOWER ORANGE

LOWER VAAL

LIMPOPO

UPPER ORANGE

GOURITZ

OLIFANTS

FISH TO TSITSIKAMMA

UPPER VAALMIDDLE VAAL

OLIFANTS/DOORN

THUKELA

INKOMATI

BREEDE

BERG

MZIMVUBU TO KIESKAMMA

USUTU TO MHLATHUZE

CROCODILE(WEST) AND MARICO

MVOTI TO UMZIMKULU

LUVUVHU AND LETABA

Western Cape dams and Western Cape dams and RHP sampling locationsRHP sampling locations

217 samples @ 92 sampling locations

Western Cape Land UseWestern Cape Land Use

Vineyards and cultivation near coastVineyards and cultivation near coast

File - WC AnalysisWithNumberOfSmallDamsNew.xls

Principal Components Analysis-1

0

1

-1 0 1

PCA axis 1 (48.8%)

PCA

axi

s 2

(16.

1%)

NO3-N

pH

TSS

ASPT

No. of families

Biodiversity measures

Water quality variables

Western CapeWestern Cape

-3

0

3

-4 2

PCA axis 1

PC

A a

xis

2

Sm Dam Impact 0Sm Dam Impact 1Sm Dam Impact 2Sm Dam Impact 3/4

WesternWesternCapeCape

Principal Components

Analysis

-3

0

3

-4 2

PCA axis 1

PC

A a

xis

2

Sm Dam Impact 0Sm Dam Impact 1Sm Dam Impact 2Sm Dam Impact 3/4

-1

0

1

-1 0 1

PCA axis 1 (48.8%)

PCA

axi

s 2

(16.

1%)

NO3-N

pH

TSS

ASPT

No. of families



Are differences significant?Are differences significant?

Separate t-tests on data from 3 subregions(geomorph. / longitudinal classification)

MountainFoothill-cobbleFoothill-gravel

…. as different substrates & inverts

Number of samples by Number of samples by SubregionSubregionWESTERN CAPEWESTERN CAPE

((Not includingNot including sites impacted by large dams)sites impacted by large dams)

Small Dam Impact = 0

Small Dam Impact > 0

Foothill-gravel

23 10

Foothill-cobble

55 17

Mountain 74 2Mountain streams with low # of impacted samples

TT--test test ((PP value)value)

ASPT ↓↓ (0.003)NO3-N ↑↑ (0.04)

GRAVEL GRAVEL STREAMSSTREAMS

Small Dam Impact = 0versus

Small Dam Impact > 0(> 5 dams / √catch area)


0

5

10

15

20

ASPT # Invert Taxa pH NO3-N TSS

Mea

n (S

D)

Small Dam Impact=0 (n = 23)Small Dam Impact>0 (n = 10)

WC Gravel Streams

**

*

TT--testtest ((PP value)value)

pH ↑↑ (0.01)

COBBLE COBBLE STREAMSSTREAMS

Small Dam Impact = 0versus

Small Dam Impact > 0(> 5 dams / √catch area)


0

5

10

15

20

ASPT # Invert Taxa pH NO3-N TSS

Mea

n (S

D)


WC Cobble Streams

*

TT--test test (all P <0.001)Mean Daily Flow ↓↓

Q90 ↓↓Q75 ↓↓Q50 ↓↓Q10 ↓↓

GRAVEL GRAVEL Discharge Discharge StatisticsStatistics

((standardisedstandardised to to catchmentcatchment area)area)

0.0

0.5

1.0

MDF / catch area Q90 / catch area Q75 / catch area Q50 / catch area Q10 / catch area

m3 .s

ec-1

.km

-2 (S

D)

Small Dam Impact=0 (n=23)Small Dam Impact>0 (n = 10)

WC Gravel Streams

*

* * *

TT--test test (all P <0.001)Mean Daily Flow ↓↓

Q90 ↓↓Q75 ↓↓Q50 ↓↓Q10 ↓↓

COBBLE COBBLE Discharge Discharge StatisticsStatistics


0.0

0.5

MDF / catcharea

Q90 / catcharea

Q75 / catcharea

Q50 / catcharea

Q10 / catcharea

m3 .s

ec-1

.km

-2 (S

D)

Small Dam Impact=0 (n=55)Small Dam Impact>0 (n = 17)

WC Cobble Streams

*

*

* * *

Mpumalanga small dams andMpumalanga small dams andRHP sampling locationsRHP sampling locations

361 samples @ 127 sampling locations

Mpumalanga Land UseMpumalanga Land Use

Farming, cultivation & timber plantationsFarming, cultivation & timber plantationsPhoto: Photo: LilLil HaighHaigh


File MPUSubstrateAnlaysisWithNumberOfDamsNew.xls

Principal Components Analysis -1

0

1

-1 0 1

PCA axis 1 (39.2%)

PCA

axi

s 2

(21.

7%)

NO3 + NO2

pH

TDSASPT

No. of families



-5

-4

-3

-2

-1

0

1

2

3

4

-7 -6 -5 -4 -3 -2 -1 0 1 2 3

Sm Dam Impact =0

Sm Dam Impact =1

Sm Dam Impact =2

Sm Dam Impact =3


File MPUSubstrateAnlaysisWithNumberOfDamsNew.xls

Principal Components Analysis

-5

-4

-3

-2

-1

0

1

2

3

4

-7 -6 -5 -4 -3 -2 -1 0 1 2 3

Sm Dam Impact =0

Sm Dam Impact =1

Sm Dam Impact =2

Sm Dam Impact =3

-1

0

1

-1 0 1

PCA axis 1 (39.2%)

PCA

axi

s 2

(21.

7%)

NO3 + NO2

pH

TDSASPT

No. of families



Number of samples by Number of samples by SubregionSubregionMPUMALANGAMPUMALANGA

((Not includingNot including sites impacted by large dams)sites impacted by large dams)



Foothill-gravel 42 32

Foothill-cobble 104 0

Mountain 100 0

Mountain and Cobble streams with no impacted samples

FOOTHILLFOOTHILL--GRAVELGRAVELSmall Dam Impact = 0

versusSmall Dam Impact > 0

(> 5 dams / √catch area)

TT--testtest ((PP value)value)ASPT ↓↓ (0.038)TDS ↑↑ (<0.001)

0

50

100

150

200

250

300

350

400

450

# Invert Taxa ASPT NO3+NO2 pH TDS


MPL Gravel Streams

*

*

TT--testtest (all P <0.001)Mean Daily Flow ↓↓

Q90 ↓↓Q75 ↓↓Q50 ↓↓Q10 ↓↓

0.00

0.15

MDF/catch area Q90/catch area Q75/catch area Q50/catch area Q10/catch area

m3 .s

ec-1

.km

-2 (S

D)


MPL Gravel

* * * **

GRAVEL GRAVEL Discharge Discharge StatisticsStatistics


SummarySummaryASPT pH NO3 TDS

WC Gravel ↓↓ ↑↑

WC Cobble ↑↑

MPL Gravel ↓↓ ↑↑

Changes associated with ↓ in discharge

Research ImplicationResearch ImplicationQuantity Quality

Limitation: threshold for small dam impacted /unimpacted sites assumed to be at arbitrary 5 small dams.km-1 => need field research to determine threshold of biological significance

Future work: • Need to determine “acceptable” decreases in

biodiversity and water chemistry variables that balance resource protection and sustainable use

• Other methods of classifying dam impact besides number of dams?

Research ImplicationResearch ImplicationQuantity Quality

Limitation: threshold for small dam impacted /unimpacted sites assumed to be at arbitrary 5 small dams.km-1 => need field research to determine threshold of biological significance

Future work: • Need to determine “acceptable” decreases in

biodiversity and water chemistry variables that balance resource protection and sustainable use

• Other methods of classifying dam impact besides number of dams?

AcknowledgementsAcknowledgements

• International River Foundation

• Rhodes Postdoc Fellowship (Rhodes University, South Africa)

• Prof. Denis Hughes and Dr. Nikite Muller

• UCEWQ-IWR staff and students

Principal Components AnalysisLarge and Small

dams impact combinedWestern CapeWestern Cape


-3

0

3

-4 2

PCA axis 1

PC

A a

xis

2

Total Dam Impact 0Total Dam Impact 1Total Dam Impact 2/3Total Dam Impact 4-9

Principal Components AnalysisLarge and Small

dams impact combined

MpumalangaMpumalangaFile MPUSubstrateAnlaysisWithNumberOfDamsNew.xls

-5

-4

-3

-2

-1

0

1

2

3

4

-7 -6 -5 -4 -3 -2 -1 0 1 2 3

Total Dam Impact =0Total Dam Impact =1/2Total Dam Impact =3/4/5Total Dam Impact =6/7

Calculation of Anthropogenic Impacts

Large Dams Storage capacity / MAR

Urban Development

Runoff increase due to development relative to natural runoff

Irrigation Irrigation water demand estimated from % irrigation area

Forestry % reduction in MAR estimated from % area under forestry

Small Dams # dams / √catch area

Correlation of Impacts with Changes in WCCorrelation of Impacts with Changes in WC

-1

0

1

-1 0 1

PCA axis 1 (49.2%)

PCA

axi

s 2

(16.

1%)

NO3

pH

TSS

ASPT

No. of families

Forestry

Sm DamsLg Dams

IrrigationUrban Dev

Correlation of Impacts with Changes inCorrelation of Impacts with Changes in MPLMPL

-1

0

1

-1 0 1

PCA axis 1 (39.2%)

PCA

axi

s 2

(21.

7%)

NO2+NO3

pH

TDSASPT

No. of families

ForestrySm Dams

Lg Dams

Irrigation

Urban Dev

% small dam area relative to % small dam area relative to catchmentcatchment



WC Gravel 23 (0.02%) 10 (0.77%)

WC Cobble 55 (0.03%) 17 (1.03%)

MPL Gravel 42 (0.87%) 24 (3.1%)

Ecological ReserveEcological Reserve… relates to water required to protect the aquatic ecosystems of the water resource (National Water Act 1998)

… refers to both the quantity and quality of the water in the resource

Research gap: links between quantity and quality => present research looking at anthropogenic impacts on water quality

Documents

Synergistic impacts of small dams on South African rivers file• Low annual rainfall (497 mm on average vs. global average of 860 mm) • >600 large dams • >500,000 dams (incl