Dr. Johnson U. Kitheka(Senior Lecturer and Research Hydrologist)

School of Environment, Water and Natural Resources Department of Hydrology and Aquatic Science

South Eastern Kenya University

THE HYDROLOGIC ALTERATION OF THE TANA RIVER AND IMPACTS IN THE LOWER TANA

BASIN

INTRODUCTION

• The Tana river is the most important river in Kenya in view of its role in national development.

• Five HEP dams in the Upper Tana Basin -Kindaruma, Kiambere, Kamburu, Gitaru and Masinga.

• The Masinga Dam is the most significant in terms of regulating the hydrology of the Tanariver due to its size.

• Studies on the effects of soil erosion and impacts in the Upper Tana Basin (e.g. Dunne and Ongwenyi, 1976; Ongwenyi, 1978; Edwards, 1979; Ongwenyi, 1985; Brown et al., 1996; Schneider and Brown, 1998; Pacini et al., 1998; Maingi and Marsh, 2001; Maingi and Marsh, 2002).

• Studies on the effects of landuse change and water abstraction on streamflow in the Upper Tana basin (Njoguet al., 2018) and Mwendwa et al 2019).

• Studies on the environmental impacts of the hydrologic alterations of the flow of the Tana river on the riverineforests (Pacini et al., 1998; Maingi and Marsh, 2001; Maingiand Marsh, 2002).

• Few studies on the hydrology of the Tana river in the Lower Tana Basin (Kitheka et al., 2003 and 2005).

Objectivesi. Determine whether there has been a

significant change in the patterns of streamflow and sediment discharge in the River Tana following construction of dams in the basin.

ii. Establish the impacts of alteration of streamflow and sediment discharge in the lower Tana Basin/Tana Delta.

iii. Provide recommendations for sustainable development of the Tana Basin.

Drainage Areas of Kenya

Area = 132,000 km2.

River runoff = 32% of the total river runoff

Source of water and electricity for nearly 65% of the Kenya’s population.

HEP Reservoirs in the Upper Tana Basin

Existing and potential reservoirs in the Upper Tana

Basin

Reservoir Gross

volume

(106 m3)

Surface Area

(km2)

Catchment

area (km2)

Mean

river

discharge

(m3s-1)

Year of

completion

Masinga

Kamburu

Gitaru

Kindaruma

Kiambere

Karura

Mutonga

Grand Falls

Usueni

Adamson’s Falls

Kora Hills

1,560

147

20

16

315

74

1,580

3,600

330

1,730

3,800

113

15

3.1

2.4

13

8

46

119

26

102

190

7,335

9,520

9,520

9,807

11,975

97.2

149.5

120.9

155.9

121.8

1981

1975

1978

1968

1988

Planned

Planned

Planned

Planned

Planned

Planned

Planned

Tana river in the Lower Tana Basin-Hola

Lower Tana Basin and Tana Delta

•Tana Delta area -225,000 ha•Important for biodiversity and ecosystem services•Population of approx. 200,000 •Low human development index (0.509) and high poverty level (77%)•High dependence on natural resources -the main sources of livelihoods•Degradation of ecosystems due to changes in the hydrology of Tanariver (Nature Kenya SEA & LUP 2012)

Global Significance of the Tana Delta

• Habitat to globally significant wildlife: Red Colobus monkeys and Mangabeymonkeys, Dugong dugong(globally categorized as vulnerable, regionally as critically endangered)

• Key Biodiversity Area (KBA) and a Global Biodiversity Hotspot

• Part of the Coastal Forests of Eastern Africa Hotspot.

• Provides dry season grazing ground for both wildlife and domesticated animals.

• (Nature Kenya SEA & LUP 2012)

Tana River important for sustainability of mangrove forests

-High plant diversity - 320 plant taxa some of them Critically Endangered globally. -21% of the plants in the delta are of conservation concern-Home to 7 plants on the IUCN Red List of Threatened Species.

Tana River important for sustainability of riverine forests

Tana Delta has important riverine forests (3,700 ha) which are important for carbon sequestration and biodiversity conservation.

METHODOLOGY

Rainfall Data

• Old rainfall data obtained from Kenya Meteorological Service.

• Rainfall data obtained for Sagana and Nyeristations.

• Establish the response of the Tana river to rainfall in its headwaters in Central Kenya.

Determination of the river discharge

• The first approach: acquisition of old data archived at the Ministry responsible for Water, WARMA and TARDA.

• The second approach: field measurements of river discharges at the river gauging stations -Kipini (RGS 4G03), Garsen (RGS 4G02), and Garissa (RGS 4G01).

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Sampling on the Tana at Hola

River Gauging at Kipini

Determination of sediment load

• The first approach: data archived at the Ministry responsible for Water resources , WARMA and TARDA.

• Data obtained for Kiambere, Garissa, Hola and Garsen.

• The second approach: involved field measurements at Garissa, Bura, Hola, Garsen and Kipini.

• Sediment concentrations were determined in the KMFRI laboratory by filtering water-sediment samples using Whatman G/F filters according to APHA methods.

Analysis of rainfall –runoff data

• River discharge data for the period 1948-1968/1972 before the damming of the river were compared to periods following construction of Masinga dam.

• The continuous rainfall and river discharge data are not readily available for most of the river gauging stations.

• Thus, only years of relatively good continuous data were used in the analysis of flow variability.

RESULTS • Shifts in the frequency of occurrence of low flows (baseflows) and

maximum (peak) flows

• Shift in the frequency of occurrence of river discharges of both base flows and peak flows.

• Before damming: The probability of occurrence of low flows <50 m3s-1 was 80% of while that for the high flows >150 m3s-1 was 20%.

• After the construction of Masinga dam: The probability of occurrence for low flows of < 50 m3s-1 was 70% while that of high flows > 150 m3s-1 is 30%.

• After damming, the probability of occurrence of high flows has increased by 10% while that of low flows has reduced by 15%.

Tana River Discharge and TSSC variability at Garsen(2002-2003)

0

0.5

1

1.5

2

2.5

0

100

200

300

400

500

600

700

800

TS

SC

(g

/l)

Dis

ch

arg

e (

m3/s

)

Discharge TSSC

maximum river discharge during long rains= 750 m3s-1. The maximum river discharge during the short rainy season =300 m3s-1.

The relationship between rainfall at Sagana and Tana River discharge at Garsen (1960-1972)

0

100

200

300

400

500

600

700

0

500

1000

1500

2000

2500

3000

3500

4000

Jan

-60

MA

Y

SEP

Jan

-61

MA

Y

SEP

Jan

-62

MA

Y

SEP

Jan

-63

MA

Y

SEP

Jan

-64

MA

Y

SEP

Jan

-65

MA

Y

SEP

Jan

-66

MA

Y

SEP

Jan

-67

MA

Y

SEP

Jan

-68

MA

Y

SEP

Jan

-69

MA

Y

SEP

Jan

-70

MA

Y

SEP

Jan

-71

MA

Y

SEP

Jan

-72

Rai

nfa

ll (m

m)

Dis

char

ge (

m3

/s)

Year-Months

Q RAIN

Significant inter-annual variabilities . River discharge largely follows rainfall pattern in Central Kenya . Phase lag of about one month to reach Tana Delta.

The relationship between rainfall (Sagana) and Tana River discharge at Garsen (1960-1972)

y = 2.039x + 170.9R² = 0.359

0

500

1000

1500

2000

2500

3000

3500

4000

0 100 200 300 400 500 600 700

Dis

char

ge (

m3

/s)

Rainfall (mm)

Longitudinal changes discharge of the Tana

0

50

100

150

200

250

300

350

400

450

500

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Dis

ch

arg

e (

m3/s

)

Tana at Garsen Tana at Garissa

Tana at Grand Falls

River discharge significantly reduces as the river flows into the Lower Tana Basin

River Discharge and TSSC at Garsen (2002-2003)

0

0.5

1

1.5

2

2.5

0

100

200

300

400

500

600

700

800

TS

SC

(g

/l)

Dis

ch

arg

e (

m3/s

)

Discharge TSSC

River Discharge and Sediment load at Garsen(2002-2003)

0

5000

10000

15000

20000

25000

30000

35000

0

100

200

300

400

500

600

700

800

Sed

imen

t lo

ad

(t/

d)

Dis

ch

arg

e (

m3/s

)

Discharge TSSload

The influence of rainfall and river discharge on TSSC

y = 0.003x + 0.068R² = 0.546

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 50 100 150

TS

SC

(g

/l)

Discharge (m3/s)

Changes in the sediment load of the Tana

• 1950-1957 Data (Before damming): Sediment load of 8.74 x 106 tons.yr-1 (Ongwenyi, 1979)

• 2002-2003 Data: Sediment load of 6.8 x 106 tons.year-1

at Garsen (Kitheka et al., 2003b and 2005).

• 2010-2012 Data: Sediment load in the Upper TanaBasin of 7.1 × 106 tons.year-1 (Njogu and Kitheka, 2018)

• A reduction of about 20% in sediment load following the construction of the Seven Folks dams.

Flow of the Tana at Grand Falls before (1963-1981) and after (1982-1996) the Masinga Dam

0

500

1000

1500

2000

2500

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Dis

ch

arg

e (

m3/s

)

Months

Mean flow before damming Mean flows after damming

Peak flow before damming Peak flow after damming

• The maximum flows and mean flows generally decreased due to damming. Significant augmentation of minimum flows (Maingi and Marsh, 2002).

• 75 - 98% of the terrigenous sediment load trapped in the Seven folk reservoirs (Maingi, 1991, Schneider, 2000).

• Flood flow alteration and changes in baseflow induces major changes in the river morphology and ecosystem (cf. Basson, 2013).

• Lack of significant dampening of the seasonal and interannual streamflow variability of the river (cf. Poffet al., 2007).

Degradation of mangrove forests

Degradation of riverine Forests

Reduced the influx of river water into the far edges of the riverine forests causes degradation of the forests (Maingi and Marsh, 2002)

Seawater intrusion into the Delta due to sea level rise

y = 0.127x + 2277.

2000

2050

2100

2150

2200

2250

2300

2350

2400

2450

2500

19

95

Jan

19

95

Ju

n

19

95

No

v

19

96

Ap

r

19

96

Sep

19

97

Feb

19

97

Ju

l

19

97

Dec

19

98

May

19

98

Oct

19

99

Mar

19

99

Au

g

20

00

Jan

20

00

Ju

n

20

00

No

v

20

01

Ap

r

20

01

Sep

20

02

Feb

20

02

Ju

l

20

02

Dec

20

03

May

20

03

Oct

20

04

Mar

20

04

Au

g

20

05

Jan

20

05

Ju

n

20

05

No

v

20

06

Ap

r

20

06

Sep

20

07

Feb

20

07

Ju

l

20

07

Dec

20

08

May

20

08

Oct

20

09

Mar

20

09

Au

g

20

10

Jan

20

10

Ju

n

20

10

No

v

20

11

Ap

r

20

11

Sep

Sea

Leve

l (m

m)

Deepening of the river and tidal channel (10-30 m deep) (Kitheka et al., 2003a-b).

Seawater intrusion in the Kipini Branch

Sea water intrusion (>10km inland)

Coastal erosion

Intensification of the erosion of the delta

Modification of coastal sediment budget leading to accelerated erosion of the shoreline of the delta

Loss of dry season grazing fields that supports livelihoods in the Delta

Conflicts between farmers and pastoralists in the Delta

Collapse of flood-recession agriculture Loss of dry season pastures (77% poverty level)

• The ultimate consequence of the alteration of the flow of the Tana River is high level of absolute poverty in the Lower Tana Basin.

• 77% of the population living below poverty line.

• Poor management of irrigation schemes in the Lower Tana Basin affects livelihoods of people.

• A large segment of the population in the Lower Tana Basin is sustained by relief food (Red Cross and GoK).

Recommendations

• The proposed further damming of the Tana river should be scaled-down.

• The operating procedures of the Seven Folk dams should be reviewed so that the flow release from the dams mimics the original flow patterns.

• Invest on the more efficient irrigation systems in the lower Tana Basin to cushion local communities from changes in the flooding patterns of the river.

• Implement landscape restoration programmes in the Delta.

Conclusions

• The most important impact of damming of TanaRiver has been alteration of the sediment load and river discharge.

• The impact of damming on the Tana River flow has been particularly significant on the mean and maximum river flows.

• The impacts of alteration of the flow of the Tanariver would be even more serious with the planned further damming of the river.

Thank you