Amadi, A.O.

Ladu, B.M.B.

Mbagwu, I.G.

English

Aquatic Commons

CONTRIBUTIONS OF THE SEASONAL AND LONG TERM FLOOD
PATTERNS OF RIVER NIGER TO THE DEVELOPMENT AND
CONTROL OF WATER HYACINTH (Eiclthornia crassipes) IN
KAINJI LAKE, NIGERIA
I.G. MBAGW[J, B.M.B. LADU and A.O. AMADI'
National Institute for Freshwater Fisheries Research (NIFFR)
P.M B. 6006, New Bussa. Niger State, Nigeria
'Hydrology Department, Jebba Hydro Power Generating Station,
National Electric Power Authority, Jebba. Niger State, Nigeria.
ABSTRACT
This paper reviewed the seasonal flood regime of River Niger, which produces
the two Annual floods — White and Black — which sustain the lake level. It further
identified a low flow period of the river, which seems to last 30 years. The annual flood
regime produces a draw-down which exposes a substantial area of the shoreline during
the year. This abandons a lot of water hyacinth strands that usually get trapped within the
rooted shoreline vegetation during the high water period. Thereafter, the stranded water
hyacinth strands die and rot.
The long low flow period reduced the water residence time of the lake from 4 to
2 within the 32 years it has existed. The implication of this is that the water surface area
became more stable. This reduced nutrient export and consequently offered pleustophytes
such as water hyacinth time to extract nutrients and build up poptilation. The envisaged
end of the low flow period will reverse this situation and discourage the establishment of
water hyacinth in the lake
INTRODUCTION The Flood Pattern of
River Niger in NigeriaThis paper reviews the flood regimes of the
River Niger as measured in Nigeria, the The River Niger flow in Nigeria exhibits a
manifestations of the flood regimes in Kainji seasonal flood regime, which is responsible
Lake and the possible contributions of these forthe annual flood characteristics of the river
flood regimes to the development and control in Nigeria and a time trend, which hitherto has
of water hyacinth in Kainji Lake not been described.
105
The Seasonal Flood Regime 2000 with a numerical technique for analysis of
trends in a time series.Nedeco & Balfour B catty, (1961) described the First WETNESS INDICES were derived for all
seasonal hydrology of River Niger. During the the years as
months of May to October, rainfalls in the
northern parts of Nigeria south of Niamey I = (QX - Qm) / Qin
(Fig. 1: inset) produce floods that quickly reach
wheçe Qx = Annual run-off and Qm = AverageKainji with a peak flow of 4,000 to 6,000 m S annual run-off for 1954 to 2000.in September to October. This floodwater is
laden with siltlclay sediments and is of high The wethess index is used to quantify the
turbidity. Due to its colour, it is locally referred relative wetness or dryness of a particular year
to as the "White Flood" with respect to the total amount of water that
entered the lake within a specific period.
The second flood originates from the river's
headwater region of high annual rainfall in the Secondly the Wetness Index series was plotted
against Time and 5-year Running AveragesFouta Dj allon highlands in Guinea, and passes fitted on the series to search for trends in thethrough the sub-arid region and deltaic swamps data. This analysis revealed a lOw flood flow
around Timbuctu. In these areas it looses much period for the river, which has lasted for about
of its silt load and water to evaporation and 30 years (Fig. 3).
infiltration, Very little water is added to the flow Manifestations of the River Niger Flood
Regimes in Kainji Lakebefore it reaches Kainji in November with a
peak flow of about 2,000 m3 s-t. The water is 1nnuuuu1 Floods
The two annual floods
— the White and Blackrelatively clear due to its low silt load and is
floods, sustain the Kainji Lake level. Floweverthus locally called the "Black Flood"
there is only one high lake level period during
Two distinct floods were thus manifested at the dry months of the year as a result of the
regulation of the river flow by Kainji dam. AsKainji annually. (Fig. 2)
water use for hydropower generation continues,
The Long Term Flood Cycle the lake level is progressively drawn-down and
a substantial area of the shoreline is exposed as
In an analysis of the discharge data of River the draw-down area (Fig. 1).
Niger from 1954 to 1977, Sagua and Fregene
(1979), opined that River Niger flow has a time The Long Term Flood Cycle
trend that was not taken into account in the
The low flood flow cycle of River Nigerdesign of Kainji Dam. However the precise
resulted in increased residence time for water
time lag of this trend was not known. in Kainji Lake. With an Active Storage
Capacity of 12.0 x m3, a replenishing ratThis study investigated the annual discharge
data of The Niger at Jideribode from 1954 to
106
10050.
15 Zbkm
Fig. 1. Map of Kainji Lake. inset shows the River
Niger basin and its tributaries.
107
Ye lwci
New Bussc
Anfcini
— Ufrjh wcitr mark
Low wntr mark
4°20 4°S0
><Iii
z
(I,
Cl,
w
z
w
3
U)
c)
LI..
Li..
92
WHITE FLOOD BLACK FLOOD
2000
I BOO
1600
1400
1200
1000
800
600
400
200
0
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
MONTHS
Fig. 2: Seasonal Flood Regimes of River Niger in Nigeria
56 58 60 62 64 66 68
YEARS
Fig. 3: Wetness Indices of River Niger Hydrological Years (1954..
2000) with Trendline of 5-Year Moving Averages
108
6YEARS
Fig. 4: RepIenshing Ratio of Kainji Lake (1954 to 2000).
io of 4.0 was designed for the reservoir using
observations in the flow of River Niger at
Jideribode from 1954 to 1960 (Nedeco &
Balfour Beatty, 1961). The flow of The Niger
potentially maintained this ratio until the first 5-
and further to 2.0 in the following years until
1995. Since then however, the replenishing ratio
h been rising. The implication of this is that in
the 32 years of the existence of the lake, its
hydrological residence time gradually increased
from 90 days at impoundment to as much as
109
180 days year period (1968-72) immediately
following the closure of the dam (Fig. 4).
Consequently, the replenishing ratio of lake fell
to 3.0 in the first decade of the existence of the
lake
Contributions of the River Niger Flood
Patterns to the Development and Control of
Water Hyacinth in Kainji Lake.
contribution of the annual fl4ods
The annual flood pattern has a profound
influence on the annual deveiopment of water
5
4
3
2
1
0
54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 '00
hyacinth in Kainji Lake. Several reports (e.g.
Fregene and Amadi, 1993) indicate that the
plant recruits upstream within the inner basin of
River Niger. During the annual floods, the
plants are flushed into Lake Kainji where most
of them get trapped within stands of rooted
aquatic macrophytes along the shorelines. Those
that reach the damsite get trapped or spilled over
to Jebba Lake. Eventually the annual draw-
down destroys those trapped within the
shoreline while those trapped at the damsite die
and rot.
Contribution of the annual floods
Longer water retention in Kainji Lake
implies more time for the mineralisation of
particulate. forms of Nitrogen and Phosphorus
and a proportional reduction in the rate of
nutrient export from the lake. Exogenous
sources of nutrients into the lake has increased
in later years from residues of fertilizer arising
from intensification of agricultural activities
around the lake since impoundment and
phosphorus released annually by bush bnnring
(Mbagwii & Adeniji, 1994). With increased
authocthonous and allocthonous inputs of
nutrients simultaneously in place alongside a
lowered flushing rate, a more stable water
column will result leading to improvement in
ecological productivity.
Obot (1984; 1986) attributed the failure of
floating macrophytes to develop in Kainji Lake
to the rapid removal of nutrients from the lake
ecosystem caused by the high replenishing ratio
of 4.0. The current proliferation of water
hyacinth in the lake can be interpreted as arising
from the prolongation of the residence time of
water in the lake, which has enabled the plants
to have more time to extract nutrients and build
up population. In contrast, the seeming end of
the low flow cycle of River Niger could reverse
this scenario and return the Kainj i Lake to a
regime of high replenishing ratio that will
discourage the establishment of water hyacinth
in the lake.
REFERENCES
Fregene, S.P. & Asnadi, A.O. (1993): Upper Niger flood
forecast for 1993. Report to the National Electric
Power Authority (NEPA) Lagos.
Haistead, LB. (1971). Freshwater bivalves of the shores of
Lake Kainji. The Nigerian Field 36 (3): 135-139.
Mbagwu, I.G. & Adeniji, H.A. (1994). A review of studies
on the lirnnology of Kainji Lake, Nigeria. A Report
to theNigerian/German ('G7Z) Kainji Lake Fisheries
Pro,notion Project. New Bussa Niger State. Nigeria.
37 pp.
Nedeco &.Balfour Beatty (1961). The Niger Dams Project.
2: Hydrology and Reservoir Operations. Federal
Republic of Nigeria. Lagos.
0bo, E.A. (1984). Studies on the productivity of
Echinocloa slagnina (Retz.) p. Beauv in the Kainji
Lake basin of northern Nigeria. Ph.D. Thesis.
University of Ife. i93pp.
Obot, BA. (1986). Ecological comparison of the pre-
impoundment macrophyte flora of the River Niger
and Lake Kainji, Nigeria. Vegeiatio 68: 67-70.
Sagua, V.0. & Fregene, S.P. (1979). Kainji Dam and the
hydrology of the River Niger. In: Proceedings of the
International Conference on Kainji Lake and River
Basins Development in Africa: Vol H, Kainji Lake
Research Institute. 223-233.
110



Contributions of the seasonal and long term flood patterns of River Niger to the development and control of water hyacinth (Eichhornia crassipes) in Kainji Lake, Nigeria

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Contributions of the seasonal and long term flood patterns of River Niger to the development and control of water hyacinth (Eichhornia crassipes) in Kainji Lake, Nigeria

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