1 Original Research Article 1 Delineation of Imo Basin of the Anambra- Imo River Basin Area 2 3 Abstract 4 This study involved the delineation of all the river sub-basins in the Imo Basin of the Anambra-Imo River 5 Basin Area. It calculated the area drained by measuring all the tributaries and their stream order. This 6 study found out that there are basically five sub-basins in Imo State, namely Imo River sub-basin, Upper 7 Orashi sub-basin, Njaba/Lower Orashi sub-basin, Otamiri sub-basin and Ogechie River sub-basin. All of 8 these rivers and their tributaries drain 5,493.45km 2 of Imo State. The longest and largest river is Imo 9 River and flow northeast ward towards the Opobo estuary where it empties into the Atlantic Ocean while 10 Orashi and Njaba flows North West and down south to the Bight of Bonny in the Gulf of Guinea. ( Gulf 11 of Guinea, is the north easternmost part of the tropical Atlantic Ocean ). Contrary to several publications, 12 the Otamiri sub-basin or catchment is 1,719.25km 2 as against the 10,000km 2 frequently mentioned in 13 literature 14 Keywords: River Basin, Catchment, Watershed, Otamiri, Imo River, Stream Order, Tributaries 15 1. Introduction 16 Generally a River Basin is the portion of land drained by a river and its tributaries. It 17 encompasses the entire land surface dissected and drained by many streams and creeks that flow 18 downhill into one another, and eventually into the Ocean. Essentially it acts as a funnel, 19 collecting water and shunting it into the ocean or into inland lakes and seas. A river basin can be 20 relatively small, or quite large, and in all cases, they are interconnected systems. In the literature 21 River Basins are sometimes called Drainage Basins, Watersheds or catchment areas. Political or 22 administrative boundaries do not determine the direction and flow of rivers, it thus requires the 23 co-operations of States/Countries within the natural hydrological unit of the river basin in 24 protecting and managing the system bringing together all interests upstream and downstream, 25 this is so because drainage basins are a very important part of the world's ecology. 26 27 Managing river basins can be extremely challenging because they cross regional and national 28 boundaries and especially so when neighboring nations have differing views about the best way 29 to manage the system. Following the 1972-74 drought in Nigeria which many describe as the 30 worst ever experienced in Nigeria, the Supreme Military Council promulgated decree 25 of 31 1976, as a swift move towards the development of Nigeria’s water resources. Accordingly 32 twelve River Basin Development Authorities, (RBDAs), were created to harness the nation’s 33 water resources and optimize its agricultural resources for food sufficiency. The RBDAs 34 include; Upper Benue Basin, the Lake Chad Basin, Benin-Owena Basin, Sokoto-Rima Basin, 35 Sokoto; Hadejia-Jema’are Basin, Kano; the Lower Benue Basin, Makurdi and the Cross River 36 Basin, Calabar. The others are; Oshun-Ogun Basin, Abeokuta; Anambra-Imo Basin, Owerri; the 37 Upper Niger Basin, Ilorin; Lower Niger River Basin and Niger Delta Basin, Port Harcourt. One 38 way of managing this resource optimally is to have an accurate knowledge of the character of the 39 total drainage systems in a particular river Basin; this may include the area extent, the underlying 40
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1
Original Research Article 1
Delineation of Imo Basin of the Anambra- Imo River Basin Area 2
3
Abstract 4
This study involved the delineation of all the river sub-basins in the Imo Basin of the Anambra-Imo River 5
Basin Area. It calculated the area drained by measuring all the tributaries and their stream order. This 6
study found out that there are basically five sub-basins in Imo State, namely Imo River sub-basin, Upper 7
Orashi sub-basin, Njaba/Lower Orashi sub-basin, Otamiri sub-basin and Ogechie River sub-basin. All of 8
these rivers and their tributaries drain 5,493.45km2 of Imo State. The longest and largest river is Imo 9
River and flow northeast ward towards the Opobo estuary where it empties into the Atlantic Ocean while 10
Orashi and Njaba flows North West and down south to the Bight of Bonny in the Gulf of Guinea. ( Gulf 11
of Guinea, is the north easternmost part of the tropical Atlantic Ocean ). Contrary to several publications, 12
the Otamiri sub-basin or catchment is 1,719.25km2 as against the 10,000km2 frequently mentioned in 13
literature 14
Keywords: River Basin, Catchment, Watershed, Otamiri, Imo River, Stream Order, Tributaries 15
1. Introduction 16
Generally a River Basin is the portion of land drained by a river and its tributaries. It 17
encompasses the entire land surface dissected and drained by many streams and creeks that flow 18
downhill into one another, and eventually into the Ocean. Essentially it acts as a funnel, 19
collecting water and shunting it into the ocean or into inland lakes and seas. A river basin can be 20
relatively small, or quite large, and in all cases, they are interconnected systems. In the literature 21
River Basins are sometimes called Drainage Basins, Watersheds or catchment areas. Political or 22
administrative boundaries do not determine the direction and flow of rivers, it thus requires the 23
co-operations of States/Countries within the natural hydrological unit of the river basin in 24
protecting and managing the system bringing together all interests upstream and downstream, 25
this is so because drainage basins are a very important part of the world's ecology. 26
27
Managing river basins can be extremely challenging because they cross regional and national 28
boundaries and especially so when neighboring nations have differing views about the best way 29
to manage the system. Following the 1972-74 drought in Nigeria which many describe as the 30
worst ever experienced in Nigeria, the Supreme Military Council promulgated decree 25 of 31
1976, as a swift move towards the development of Nigeria’s water resources. Accordingly 32
twelve River Basin Development Authorities, (RBDAs), were created to harness the nation’s 33
water resources and optimize its agricultural resources for food sufficiency. The RBDAs 34
include; Upper Benue Basin, the Lake Chad Basin, Benin-Owena Basin, Sokoto-Rima Basin, 35
Sokoto; Hadejia-Jema’are Basin, Kano; the Lower Benue Basin, Makurdi and the Cross River 36
Basin, Calabar. The others are; Oshun-Ogun Basin, Abeokuta; Anambra-Imo Basin, Owerri; the 37
Upper Niger Basin, Ilorin; Lower Niger River Basin and Niger Delta Basin, Port Harcourt. One 38
way of managing this resource optimally is to have an accurate knowledge of the character of the 39
total drainage systems in a particular river Basin; this may include the area extent, the underlying 40
2
geology, the total number of streams/rivers and their stream order, bifurcation ratio, time of 41
concentration, discharge etc. 42
43
The Anambra – Imo River Basin is one of the river basins in Nigeria covering the rivers in 44
Anambra, Imo and Abia States. It has been discovered in literature that knowledge of the rivers 45
and streams in Imo State which make up part of the Anambra – Imo River Basin is sketchy and 46
sometimes faulty. For example Onweremadu [1] asserted that Otamiri sub-basin is 10,000km2 47
and on an elevation of 152m. Many other authors hold this same view [2,3,4,5,] Some of these 48
authors ( e.g. 4, 5]also assert that Otamiri flow into the Atlantic Ocean; that Okitankwo River is 49
an offshoot of Imo River, the basis for these conclusions were not well defined. Furthermore, 50
Okoro et al., [5] delineated the rivers in the Imo Basin into five watersheds namely Okigwe 51