Hydroiogical Sciences-,)'ouriwl-des Sciences Hydrologiques, 46(1) February 2001 13

Regional hydroiogical research perspectives in the Niger Delta

T. K. S. ABAM Niger Delia Environmental Survey. 20 Marina Street, NAL Towers, Lagos, Nigeria e-mail: ]JiSihM!i£!i2iIQiÛLSMÏÏ

Abstract The Niger Delta at the southernmost part of Nigeria, is a large and ecologically sensitive region. Natural and anthropogenic activities, such as subsidence, sea-level rise, upstream dams and reservoirs, dredging and over-abstraction of groundwater, disturb this equilibrium, leading to adjustments in ecological conditions and boundaries. Ecological changes affecting large areas cannot be explained by local observation. Relating observed effects to causes of environmental behaviour therefore requires a regionalized data collection, that provides a broader perspective of the issues involved with resultant ease in the identification of common problems and formulation of effective solutions. Key words environmental impact assessment; ecological changes; human impacts; ecohydrology of deltaic areas; environmental sensitivity indices; saline water/freshwater interaction

Perspectives de la recherche hydrologique régionale dans le delta du Niger Résumé Le delta du Niger, à l'extrême sud du Nigeria, est une vaste région écologiquement sensible au sein de laquelle diverses espèces biologiques, dans l'eau et dans le milieu souterrain environnant, sont en équilibre dynamique. Les processus naturels et les activités humaines, comme la subsidence, les marées, les barrages et réservoirs, le dragage et la surexploitation des eaux souterraines perturbent cet équilibre, et sont la cause de modifications des facteurs écologiques et de leurs conditions aux limites. Les modifications écologiques affectant de vastes régions ne peuvent pas être expliquées par des observations locales. La relation des effets aux causes implique donc la collecte de données à l'échelle régionale, qui fournit une perspective plus large et facilite l'identification des problèmes et la formulation de solutions efficaces. Mots clefs évaluation des conséquences environnementales; modifications écologiques; mpacts anthropiques; écologie des régions deltaïques; indices de sensibilité environnementale: nteraclion eau salée/eau douce

INTRODUCTION

The Niger Delta, a 30 000 km2 wetland of global ecological significance is located at the southernmost part of Nigeria. The delta is dissected by a dense network of rivers and creeks, which maintain a delicate but dynamic equilibrium between saline, estuarine and freshwater surface bodies with complex underground extensions. The region which is rich in biodiversity and natural resources is ecologically fragile and is presently inhabited by about seven million people. This high resident population, concentrated in the two urban cities of Port Harcourt and Warri is attributed to the rapid growth in the oil and gas industries.

Now at: Institute of Geoscience and Space Technology, Rivers State University of Science and Technology, PMB 5080, Port Harcourt, Nigeria.

Open for discussion until I August 2001

14 T. K. S. Abam

t<M NIGER DELTA 1:50,000 MAf» INDEX

0 10 20 30 40 50

Fig. 1 1:50 000 scale topographical map index of the Niger Delta study area.

Diverse environmental problems have been associated with this growth (World Bank, 1995) and considerable effort, sometimes recurrent, is expended com­bating these problems, albeit with little success. River pollution and the perennial ero­sion and flooding problems, which have defied control measures (NEDECO, 1961; Zinkcon International, 1980; Abam & Okagbue, 1986) are ready examples. The reason for the low level of success is due in part to the inadequacy of technical information on the region and the apparent defect in the study strategy, particularly, the sampling strategy which had encouraged limiting sampling to impacted areas.

Figure 1, which shows the coverage of the 1:50 000 topographic, vegetation land use map series, indicates that technical information sections of the delta are still not available. The combination of a poor information base and localized sampling can lead to a confused impression of environmental interactions. In this process, cause and effect of environmental conditions cannot be distinguished and the mechanisms for propagating impacts would also not be understood. The recognition of the hydrological process as a dominant mechanism for ecological change can enhance the understanding of environmental responses to changes in hydrological conditions. A regionalized sampling strategy is therefore proposed for a reliable environmental diagnosis and pollution abatement, supported with local examples from the Niger Delta.

PHYSICAL ATTRIBUTES OF THE NIGER DELTA

The River Niger drains a large part of West Africa (Fig. 2) and discharges its waters, sediments and other loads including exotic species into the Niger Delta and its

Regional hydrological research perspectives in the Niger Delta 15

Fig. 2 The Niger River drainage area and the major distributary river system of the Niger Delta.

extensions into the Atlantic Ocean. Over the years, this process has resulted in the formation of a complex and fragile delta with rich biodiversity.

In order to accommodate this natural diversity, the delta is divided into ecological zones. Although the ecological boundaries separating the zones are indicated as lines, in actuality, there exists in the field, a transitional zone of variable width, characterized by mixed attributes. The extent of the variation is usually a function of hydrology,

16 T. K. S. Abam

topography and soil type. Changes in any of these factors may alter existing ecological conditions and boundaries.

The surface of the delta is dissected by a dense network of rivers and creeks, which creates a condition of delta-wide hydrological continuity. Disasters such as oil pollution in one part of the delta can readily be felt in other parts. The river network conveys water and sediments through some 20 estuaries into the Atlantic Ocean to support the dynamic equilibrium of the coastal hydrological processes. This means that a spill in an upstream location beyond Asamabiri, which is not attended to, has the possibility to spread across these estuaries. This may affect a significant part of the delta, comprising sensitive low-lying swamps and over 180 km of coastline and conti­nental shelf regions in a matter of two weeks. Also, the region is dominated by south­westerly air currents and complex ocean water circulation patterns, extending the reach of pollution beyond the immediate confines of the coastal area. Environmental problems can thus be hardly limited to an ecological boundary.

HYDROLOGY AND DELTA-WIDE ENVIRONMENTAL PHENOMENA

The driving force of several ecological problems in the Niger Delta derives from surface water dynamics. Among the major ecological problems of the region are river and coastal erosion, flooding, saline intrusion, buoyant pollutant transport (e.g. oil spill or water hyacinth), water-borne diseases, fisheries conservation, etc. Figure 3 gives, in hierarchical order, the inter-relationships of these problems with the dynamics of surface water. It therefore follows that appreciation of the regional hydrology is essential for understanding the inter-relationships between space, time and environ­mental events. This is exemplified by a recent 40 000 barrels crude oil spill from a Mobil off-shore platform at the eastern end of the Nigerian Continental shelf. Contrary to the prediction of theoretically computed oceanic currents (NEDECO, 1961), the oil streak travelled over 300 km westward into the Lagos coast and lagoon network and also penetrated over 50 km upstream of the coastline.

The derivation of Environmental Sensitivity Indices (ESI) of oil spills in estuarine, coastal or off-shore operations also hinges on regional current patterns. Strategies for effective pollution control and remediation in a region largely submerged by surface water must derive from a holistic and regional process, in addition to a high level appreciation of the dynamics of surface water. Such a process requires integration of multi-sectoral work programmes covering biological environment and resources, pollution, human health assessment, and socio-economics, with a Geographic Information System (GIS). The quality of the results could be doubtful if, for reasons of cost, sampling areas and density are limited. This is also the case, in model studies, where significant errors could be introduced in cases in which data are limited in size and space. The sectoral inter-relationship of these with an indicative management plan is given in Fig. 4.

The current popularity of the application of GIS techniques holds advantages for the derivation of regional strategies. The advantage of GIS in this connection is that spatial change detection analysis can be applied to time sequenced databases, to track environmental changes and trends in terms of land use, coastline geometry, river alignment, vegetation drift, water regimes, flood plain sizes, fisheries potential, pollution levels, etc.

Regional hydrological research perspectives in the Niger Delta 17

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18 T. K. S. Abam

Biological environment and resources • Biodiversity inventory • Forest resources • Fisheries resources • Ecological zonation/habitat

classification

Human health assessment • Identification of study areas, obtaining information

from existing medical reports, and determination of health system in the Niger Delta

• Field assessments to provide the health status of the study communities

Socio-economics •Settlements and infrastructure •Oil industry and communities •Women as agents of development • Appropriate/indigenous technology

POLLUTION • Degraded lands • Air quality • Water quality and classification

PRA/PLA •Reconnaissance •Training workshop •Actual field activities

Hydrology and hydrodynamics • Changes in hydrology of the Niger flood • Coastal erosion • Problems and issues raised by PRA

activities and other stakeholders

ESTABLISHMENT AND MAINTENANCE OF GIS DATABASE • Interaction with sector specialists • System support and maintenance

• Acquisition and installation of hardware and software

• Recruitment and training

Fig. 4 Inter-sectoral relationships in regional environmental investigations.

DYNAMICS OF SURFACE WATERS

A review of the dynamics of surface water in respect of some recent hydrological developments can illustrate their environmental implication and emphasize the necessity for regionalization in data sampling and modelling.

Based on isolated site investigations, IGST (1988) indicated that there was a strong flux of flow and sediments to the coast. This was facilitated by the higher freshwater discharge in comparison to the upstream directed flood tidal flux, the higher ebb tide velocity and duration compared to equivalent flood tide attributes. The picture portrayed by IGST (1988) does not accurately reflect what is known today, because the river network system that was responsible for redistributing flow and sediment from upstream sources was not recognized in their analysis. A subsequent analysis with this

Regional hydrological research perspectives in the Niger Delta 19

perspective of the characteristics of networked rivers explained the presence of large sections of the coastline which are starved of sediments and are, as a result, experiencing erosion.

Water level records were first set up in the early 1950s by the Nigerian Inland Waterways Department and became popular in several parts of the Niger Delta after the publication of the NEDECO report (NEDECO, 1961). Unfortunately, the records available today are few and discontinuous and hardly helpful, even in routine flood frequency analysis. Efforts to re-activate old stations and set up new ones by the Niger Delta Basin Development Authority, to meet the minimum hydrodynamic network density, were not successful. Even the on-going Niger Delta Environmental Survey (NDES) process, which is a private sector initiative in the region, has budgetary constraints which have not permitted a full-scale hydrological survey of the region. This is in spite of the critical importance and diverse potential applications for such data. For example, water level records can give clues to certain regional environmental phenomena such as floods or subsidence.

Comparison of water level records m locations on interconnected river systems shows upstream areas, e.g. Anieze, having a lower water level compared to tidal areas, such as Forcados, possibly due to subsidence (Fig. 5) induced by oil and gas pro-

January June Fig. 5 Comparison of water level variation at Anieze and Forcados.

20 T. K. S. Abam

duction in the region. Two million barrels of crude oil and over 300 million standard cubic feet of gas are abstracted daily from the underlying sedimentary basin. Although produced from depths between 2.7 and 8 km, the deformability of the intervening geological formation suggests that such high production levels, sustained over time, are likely to induce significant subsidence. Although no authoritative subsidence measurements have been earned out, subsidence rates ranging from 2.5 to 12.5 cm year' have been quoted by different researchers (Fubara, 1986; Ibe, 1988). The relatively higher water level in some coastal areas indicates a reversed river bottom gradient and implies the presence of a sustained hydraulic head driving sea water towards subsiding inland areas and extending saline contamination to far upstream locations.

Some of the hydro-ecological peculiarities of the Niger Delta that recommend a regionalized approach to environmental investigations are discussed further below.

SALINE WATER-FRESHWATER DISCHARGE

The estuary is the meeting point of freshwater and saline water discharges. Most estuarine rivers are salinized during high tide by the influx of sea water upstream to distances which depend on discharge, river bed slope, channel shape, tidal level and fluctuations, and wind velocity and direction. The distances salinized upstream may range from less than one kilometre to tens of kilometres and could be increased by subsidence. The variability in these distances across the coastline is the result of variations in these parameters. For instance, the dredging of an upstream channel or construction of a new dam upstream could affect downstream discharges and sediment transport, which may invariably alter the location of the saline water-freshwater interface at the estuary. The upper limit of saline water in the region, traced from the NEDECO (1961) report, appears significantly altered in comparison with the present profile mapped from satellite imagery data or by other remote sensing techniques covering the area as demonstrated in NDES (1995). These changes are the result of anthropogenic intervention, involving upstream dams, reservoirs and canal construc­tion discussed in greater detail in a later section.

Tidal fluctuations within the estuaries are accompanied by a cyclic diurnal change in water quality due to the mixing, in continuously varying proportions, of original river water with sea water. In the lower reaches of the estuary, the water quality approaches that of sea water and at the farthest point inland, that of primary river water. The damming effect of sea water causes a freshwater tide or back-water effect in the upper reaches, where, although the river stage responds to tidal effects, the quality remains essentially unaffected. This scenario is illustrated by monitoring results at Peremabiri, where the hydrographs, over the years, show seasonal fluctua­tions throughout the year, but its water quality is essentially fresh with only 4% increase in conductivity of the wet season value.

Within the tidal reach, the change in water quality depends on the composition of the original river water and sea water and the relative quantities involved in the mixing. A zone of mixing and diffusion migrates up and down the river, the maximum saline contamination occurring during high tide and the minimum during low tide. Delineating this zone requires geophysical/geochemical sampling over a wide section of the delta.

Regional hydrological research perspectives in the Niger Delta 21

IMPOUNDMENT IN DAMS AND RESERVOIRS

The hydrological fate of the Niger Delta is intrinsically linked to the operations of upstream dams and reservoirs, particularly large dams such as the Kainji and Jebba dams, which are on average more than 600 km from the Niger Delta. Downstream water levels, discharge and sediment loads are all adversely affected by impoundments in dams (Abam, 1999b).

Water levels and discharge

Before the Kainji hydro-dam (capacity = 2700 x 10" m') came into operation, the average yearly discharge at the confluence of the Benue and Niger rivers (Lokoja) (Fig. 6) was 21 x 109 m1, that is a daily average of 7000 mJ s"'. Beets (1988) reported an average drop in discharge of 20% immediately following the commencement of operations at the Kainji Dam due to impoundment in the reservoir. This discharge rate was constant until 1981 when another drastic drop of 45% was noticed. This later drop in discharge can be attributed to the operations of Bakolori, Kiri and Pankshin dams commissioned in 1982, which have a combined reservoir capacity of 5589 x 106 mJ.

These variations in total discharge induced mainly by the runoff regulation have lowered peak water levels at Lokoja from about 11.13 m to 9.6 m and this is further reflected in the coastal areas of the Niger Delta, as exemplified by Peremabiri.

Fig. 6 Map showing the location of some major dams in Nigeria.

oo T. K. S. Abcun

As a result of the lowering of the water level, the flood plain of the Niger River system has shrunk in size. The shrinking of the flood plain is gradually engineering a change in the soil moisture conditions, with a high possibility of corresponding change in plant speciation. Already, several of the fish spawning areas within the freshwater swamps have been lost due to low flood levels. Also, silt nutrient supply to the forest has been drastically reduced for the same reason. A variety of wildlife species intolerant of ecological change are bound to suffer stress and eventually extinction. Ecologists have already made several claims to the disappearance of some species of fish, birds and other wildlife downstream of dammed rivers (Crisp, 1985).

The low flood discharges have also reduced the extreme maximum flow velocities and sediment transport capacity. Consequently, early deposition of bed load is now encouraged in the upper part of the Niger Delta, thus depriving the coastline of sediment requirements for delta progradational processes.

Coastal erosion

The progradation rate of the delta can be estimated by comparing the positions of palaeo- and present-day coastlines of the Niger Delta on a regional scale, as was indeed attempted by Allen (1965). More recent investigations, especially by Ibe (1988), Beets (1988) and Awosika et al. (1990), have revealed a net retreat of coastline in the eastern section of the Niger Delta and only marginal accretion in the western Niger Delta.

These patterns of coastline development contrast with that of an accreting coastline reported by Allen (1965) and Whiteman (1982), which reflected the pre-Kainji Dam status. Recent observations of the coastline suggest a strong erosional stress and a shift in existing ecological balance. The presence of strong erosional stress suggests that there is not enough sediment reaching some sections of the coast to sustain the existing ecological balance between coastal erosive processes and constructive river morpho­logical processes. These imbalances in coastal equilibrium which have led to relative stagnation in the progradation of the Niger Delta are attributable to impoundment of water and sediments in upstream dams and reservoirs.

An inspection of the drainage zones in the Niger Delta reveal drainage rivers of limited extent and catchment size in the eastern Niger Delta, implying impediments in sediment supply, thus making the problem more severe in this eastern area (Abam, 1995). This deduction is strengthened by the skewed discharge and sediment distribution in favour of the western Delta (Table I) up to 1992, which resulted in the invasion of large expanses of mangrove swamps in the western Niger Delta by tropical forest.

RIVER DREDGING AND CANALIZATION

The equilibrium attained in many decades of stable, low-level anthropogenic activities is being upset. First, the dredging of a 2.5 km riverbed stretching from Asamabiri to Agbere-Odoni has deepened the River Nun and introduced higher flow velocities. Although the channel width was narrower, the discharge became considerably higher than in the Forcados. This development, if sustained, will certainly introduce drastic changes to the ecology of the entire Niger Delta. The mangrove swamp in the western delta, depleted through many years of freshwater and sediment invasion associated with the higher discharge of the Forcados River, will commence a recovery process. This

Regional hydrological research perspectives in the Niger Delta 23

Table 1 Distribution of discharge between rivers Forcados and Nun.

Survey

1953/1954 1959/1960 1978 1979 1980 1981 1982 1983 1984 1985-1997 1998

NA: not availablf

Annual dtschai River Nun (m5

NA NA 2499 2464 2358 2364 1871 1267 1353 No data 5189

:ge through s"')

Discharge River Fori

55 60 55 55 55 55.01 54 55.01 55 No data 46

through _~ados (%)

Discharge through River Nun (%)

45 40 45 45 45 44.99 45.01 44.99 45 No data 54

process is very slow and the consequent expansion in the mangrove colony is not likely to be significant even after several years, because of the slow rate of mangrove regeneration.

The size of the mangrove colony at the major estuary of the River Nun and the surrounding areas is expected to shrink in response to increased freshwater and sediment discharge through the zone. However, the erosional stress being experienced at the coast at present will gain further strength with the anticipated rise in mean sea level because of the associated stronger coastal hydraulic forces. The influence of the increased discharge through the River Nun is not likely to extend to these areas, since they are not directly connected to the drainage network of the River Nun.

Secondly, the over 300-km canal network undertaken by governments and oil companies between 1980 and 1988 has created new conditions that have affected both flow patterns and the eco-system (World Bank, 1995). The problems associated with canal construction in the region include: (a) destroyed fishing grounds and enclosures; (b) changed salinity leading to forest dieback; (c) changed water flow patterns, dis­rupting erosion and sediment deposition; (d) temporarily increased turbidity and decreased dissolved oxygen from dredging resulting in possible reduction of organic soils resulting in possible reduction of fish biomass; (e) dredge spoils eroding during rains, increasing turbidity and, potentially, acidity; (f) temporarily increased biochemical oxygen demand (BOD) from dredged material and houseboat sewage; (g) reduced farm yields because of dredge spoil runoff acidifying fields; (h) reduced farm yields because of water logging of fields; and (i) destroyed mangroves and freshwater forests, including valuable timber and tree crop species.

The long-term problems from canals modifying the hydrological regimes are begin­ning to emerge (Powell, 1995). Perhaps the greatest impact is the alteration of discharge distribution, causing flooding in some areas and depriving others areas of water.

Beets (1988) also reported several avoidable adverse impacts of canal construc­tion. The Apoi-Gbaraun canal flooded Gbaraun town, displacing a population of over 25 000 and disrupting fishing and the sediment/erosion balance downstream.

Until the 1980s, the Ogbia area immediately south of Otuaka, situated some 15 km SSE of Yenegoa, was a pure freshwater area. Through a number of short-cut canals, intrusion of saline waters has occurred. The freshwater vegetation soon disappeared in preference to a mangrove vegetation that is presently dominant. Similarly, dredging activities of Chevron Nigeria Limited allegedly resulted in salt water intrusion into an

24 T. K. S. Abain

otherwise freshwater swamp forest, leading to the complete destruction of vast areas of land (over 20 km2) in the Opuekeba area of Tsekelewu (World Bank, 1995).

A regional approach to environmental impact assessment (EIA—an appraisal of the likely effects of a proposed project on the environment) could have foreseen and mitigated these ecological impacts.

Saline intrusion of groundwater

Increase in mean sea level, meteorological drought and regional subsidence of the Niger Delta act in combination to enhance the influence of saline water upstream. The effects of the incursion of saline water are expected to be worse for major urban centres such as Port Harcourt and Warri, which are located in the estuarine areas, because of the increased pressure on underground water resources imposed by high industrial consumption and high population.

Abstraction rates have varied over the years in these cities, and so have their effects. Increased abstraction rates and volumes are accompanied by increased chloride concentrations in neighbouring inland aquifers (Table 2). Average chloride concentra­tion in groundwater from the Moscow Road, Port Harcourt pumping station increased from 11 mg 1"' in 1995 to 37 mg 1" in 1996 and then dropped to 17 mg 1 ' in 1997. In the same period, groundwater abstraction from the major pumping station in Port Harcourt increased from 18.8 x 106 1 in 1995 to 3857 x 106 1 in 1996 due to a programme of World Bank intervention in water rehabilitation (Abam, 1999a). In 1997 abstraction dropped to 2926 x 106 1. Relatively higher chloride concentration was similarly observed in Forcados and Excrafos in the western Niger Delta (Otezi, 1990).

These two rapidly growing coastal communities are also located in the estuary and depend entirely on potable groundwater resources that may now be severely depleted and threatened.

Over-abstraction in coastal/estuarine aquifers may result in the reversal of hydraulic gradients of groundwater. When this happens, the saline water-freshwater interface moves towards a freshwater zone, thereby extending the zone of saline contamination. A similar effect is readily created by a rise in the head of sea water relative to that of freshwater, for example during high tide.

The freshwater-salt water interaction in the coastal aquifer is not merely of hydrostatic equilibrium, but one involving movements of freshwater and saline water adjusting to a hydrodynamic equilibrium. Disturbing the hydrodynamic equilibrium between salt water and freshwater in the coastal aquifers by heavy pumping results in the invasion and contamination of freshwater aquifer by saline water. Within the estuarine reach of the stream, saline water may encroach into the underlying freshwater aquifers in a variety of ways, depending on the relative positions of the water table, the river stage and the degree of interconnection between the stream and the aquifer. There is a potential threat to surface freshwater resources in the Niger Delta from enhanced

Table 2 Influence of heavy abstraction on chloride concentration in Port Harcourt, Nigeria. __^ . _̂_ ^ _ _ ^ _ .

Annual abstraction (x 10" m) 18.8 3857 2926 Average chloride concentration (mg l'1) 11 37 17

Regional hydrological research perspectives in the Niger Delta 25

upstream movement of sea water due to reversal of riverbed gradient produced by subsidence. The inland incursion of tidal waters have been strengthened by the reduction of freshwater discharge due to impoundments in upstream dams and reservoirs.

CONCLUSIONS

The dynamic riverine ecological equilibrium is under threat as a consequence of dredging and the operation of upstream dams and reservoirs. The consequent reduced water levels and discharge and lower sediment transportation capacity have not been sufficient for maintaining ecological equilibrium, especially around the coastline. Ecological conditions and boundaries are continuously adjusted to the changing patterns of discharge. Adjustment rates are variable, some slow and others fast. Some cases involve irrecoverable losses.

A number of the ecological consequences of environmental change as described above are caused by factors external to the delta, yet their effects are transmitted several hundred kilometres downstream. This underscores the importance and relevance of regional strategies to environmental research.

REFERENCES

Abam, T. K. S. (1995) Coastal zone management strategy for Hooding and erosion in the Niger Delta. A report prepared for the World Bank.

Abam, T. K. S. (1999a) Dynamics and quality of water resources in the Niger Delia. In: Impacts of Urban Growth on Surface Water and Groundwater Quality (ed. by J. B. Ellis) (Proc. Birmingham Svmp., July 1999). 429-437. 1AHS Ptibl. no. 259.

Abam. T. K. S. (1999b) Impact of dams on the hvdroloev of the Niger Delia. Bull. Int. Assoc. Engni> Geol. & Environ. 57, 239-251.

Abam. T. K. S. & Okagbue, C. O. (1986) Construction and performance of river bank protection structure in the Niger Delia. Bull. Assoc. Engng Geol. (France) XXIIK4), 499-506.

Allen, .1. R. L. (1965) Coastal geomorphology of eastern Nigeria: Beach Ridge Barrier Islands and vegetated flats. Geologic en Mijubou 44, 1-21.

Awosika. L. F., Ibc, A. C. & Udo-Aka, N. A. (1990) Impact of sea level rise on ihe Nigerian coastal zone. In: Changing Climate and the Coast, vol. 2, 67-78.

Beets, C. ! 1988) Field studies for Hood and erosion in the Niger Delta. Internal report, Institute of Geosciences and Space Technology (IGST), Rivers Slate University of Science and Technology, Port Harcourt, Nigeria.

Crisp, D. T. (1985) Effects of impoundment and regulation of the River Tees at Cow Green upon fish populations in tributaries. In: Habitat Modification (ed. by J. S. Alabaster), 42—15.

Fubara, D. M. J. (1986) Flood and erosion: human contributions and remedial environmental policy. Mimeographed paper, Institute of Flood, Erosion, Reclamation and Transportation, Rivers Stale University of Science and Technology, Port Harcourt, Nigeria.

Ibc. A. C. ( 1988) Coastline Erosion in Nigeria. University of Ibadan Press, Ibadan, Nigeria. IGST (Institute of Geosciences and Space Technology) (1988) Master plan and design of flood and erosion control

measures in Ihe Niger Delta. Technical Progress report no. 1, IGST, Rivers Stale University of Science and Technology, Port Harcourt, Nigeria.

NEDECO (Netherlands Engineering Consultants) (1961) The Waters of the Niger Delta. NEDECO, The Hague, The Netherlands.

NDES (Niger Delta Environmental Survey) (1995) Phase i Report, vol. 1-4. NDES, Lagos, Nigeria. Otczi, A. U. ( 1990) Delineation of sea water intrusion in a coastal beach ridge of Forcados. J. Mining Geol. 26(2), 225-229. Powell. C. B, (1995) Wildlife species known/suspected in Upper Orashi Forest Reserve, threatened by Ihe EU sponsored

RISONPALM lowland oil palm project, Yenagoa, Nigeria, Unpublished report. Whiteman. J. (1982) Petroleum Resources of the Niger Delta. Report. World Bank (1995) Defining an Environmental Development Strategy for the Niyer Delta, vols 1 & 2. World Bank.

Washington DC, USA. Zinkeon International (1980) Erosion and Hood control works for 16 town and villages in Ihe Niger Delta. Zmkeon

International BV. Received 8 March 1999; accepted 28 February 2000