# Ikpoba River

Ikpoba River is a waterway in southern Nigeria, traversing Edo State and providing resources for the inhabitants of Benin City and nearby areas. Rising from the Ishan Plateau, the river flows through Benin City and drains a basin area of approximately 922 km$^2$ (356 sq mi). It supplies freshwater, generates hydroelectric power, and supports agricultural irrigation. The river's hydrology is influenced by seasonal rainfall variations. Baseflow contributes about 56% of its total discharge, while the remaining 44% comes from overland flow. In its upper sections, the river exhibits a dendritic drainage pattern and is sustained by groundwater from the Benin Formation, ensuring a relatively consistent flow throughout the year. Studies have detected heavy metals like cadmium, lead, and mercury, raising concerns over industrial and domestic waste pollution.
Beyond its environmental importance, the Ikpoba River holds historical and cultural significance. Since the 15th century, it has been considered a spiritual guardian of Benin City, especially after Oba Ewuare the Great was believed to have fortified it against external threats. Over time, the river has become an object of religious reverence and is often worshipped during times of crisis. Its name is embedded in the traditions of the Bini people. An infrastructure on the river is the Ikpoba Dam, initially impounded in 1975 and officially commissioned in 1987. The dam functions as a reservoir for public water supply, delivering an estimated 90,000 m$^3$/d (37,000 cu ft/ks) of water to residents of Benin City. However, the reservoir has experienced siltation, resulting in a 23.16% loss of capacity due to sediment deposition. Periodic dredging has been recommended to maintain its efficiency and extend its operational lifespan.
Environmental issues affecting the Ikpoba River include deforestation along its riparian zones, primarily driven by agricultural expansion and urban development. The presence of pollutants from breweries, abattoirs, and sewage discharge points has impacted the river's ecology. To mitigate these impacts, the Edo State Government has initiated efforts to dredge and desilt the river, aiming to reduce flooding in low-lying areas of Benin City. Despite these environmental challenges, the Ikpoba River remains a resource for fishing, transportation, and small-scale hydroelectric power generation. Plans to utilise its hydropower potential have progressed, with a feasibility study completed in collaboration with the United Nations Industrial Development Organization (UNIDO) to generate renewable energy from the dam. The river plays a role in regional development and climate resilience strategies.

## Course and hydrology

The Ikpoba River, a sixth-order river flowing through Benin City in Edo State, Nigeria, originates from the Ishan Plateau and spans a catchment area of approximately 922 km$^2$ (356 sq mi). It follows a southward course, beginning in highland regions and meandering through both urban and rural landscapes before joining a larger drainage system. The river's coordinates range from 6°19′12″N, 5°24′0″E to 6°22′48″N, 5°51′7.2″E, indicating its reach across multiple ecological zones. Hydrological studies suggest that, despite minor seasonal variations, the river maintains relatively stable water levels year-round due to a combination of groundwater inflows and surface runoff. In studied sections, the river's width ranges between 10 and 12 meters, with depth fluctuations influenced by precipitation levels and local topography.
The main sources of the Ikpoba River's water include direct rainfall, groundwater seepage from the Benin Formation, and contributions from numerous smaller tributaries. Hydrograph analyses reveal that baseflow constitutes about 56% of its total discharge, highlighting the role of subsurface water inflows in sustaining the river's volume. The remaining 44% comes from direct runoff due to precipitation events and surface drainage, contributing to seasonal water level variations. During the wet season, typically from April to October, heavy rainfall increases flow rates, while in the dry season, flow levels decrease, occasionally exposing riverbanks and sediment deposits.
As the river passes through Benin City, it is affected by various urban hydrological factors, including stormwater runoff, industrial effluents, and domestic wastewater discharges. Activities such as car washing, bathing, and fishing—particularly downstream of the Ikpoba Bridge—have altered its natural hydrology, introducing contaminants that impact water quality. The interaction with urban environments has led to changes in flow characteristics, including localised erosion and sediment accumulation. Hydrodynamic modelling indicates that areas with higher levels of urbanisation exhibit reduced infiltration rates, leading to increased surface runoff and a heightened risk of flash flooding during heavy rainfall events.
A significant hydrological feature of the Ikpoba River is the Ikpoba Reservoir, created by the Ikpoba Dam, which was first impounded in 1975 and serves as a primary water supply source for Benin City. The dam helps regulate water flow, mitigating seasonal discharge fluctuations while providing water for domestic and industrial use. However, sedimentation has become a concern, with studies estimating that 23.16% of the reservoir's storage capacity has been lost due to silt accumulation. This has necessitated periodic dredging and desilting efforts to maintain the dam's functionality and ensure continued water availability.
The river's hydrological dynamics are affected by climate variability and land-use changes within its watershed. Deforestation in upstream areas has led to increased surface runoff, reduced infiltration rates, and exacerbated erosion along the riverbanks. Additionally, the urbanisation of Benin City has altered the river's drainage patterns, with more paved surfaces reducing natural groundwater recharge. Studies have recommended sustainable water management strategies to address these issues and support the river's ecological and economic functions.

## Geology and geomorphology

The Ikpoba River traverses the Benin Formation, a geological unit primarily composed of unconsolidated sand, gravel, and clay deposits dating back to the Late Tertiary period. This formation is part of the Niger Delta Basin and is noted for its high porosity and permeability, which facilitate groundwater recharge into the river. The interaction between the river and these underlying sediments affects water retention, sediment transport, and the stability of the riverbed. The coarse-grained nature of the sediments supports a consistent baseflow, allowing the river to maintain flow even during the dry season when surface runoff decreases. Human activities such as sand mining and deforestation have modified sediment distribution, resulting in localised erosion and variations in flow velocity.
The river's geomorphology is influenced by both natural fluvial processes and human interventions. In its upper reaches, the river meanders through forested areas where vegetation stabilises the banks and reduces erosion. As it flows through urbanised regions of Benin City, the channel becomes more altered due to embankment construction and artificial reinforcements. The installation of these barriers has disrupted natural sediment transport, causing deposition in some areas while accelerating erosion in others. The construction of the Ikpoba Dam has further impacted the river's geomorphology by trapping sediments upstream and altering downstream flow dynamics. The dam has led to the accumulation of fine-grained sediments within the reservoir, reducing its storage capacity over time. Studies indicate that 23.16% of the reservoir's total volume has been lost due to siltation, necessitating periodic dredging.
Erosion patterns along the river vary based on land use and hydrological conditions. Higher erosion rates have been observed near the Ikpoba Bridge, where human activities such as vehicle washing, cattle grazing, and sand dredging have destabilised the banks. These practices increase sediment load and lead to bank undercutting. In contrast, areas further downstream experience significant sediment deposition, primarily due to reduced water velocity and the influx of industrial effluents. Brewery industries in this area discharge wastewater into the river, leading to chemical interactions that enhance sediment coagulation. This has created a depositional environment where fine sediments accumulate, forming mudflats and altering the river's course.
The riparian zone, once covered with native bamboo (Bambusa vulgaris), has undergone degradation due to deforestation for agricultural and urban development. The loss of vegetation has increased runoff, reduced groundwater infiltration, and intensified erosion. In certain areas, the riverbanks have been reinforced with concrete embankments to prevent further land loss. While these structures offer temporary stability, they have altered the river's hydrodynamics, increasing flow velocity and contributing to downstream erosion. Additionally, the removal of riparian vegetation has impacted habitats for aquatic and semi-aquatic species, affecting biodiversity.
Sedimentation within the reservoir remains a concern. The total volume of silt in the reservoir is estimated at 347,378.41 m$^3$ (12,267,553 cu ft), reducing its capacity to store water for hydroelectric power generation and urban supply. If unchecked, sediment accumulation could lead to increased overflow risks and reduced efficiency of the dam's operations. The Edo State Government has recognised this challenge and directed dredging of the Ikpoba River to restore its capacity and mitigate flooding risks. The Benin City Rainstorm Master Plan is being updated to integrate erosion control measures specific to the Ikpoba watershed. These efforts aim to balance development needs with ecological sustainability.

## Biodiversity and ecology

The Ikpoba River supports an ecosystem that includes various aquatic and terrestrial organisms. The river provides habitat for fish species, benthic macroinvertebrates, riparian birds, and aquatic vegetation. However, heavy metal pollution, urban expansion, and deforestation have impacted the biodiversity of the Ikpoba River, leading to changes in species composition and population levels. Studies have found that industrial effluents and agricultural runoff introduce contaminants such as cadmium (Cd), lead (Pb), and mercury (Hg) into the river, resulting in bioaccumulation of these substances in aquatic organisms. The presence of these pollutants has been linked to alterations in fish tissues, including effects on gills, liver, and kidney functions.
The fish community in the Ikpoba River includes Tilapia zillii, Clarias gariepinus, Clarias anguillaris, and Oreochromis niloticus—species harvested by local fishermen. Tilapia populations have shown signs of heavy metal bioaccumulation, with higher concentrations of lead and cadmium detected in their tissues, especially in specimens collected from downstream sections near industrial discharge points. Fish in these areas also exhibit morphological features and reduced reproductive success, highlighting ecological risks associated with pollution. The benthic invertebrate community, serving as a bioindicator of water quality, has also been affected. Research has identified a decline in species richness in sections of the river exposed to industrial effluents, with pollution-tolerant species becoming more dominant. In contrast, upstream areas with less pollution continue to support a more diverse macroinvertebrate population, including sensitive species that require high dissolved oxygen levels.
Habitat degradation and pollution have also affected riparian bird populations along the Ikpoba River. Bird species such as the Pied Kingfisher (Ceryle rudis), Little Egret (Egretta garzetta), and African Fish Eagle (Haliaeetus vocifer) rely on the river for food, nesting, and breeding. The loss of riparian vegetation and increased human disturbances have reduced available nesting sites, leading to declines in certain bird populations. Studies have found that bird diversity is highest in river sections with intact vegetation, while areas experiencing deforestation and urban encroachment show a reduction in avian species. The removal of bamboo groves along the riverbanks—which once provided shelter for birds and other wildlife—has contributed to declining bird populations. Additionally, industrial pollution has affected prey availability for piscivorous birds, leading to reduced feeding success among species such as kingfishers and egrets.
The aquatic vegetation of the Ikpoba River has also changed due to nutrient loading from agricultural runoff and industrial discharges. Excessive nutrient inputs have led to the proliferation of algae and invasive aquatic plants, some of which have disrupted the natural flow of the river and reduced oxygen levels in the water. The spread of invasive plant species, such as water hyacinth (Eichhornia crassipes), has created challenges by blocking waterways and altering habitats for aquatic species. The increased presence of algal blooms has also led to localised oxygen depletion, a phenomenon known as hypoxia, which can result in fish kills and reduced biodiversity.