Spatial and Temporal Variation of Hydrological and Geometrical Parameters Across River Oyan, Ogun State, South Western Nigeria

Channel erosion and sedimentation are major environmental threats to the sustainability and productive capacity of surface water. Sedimentation poses a significant threat to dam functionality, affecting water storage capacity, structural integrity, and downstream ecosystems. River Oyan has records of yearly flooding and Oyan Dam has drastically reduced in reservoir capacity due to the siltation. There was a paucity of information about erosion and sedimentation studies along the river. Objectives: Therefore, this study aimed to analyze the spatial and temporal variation of hydrological and geometrical parameters on River Oyan as input variables for sediment modelling across the river. Method: Twenty-two (22) years of hydrological and rainfall records were obtained from the Ogun-Osun River Basin Development Authority. Hydrological parameters (flow velocity, discharge), geometrical parameters (breadth, depth), and physicochemical properties of water samples were measured at six sampling points along the river from April 2021 to March 2024. The physico-chemical properties measured include turbidity, Total Suspended Solids (TSS), pH, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Dissolved Oxygen (DO), and hardness. Descriptive statistics and t-tests were used to establish the variation in hydrological and geometrical parameters at a 0.05 significance level. Findings: During the wet season, water samples from River Oyan showed turbidity levels between 84.0 to 88.5 mg/L compared to 8.5 to 16.0 mg/L in the dry season. The Total Suspended Solid (TSS) was higher in the wet season (126-129 mg/L) than in the dry season (83- 86 mg/L). Conversely, temperature and Total Dissolved Solid (TDS) were lower in the wet season. PH levels of water samples S4, S5, and S6 increased from 7-9, 7-9, and 8-10 respectively, from the wet to dry season while that of S3 decreased from 9-8. Dissolved Oxygen (DO) ranged from 8.9 to 12.0 mg/L in the wet season and 9.6 to 11.7 mg/L in the dry season. Sampling points breadth and depth were greater in the wet season, as were the velocity and discharge. Cumulative Peak Rainfall was 512 mm and the runoff ranged from 202.3- 512 mm. The t-test statistic reveals that the True Value (TV) of all parameters was greater than the Critical Value (CV) of 2.571 at the 95% confidence level (α=0.05). There is temporal and spatial variation in the hydrological and geometrical parameters along River Oyan. Hydrological and geometrical measurements along the river can ensure the development of a reliable sediment transport model. There is a paucity of information on sediment transport studies on River Oyan. Novelty: This is one of few studies on sedimentation in River Oyan and will serve as baseline information for other research in this domain.