Spatial variability of sediment grain size across stream orders in the headwater catchment of Omi River Basin, southwestern Nigeria

Sediment grain sizes vary with channel material and morphology, hydraulic processes and environmental controls in the basin and may influence basin ecological health. In this study, we assessed the variation in particle size distribution across stream orders and along the longitudinal profile of the Omi River basin in Ibadan, southwestern Nigeria. The basin was partitioned into upstream, midstream and downstream segments along the longitudinal profile. Forty (40) reaches were sampled across the first-, second-, and third-order streams. Sediments from the riverbed were collected at the reaches using a sediment grabber and analyzed for particle sizes using standard laboratory techniques. Fine-grained sediments (75–210 µm) exhibit dominance across the stream orders, although it is intermixed with silt and pebbles in the mid-stream and occasioned by silt downstream. We identified no significant difference in particle size distribution across stream orders (X² (16) = 13.234, p > 0.05) and along the river longitudinal profile (X² (16) = 21.963, p > 0.05) due to the dominance of fine sand. Further, a significant variation in the distribution of sand (F_(2,37) = 8.981, p < 0.05) and clay (F_(2,37) = 4.110, p < 0.05) particle size components along the longitudinal profile was obtained. The prevalence of fine-grained sediments suggests that the basin may be experiencing considerable soil erosion and sediment transport, impacting water quality, stream stability, and aquatic ecosystems. Therefore, we recommend further research on the sediment grain size dynamics relative to river water quality and channel geometry in the basement complex terrain of humid tropical environments.