Geological Processes, Statistical Characterization and Downstream Fining of Alluvial Sediments of River Osun, Southwestern Nigeria

Abstract

The geological processes, statistical characteristics and downstream fining of River Osun in southwestern Nigeria were studied. The  objectives were to determine the variation in particles sizes, contributory effects from tributaries and, to determine the deposition  characteristics of these sediments. A total of 10 samples were collected along the River Osun, from Osogbo to Omitutu, covering a total distance of 140 km. Sampling was done within the river bends, meanders and exposed bars adopting purposive sampling method. Each  sample was washed, through 0.063mm sieve and mechanically sorted into a set of US mesh sieves of 2.360, 2.00, 1.00, 0.50,.250, .125 and  0.063 mm and a receiving Pan (Wentworth Classification) using a Ro-tap shaker in Soil Mechanics laboratory, at National Water Resources  Institute, Kaduna. The various percentiles were calculated. The graphic mean of River Osun sediments range between -0.7 to  0.93ф with an average of 0.07ф. This indicates: a very coarse to coarse sand. It contains 40.79% fine gravel, 20.90% coarse sand, 21.29%  medium sand and fine sand account for 16.90%. This showed that particles are carried as bed load and are either close to their source or there were mixing from tributaries. The sorting varied from 0.74 to 1.25 ф, with an average of 1.26 ф. It indicates a moderately to poorly  sorted sand, which implies a rapid deposition or lateral addition of sands from other sources. The skewness values range from -0.06 to  0.42 ф (platykurtic to leptokurtic). This is a reflection of the flow characteristics of the deposition medium from low to high kinetic of  energy of sediment transport causing deposition. The downstream fining showed a virtual departure from and exponential order; the  effect could be attributed to the lateral addition/mixing, as well as anthropogenic factors like construction of dam, irrigation, and sand  dredging along the channel. This study could be applied in the study of transportation and sizes of heavy minerals along the channel of  River Osun.