Factors controlling gully morphology on the quartzite ridges of Ibadan, Nigeria

Gullying processes on sedimentary terrains have received significant attention from researchers while such processes are often overlooked in Basement Complex areas because of their perceived resistance to denudation. In an attempt to address this deficiency, we evaluated the nature and variation in gully morphology on a Basement Complex. Three gully systems on prominent quartzite ridges of Ibadan, Southwestern Nigeria were examined: Mokola, Mapo, and Eleyele quartzite ridges. Slope angles of gullied areas were recorded along with observations on slope convexity/concavity and vegetation cover. Gully morphological characteristics of length, width, depth and depth of gully head were measured in the field, and were used to calculate width/depth ratios and gully sinuosity, shape, and volume. Gully bank soil was sampled at a depth of 0-30 cm and 23 samples were analyzed for particle size composition, pH, Electrical Conductivity (EC), Organic Matter (OM), Sodium Adsorption Ratio (SAR) and Exchangeable Sodium Percentage (ESP) using standard laboratory techniques. These parameters showed considerable variation within and between sites. The mainly reddish indurated layer of the channel beds was predominantly composed of sandy clay and gully banks exhibited significant evidence of piping and tunneling. Our study showed that SAR (r = 0.794) and ESP (r = 0.867) were positively correlated with gully cross-sectional area, and SAR and ESP show positive relationships with total gully volume (an indicator of soil loss) of r = 0.661 and r = 0.758 respectively. The relationship between gully volume and maximum depth reveals a Durbin-Watson value of 2.67 where R2 = 0.826. The dominant factors of gully development on the ridges were identified as topography (slope shape and angle), soil properties (SAR and ESP), and land use/ human activities (urban growth). This study has identified the need to quantify the impact of lineaments on gully morphology dynamics on Basement Complex terrains.