90A JOURNAL OF SOIL AND WATER CONSERVATION JULY/AUGUST 2018—VOL. 73, NO. 4 L.A. Oparaku, D.A. Oluwaseun, and G.O. Ogbeh Anthropogenic factors infuencing the development of gullies on the Idah-Ankpa Plateau of north central Nigeria doi:10.2489/jswc.73.4.90A T he Idah-Ankpa Plateau (IAP) of north central Nigeria is beset by land degradation in the form of gullies. The proliferation of gullies on this plateau landscape has been spectacular and aggra- vated since the end of the Nigerian Civil War in 1970 (ECAN 1982). On the IAP, gullies scarify urban and agricultural lands and silt up streams and reservoirs, thus dam- aging the productive and aesthetic qualities of the environment.Tackling the gully ero- sion problem requires adequate information on the causative factors of this environ- mental phenomenon. Oparaku et al. (2015) carried out a study of the land degradation problems of the IAP and reported that many urban drainage structures, high-value buildings, roadside drains, and culverts have been lost to gully erosion at Ankpa, Dekina, Ayangba, Ofu, and Ogwulawo. There are two broad factors of gully erosion, namely, physical and anthropo- genic (human) factors.The physical factors include geology, soil, topography, climate, and vegetation; these have been exten- sively discussed in the literature (Ofomata 1965; Lal 1990; Okogbue and Agbo 1990). On the other hand, the anthropogenic fac- tors comprise all the activities of man that tend to disturb the soil and strip it of its vegetative cover, thus exposing it to rapid, accelerated erosion. In fact, gully erosion is the most spectacular expression of accel- erated erosion on the land (Michael and Ojha 2013). The opinions expressed by research- ers about the total contribution of gullies to sediment production in catchments are contradictory. However, it has been reported in the literature that gullies con- FEATURE Louis Alimonu Oparaku (corresponding au- thor) is a senior lecturer in the Department of Agricultural and Environmental Engineering, Federal University of Agriculture, Makurdi, Nigeria. D.A. Oluwaseun is a lecturer in the Department of Agricultural Engineering, Fed- eral Polytechnic, Nasarawa State, Nigeria. Gabriel Ogwuche Ogbeh is a lecturer at the Department of Agricultural and Environmental Engineering, Federal University of Agriculture, Makurdi, Nigeria. tribute between 10% and 95% of sediment production in a catchment (Valentin et al. 2005; Poesen 2011), whereas gully chan- nels constitute not more than 5% of the catchment surface area (Poesen et al. 2003).This review assesses the influence of anthropogenic factors in the development of gullies on the IAP of north central Nigeria.These influences are considered as they affect both the rural and urban envi- ronments in the area. THE STUDY AREA The IAP comprises the western Ankpa High Plateau and the Idah floodplains. It is located in the middle belt of Nigeria and lies between latitudes 7°17'00" N and 7°23'30" N and longitudes 8°20'20" E and 9°00'00" E.The total land area is estimated at about 5,675 km 2 (2,192 mi 2 ) with a perimeter of 793,531.76 km (493,077.8 mi) (Oparaku 2015). The area is consid- ered remote and inaccessible. As a result, it has attracted few environmentally based research activities. The climate is tropical, and the mean annual rainfall is 1,260 mm (50 in). There are two major seasons in a year: the rainy season from mid-April to late October and the dry season from November to mid-April. The average mean annual temperature is 32.6°C (90.7°F), and the relative humidity may be as high as 98.7% in October and as low as 75.2% in January. Annual evaporation ranges from 73.4 to 166.9 mm (2.9 to 6.6 in). The underlying geology of the area con- sists of Cretaceous sediments made up of three uniquely homogeneous formations (figure 1), which underlie 100% of the pla- teau landscape: the Upper Coal Measures (UCM; 36%), the Ajalli Sandstones (AS; 44%), and the Lower Coal Measures (LCM; 20%).The geological successions of Figure 1 Geological map of the Idah-Ankpa Plateau. Town Legend Study area boundary Roads Upper Coal Measures Lower Coal Measures Ajalli sandstones Rivers Ofu Ejule Allomo Egume Ankpa Ofugo Orokam Otukpa Okpo Ogugu 7 7 8 8 8 8 7 Dekina 9 9 9 9 8 9 9 9 9 Idah 12 0 12 24 36 48 km 8°30′ N 8°00′ N 7°30′ N 7°00′ N 6°30′ N 8°30′ E 9°00′ E 9°30′ E 10°00′ E 10°30′ E N Copyright © 2018 Soil and Water Conservation Society. All rights reserved. www.swcs.org 73(4):90A-95A Journal of Soil and Water Conservation