Study of Some Watershed Characteristics Associated with Gully Erosion Development Using Google Elevation Services in Girei Basin Adamawa State, Nigeria

Abstract

A field study was conducted in 2020 to characterize watershed features and 
digital terrain attributes of the Girei area. Digital mapping tools were applied 
to assess altitude, basin size and number, slope gradients, drainage channels, 
and topographic elements. Results showed that altitude ranged from 152 m in 
lowland areas to 488 m at hilltops. Four basins (watersheds) were identified, 
with areas of 2,288.1 km², 190.6 km², 41.1 km², and 13.7 km². Vegetation was 
dominated by grasses and shrubs typical of the savannah, with scattered shea
butter, acacia, eucalyptus, and locust bean trees. Land use was largely rice 
and sorghum cultivation, often practiced interchangeably. Drainage patterns 
reflected the relief, with streams and rivers originating from highlands and 
channel lengths totaling 355.8 km, comprising trunk routes A and B as well as 
service routes connecting communities. The slopes were classified as level 
(<2%), gentle (2–6%), moderate (6–13%), steep (13–25%), and very steep 
(>25%). The long slope distance from the highest elevations to gully areas 
facilitated runoff, accelerating gully headcut formation and expansion. These 
findings highlight the susceptibility of Girei to gully development driven by 
slope gradient, slope length, runoff velocity, and water volume. Management 
interventions such as runoff barriers and slope stabilization are recommended 
to mitigate erosion risks.