Some effects of the Tiga Dam on valleyside erosion in downstream reaches of the River Kano

Some of the effects of a lower river stage on valleyside fluvial processes are examined in downstream reaches of the Tiga Dam in the Kano River basin. The focus is on valley-bottom gullies which responded clearly and quickly to the lower height of flow in the Kano River channel in these reaches. The characteristics of the valley-bottom gullies at a time before the construction of the Tiga Dam were compared with those of the same gullies when the Tiga Dam was in operation.It is clear that valley-bottom gullies along the Kano channel changed from the typical, savanna shallow types (pre-Tiga) to deeper ones (post-Tiga). For example, the depth of the gully mouth increased from 1.3 m to 2.02 m, the gullies lost their alluvial deposits on the gully floors and a dual-cycle gully shape was established. The incision was rapid and intense, lasting not more than three wet seasons.It is argued that the construction of major dams on savanna streams would usually escalate fluvial processes on the valleyside slopes downstream, and that the maintenance of perennial flow in the relevant river channel would shorten the relaxation time required for a new equilibrium to occur.