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. 15 . 2.2.2.2.2 Lowering of Base Level for Tributary Stream. The average water surface elevation in the main channel acts as the base level for the tributary. The lowering of base level will increase the energy gradient in the tributary. This increased energy gradient induces headcutting and causes significant increase in water velocities in the tributary stream. Headcutting is the erosive process hy which a drop in gradient moves upstream. Applying Equation 2-8 to the tributary stream, it can be seen that the increase in slope must be balanced by an increase in sediment transport if fall diameter and water discharge remain constant (Figure 2-4). . . . 2.2.2.2.3 Raising Base Level for Tributary Stream. The raising of stream base level will decrease the energy gradient in the tributary. This decreased energy gradient, in most cases, causes significant deposition. This can be seen utilizing Equation 2-8 where a decrease in slope is accompanied by a decrease in transport capacity assuming constant conditions of water dis- charge and size of bed material. For example, an alluvial fan develops which in time can divert the river or reduce the waterway. A similar situation occurs naturally where a steep tributary stream draining an upland region reaches the flatter floodplain of the parent stream (Figure 2-5). . . . 2.2.2.2.4 Straightening of a Reach by Cutoffs. Straightening a chan- nel will significantly increase the channel slope. In general, this causes higher velocities, increases bed material transport, degradation, and pos- 5i bl e heaucutt ing of the reach. Thi scan resul t in unstab Ie river banks and a braided stream form. Straightening of a reach by cutoffs is very common in urban development. In order to design a straightened channel so that it behaves essentially as the natural channel in terms of velocities and.mag- nitude of bed material transport, it is necessary, in general, to build a . . .