The Ox-bow lake is a typical water feature formed in the ____ course of a river.

Ox-bow lakes are formed in the lower course of a river where the gradient is gentle and the river meanders widely. When a meander loop becomes extremely curved, the river may cut across the neck during floods, abandoning the loop and creating a crescent-shaped lake. In the upper course, rivers are too fast and steep to form meanders, while the middle course is where meandering begins but ox-bow lakes are less common. The lower course's slow-moving water and flat land are ideal for ox-bow formation.