Features in the middle course of a river system

In the middle course of a river system, several features can be observed. These features play a crucial role in shaping the landscape and influencing the flow of water. Among the given options, features such as interlocking spurs, meanders, and river cliffs are commonly found in the middle course of a river system. Rapids and waterfalls, on the other hand, are more commonly associated with the upper course of a river system.

1. Interlocking spurs
Interlocking spurs are elongated ridges that protrude into the river valley and alternate from one side of the valley to the other. They are formed when the river tries to erode the landscape but encounters hard rock formations. As a result, the river is forced to meander around these obstacles, creating a series of interlocking spurs. Interlocking spurs are commonly seen in the middle course of a river system.

2. Meanders
Meanders are sinuous bends or curves in a river's course. They are formed as the river flows through a gently sloping valley in the middle course. The river erodes the outer bank of a bend and deposits sediment on the inner bank, causing the bend to become more pronounced over time. Meanders are characteristic features of the middle course of a river system.

3. River cliffs
River cliffs are steep, vertical or near-vertical slopes that border the edge of a river. They are typically formed through erosion processes, such as the undercutting of the riverbank by the flowing water. River cliffs can be found in the middle course of a river system where the river has enough energy to erode the landscape but is not yet at its full force.

4. Rapids and waterfalls
Rapids and waterfalls are usually associated with the upper course of a river system, where the gradient is steeper and the river has more energy. In the middle course, the river tends to have a gentler gradient, and the flow of water is more calm and gradual. Therefore, rapids and waterfalls are not typically found in the middle course of a river system.

Overall, the correct answer is option 'D' - 1, 2, and 3 only. Interlocking spurs, meanders, and river cliffs are commonly found in the middle course of a river system, while rapids and waterfalls are more commonly associated with the upper course.