As you go from upper course to middle course to lower course:
The upper course, also called as the upper stage, is characterized by valleys and where the water flows with very high velocity. The upper stage is also called the mountain stage because this stage is in a very steep part of the river.$^4$
In the mountain, there is not a singular start of the river. Rather many smaller streams feed into the river. This smaller streams are called Tributaries.$^4$
When two rivers join together, that point is called the confluence. When two river systems are draining on either side of a mountain, that mountain is called the water divide.$^4$
The middle course is also called the matured stage. The large amounts of vertical erosion and the deep valleys of the upper course is not present in this stage. In this stage, lateral erosion becomes dominent. As a result the river widens. The volume of the water also increases. The river is still deep. At this point, the slope is moderate.$^4$
At this stage, the valleys are extremely broad and has a very small slope. At this point, the dominant physical process is deposition. The river is shallow. Later in this stage, the valley may become entirely flat. When this happens, the valley is called a floodplain. When the main river splits into many different smaller rivers, the smaller rivers are called distributaries.$^4$
The drainage basin/watershed area is the area in which the river system is. The area of land in which water drains into the river is called the drainage basin. Any flowing water within the drainage basin will end up within the river. The watershed boundary is the boundary of the drainage basin.$^6$
In the stages section, I referred to physical processes of rivers. But what are they? There are three main physical processes in a riverine system. Erosion, transportation, and deposition. These three depend on the amount of energy present in the river.$^7$