The upper course : closest to the source of a river. the land is usually high and mountainous , and the river has a steep gradient with a fast-flowing water. there is a lot of vertical erosion.
The middle course - where the river starts the slow down. it gets wider. and travels in winding loops. there is a lot of lateral erosion, deposition and transportation. the middle course can take up most of the river
The lower course - the last course if a river . the gradient is at its gentlest, as the river approaches the sea. rivers usually have a wide, smooth channel, and water moves it slowest.
I hope this helped a bit !