When a piece of the Earth's crust first emerges from the sea and rain begins to fall on it, the water running off begins to erode it. As the water runs down hill, it erodes channels that become organized into a drainage pattern. As running water deepens its channel, it encounters rocks of different hardness and adjusts its channel in response.
Soft layers erode faster than hard layers.
This simple statement is an important principle of erosion which that applies to almost every natural scene. Eventually, hard layers or other hard parts of rocks stand out in bold relief, and softer layers or parts retreat into swales, gullies, and valleys. Geologists call the process differential erosion. For much of their history, the major rivers may be able to maintain their courses across rocks of different hardness. Differential erosion will produce rapids or falls where the rivers cross harder rocks, but for the most part the rivers will maintain their general course.