In the human circulatory system, for example, or in human lungs, like those represented to the left. The Meramec River is a medium sized river. It comes as no surprise that the widest rivers have the biggest watersheds. The Mississippi watershed drains about half the Midwest and eastern United States. Think about rivers as excess water disposal machines.
Rivers transport water from wherever it is to the ocean.
In places where it rains more, such as in the temperate deciduous forest and the rainforest, there are more rivers and streams to deal with the large and consistent annual rainfall. In the desert, however, there is significantly less rain. Logically, fewer rivers exist in the desert. When it does rain, the rains are very short and intense. Since there is no vegetation to absorb the water, almost all the water drains immediately into dry river basins that for most of the year look like flat plains.