Watersheds are defined by ridges. If you could stand in a river and watch the rain strike the exact top of the ridge, you would notice that the raindrop either flowed downhill into your watershed or over the ridge into an adjacent one. In Tennessee, the ridges defining watersheds are much more obvious in East and Middle Tennessee than in West Tennessee, which is predominantly flat. In West Tennessee, some watersheds are prone to flooding because the land is so flat that water covers adjacent fields when rivers top their banks. In East Tennessee, some watersheds are prone to flooding because the land is so steep that rainwater flows to the river faster than it can be carried on to the flood zone.
Some soils can absorb rainwater faster than others.
In West Tennessee, where the soils have a high sand content, the ground quickly soaks up rainwater. In the ridges and valleys of East Tennessee, where the soils have a higher clay content and limestone is exposed, rainwater quickly rolls off the landscape. A drop of water falling on a farm in Wayne County hits a tin roof. It slowly travels over the farm and eventually may make its way into the Green River, placing it in the Green River watershed. From there, the Green River carries it to the Buffalo River, so it's in that watershed as well.