WATERSHEDS A watershed is an area where the surface runoff from precipitation onto the land flows together toward lower areas such as lakes or oceans. As you can see in Figure 7-4 on p. 192, watersheds have a connected system of streams that work like the veins in your circulatory system; small veins collect blood in
your body's extremities, the small veins flow together and merge into larger ones, and the largest veins deliver blood back to your heart. Surface water is collected and moved through a watershed in a similar way. Small streams in the watershed’s extremities flow together and form larger and larger streams. Streams that flow into another are called tributaries, and tributaries merge into the watershed’s main trunk river. The watershed is all the land that contributes water to a trunk river, including all the tributary streams. The boundaries between watersheds of all sizes are elevated areas called divides. Watersheds vary tremendously in size and shape, from small mountain valleys to vast continental plains.