As we move downstream, more and more medium-sized streams have emptied into the main channel, contributing to the flow of what eventually becomes a river.
The channel flows wider and deeper flowing through a wide flat floodplain along the bottom of the river valley. Because the water runs more slowly here, some sediment settles out, forming a sandy or muddy substrate on the river bottom.
Eventually we reach the river's mouth, where it empties into yet another body of water, perhaps a larger river or a lake. For example, in Ohio all rivers and streams flow either into the Ohio River or Lake Erie.
Any sediments, debris, or contaminants still carried by the river, even from the farthest reaches of the headwaters, will be emptied into the receiving waters. As a result, what we find, or don't find in our major rivers, Great Lakes, and coastal areas will provide us with clues about the health of all of our nation's diverse ecosystems.
Bringing it all together - Watershed Management.