Sources of Beauty and Life
In order to describe the formation of a river system, perhaps it is best to start with a brief explanation of the essential role they play in not only shaping the landscape all over the globe, but also the vital function they have in sustaining life in the many forms it takes on our planet. As rivers move downhill and pick up more water from streams and tributaries, they become such a great source of power that they're able to cut through rock and form V-shaped valleys and canyons which create scenery that has awed the human race for centuries.
Many poets have likened a river to a human soul as it travels this world and onto some greater unknown source. The geography molded and carved out by the erosive power of rivers has never stopped being a muse for the artistic human race.
Far more tangible however, is the life sustaining benefits of a river system. Rivers provide food for countless species of plants, trees, birds, mammals (including many of those listed in our comprehensive guide to endangered mammals), vertebrates, and animals (to include humans) that live in or around them.
Obviously, many on that list don’t live in rivers, but they rely on it for food to eat and water to drink. Areas around rivers also provide the fertile fields we need to grow crops and the opportunity to use the technology we developed to divert the water from rivers to irrigate what we grow.
The Role of Gravity
In the beginning, there is a source of water, which is so minute that it is hard to fathom how such a small trickle of water can end up being a vast and sometimes raging torrent by the time a river runs its course and joins a larger body of water such as a lake, sea, ocean or bay.
That transition zone where the waters meet is known as a river delta. Many rivers have deltas, which are marked by a multitude of divisions in the main body of the river, known as channels, that then meet the larger body of water at different places. Let’s return to the beginning, however, since that is the focus of our article.
A river is defined as a fairly large natural stream of water following a definitive channel and fed along its course, which may be marked by a series of diverging and converging channels or by smaller tributaries such as creeks, streams, and brooks, and emptying into the larger body of water we've already discussed. A river functions as a drainage system no matter where it is or what the topography of the landscape might be. As such, gravity is a leading factor in where it begins and where it's going to run to. Therefore, most rivers begin in the hills or mountains where snowmelt or rain water, depending upon the climate, can no longer be held in check by the land.
Origins, Headwaters, and Continuous Feeds
As the snowmelt, runoff (known as rills), and rain collect up there, eventually the land can't hold the water and it flows downhill in gullies and streams. The rivulets of surface water, or rills, gain momentum and the first stage of erosion takes place as the rills dig into silt and soil forming channels known as runnels. Runnels converge into continuously flowing channels known as brooks, which will later become rivers as they gain water from this same process downstream of its course. Some rivers also begin at springs, marshes and lakes. In the tall mountains, glaciers can be the origin of a river.
The point of origin is necessarily the highest point of the watercourse in almost all cases. Whatever the source of the headwaters, streams move down by way of the easiest course due to the force of gravity.
Streams will then meet other streams that are serving as drainage systems for wherever they emanate from. The drainage basin for a given area is also known as a catchment area. The meeting place of these water flows is known as a confluence and the smaller stream is designated as a tributary. Tributaries are, therefore, the key to the formation of a river system, and by their definition, they are smaller streams flowing into bigger ones.
Onward to the Sea
A river builds volume and momentum and becomes the main channel for however many drainage systems it passes through until it dumps into the sea. So before a river reaches its final destination, a river system is the totality of all those streams, brooks and tributaries that add to it along the way.
The river itself is the result of an immense amount of drainage for all the land it serves. As a great example, read about the formation of the Colorado River to see how that mythical river started and carved its way through the awe inspiring Grand Canyon.
Science Clarified: https://www.scienceclarified.com/Qu-Ro/River.html
Cameron, Ward, "A River Runs Through It – River Mechanics," Mountain Nature https://www.mountainnature.com/geology/Rivers.htm
Image credits/Daniel P. McGoldrick