Rivers begin life as a trickle of water that originates high on a hillside or in a mountain range. Most of these thin runs of water, known as headwaters, emerge through the earth from underground streams. These submerged bodies of water are formed when rain or snow seeps into the ground, before the pressure of being trapped underground bubbles it back to the surface. This water is called surface run-off.
The course of a fledgling river’s surface run-off is steered by gravity, which will initially send the water flowing downhill in trickles, which will eventually meet with other parallel rills or tributaries, as it gathers momentum. Once these parallel rills unite, they form a stream. When more rills converge with the stream, a bigger flow of water is formed; a brook.
Guided by gravity and the surrounding geology, the brook flows on through the valley, its volume of water swelling with rain and groundwater. The brook becomes wider as it travels, and as its water level rises, it becomes a river.
It isn’t the weight of the moving water alone that dictates how wide or deep a new river is to become. It is the river’s load which will gorge out its path in the geology that surrounds it. A river’s load will include any rocks, stones, and other large particles, which wash along the new riverbed. As the river water pushes its load along, the bed of the river will deepen.
The speed of the moving water determines how quickly the load will erode the river’s banks, and how wide the river will become. As the river winds through the landscape, it carves out deep valleys in solid rock and deposits huge amounts of debris on either side of it.
Not all springs, brooks and streams that form into headwaters on high ground will become rivers. Many will remain small creeks, brooks, rivulets or tributaries. Those that do become rivers will forge their way through the geology of the earth until they reach, and sometimes merge with, other rivers, and then ultimately meet the sea.