When it rains water collects in little puddles and hollows in the uneven earth. Sometimes the water sinks into dirt until the dirt can hold no more and the water forms the puddle. If the rain continues to fall it forms a surface, rippled and blown by the wind. The puddle may be muddy, or it may be clear depending on whether the water is stirred enough by the wind or by an animal or maybe by a child, wrapped up for the weather and wearing rubber or plastic boots.
Some rain falls at altitude in mountains or hills and falls on rocks, trickling down into cracks and crevices. Some rain falls on plants and is absorbed or runs down to the roots and is absorbed by them. Such rain takes the shortcut back into the air as the plant pumps it to the leaves and it is expired into the air.
Much of the rain that falls is not seen again for a while. It trickles through the soil, through open spaces between and within rocks. Some it soaks into cracks which take it deep within the earth to aquifers which can hold the water for years, decades, or centuries. Some it soaks into soft and soluble rocks like limestone and can form caverns underground. Sometimes this water evaporates underground and creates the beautiful rock forms that we can stalactites or stalagmites.
Some of the water remains on the surface and is joined by water seeping through the soil or the rocks to form little streams. The visible part of the stream is not all there is to a stream. Most streams have rocky or pebbly beds, or gravel or sand beds and water flows through those, as well as the free-flowing visible water. Indeed, some streams are not visible at the surface with the water running under the pebbly bed. Unless the stream is in flood that is, when the water fills the channel perhaps to overflowing.
Up in the mountains or the hills the small trickles of water tumble over or under the rocks and gradually merge to form small streams. Water oozes from wet soil and sometimes springs from rocks where the aquifers reach the surface. Rocks stick up through the water, little waterfalls chute the water in glistening curves and air mixes with the water to produce little white water patches. Vegetation hangs over the little streams, moistened by the stream and dripping water back into. Water loving mosses abound, and trees dip their roots.
The stream is continually renewing. This is not the same steam that you saw here yesterday. That water has flowed on perhaps by now reaching the sea. This water was yesterday floating in the sky in the form of a fluffy white cloud, or a grey dense cloud, or merely as an unseen vapour, measured only by humidity. Gravity’s imperative call pulls the water down from the hill.
There may be life in the water. Small insects and similar tiny organisms survive in the tumbling water, and even smaller little animals, from single cells animalcules to tiny multi celled beings live in it. They creep over the rocks and pebbles, feeding on debris that falls into the water or on little plants that also cling to the rocks, or each other.
As the stream progresses downhill it gathers more and more water from tributary trickles and merging flows. It deepens and flows over rocks which higher up it would have flowed around. Pools form and pools make it easier for life to exists. Maybe crustaceans and fish could now be lurking in the pools and even in the deeper less turbulent flows. Such life would also attract birds such as ducks.
All this life lives on a knife edge. A sudden storm upstream may swell the flow to many times the normal rate, moving rocks and boulders and even reshaping the banks of the stream. This would be a disaster for the wildlife in the stream sweeping it downstream and out of its usual environment into areas to which it is not adapted. Such storm events shape the little streams much more than the every day trickle.
A stream which I’m describing has a steep course. It may sweep past last stones and boulders and drop many metres for each dozen or so metres traversed. In many places it will reach places where the underlying geology changes and a waterfall forms or a cascade. A cascade is where the stream drops several metres in a short distance and the bed is full of boulders and rocks which may be totally or partial covered by the stream.
A waterfall usually drop into a pool caused by the water falling on and removing rocks and boulders. The spectacle of the drop and the pool makes it very attractive and people will sometimes hike for miles to see a waterfall.
As the stream becomes a river it becomes more placid. More life lives in it, it is broader and deeper. We can fish in it, float boats on it, and use it as a highway. We often live by rivers as they generally follow the easiest route through a sometimes tricky landscape. Large cities are more often than not found on rivers for this reason. Farmers on the banks of a big river want to trade with people up of downstream.
We tend to forget, in these days when we travel mainly by road, that rivers were once the prime highways, and indeed our principle roads frequently follow the route of a river.
As a river reaches lower altitudes it become more placid, as I have mentioned. People live next to it, and so they are in danger of flooding if a storm hits up river. A river is constrained by nature banks, but these many be breached. Consequently we have tamed many rivers, building large earthworks to constrain it to its usual course in the case of floods. However, the flooding of a river and the bursting of its banks deposits nutrients on to the soils around the river.
The ancient Egyptians knew this and worked with the annual flooding of the Nile. They developed geometry and mathematics to reinstate farm boundaries after floods. They also developed a legal system at least in part to legislate the inevitable disputes.