I’m essentially asking how a river “starts”, since I generally know that rivers mostly ends flowing into the ocean.
If you could, please use the Yangtze river as an example.
In: Physics
Rain, mainly, flooding off land into small streams which come together to make rivers. There are also rivers from lakes that flow to the sea, with the lakes getting water from surrounding land and rivers going into them.
Rivers have a source. These typically start in higher elevations and are fed from melting snow/ice and rain. It could be a lake or stream or the like. The tributaries (streams or smaller rivers) combine and create larger rivers. The Yangtze’s source is tributaries in the Tibetan Plateu fed by glaciers melting and snow melts.
Rain, snowmelt, lakes, and springs.
With rain and melting snow, the water falls from the sky, flows downhill, and because of how hills work the water aggregates into tiny streams. Some also absorbs into the ground and is taken up by plants, but that’s not the focus right now. The streams join up into creeks which become increasibgly larger rivers as they join. Basically, a river starts wherever the cartographer decides he wants to stop drawing branching streams, and where those paths usually conjoin. The Yangtze, which you asked to use as an example, is formed this way from snowmelt in the Quinghai mountains in Tibet.
Lakes can start rivers too by basically overflowing their banks or if a bank gives way. Water takes the easiest path downhill from there. Lakes are (usually) fed by rivers though so this might not count.
Springs are where water from the groundwater breaches the surface. These usually form small ponds with no apparent source before overcoming their banks and making creeks/streams/rivers depending on just how much water is coming out.
It’s all rain, or snow!
Rain falls across the land and flows down hill. It collects first into tiny rivulets, then into streams and creeks, and then into rivers which can merge with each other.
The same with snow, as it melts.
The region of land that collects all of the water into a particular river is refered to as that rivers “watershed”, because that land is shedding water into it.
Water evaporates from the oceans, becomes clouds, which drifts over the mountains and falls on the mountains as snow.
In the spring and summer, the snow melts and becomes streams which become rivers which eventually flow into the oceans and the cycle repeats itself ad infinitum.
Snow melt, rain water runoff, lakes and other bodies of water can all supply rivers with water.
The water itself comes from precipitation; rain, hail, sleet and snow, for example. This either flows directly into the river or forms ice at higher elevations which slowly melts and feeds into the river.
Precipitation starts from evaporation; typically water from the ocean evaporates into water vapour and floats into the sky (although water can also evaporate from rivers, lakes or anywhere else water is exposed to the air).
The Yangtze, however, is complicated. The “source” (or “start” of the river) is debatable. To make matters worse, it’s actually composed of several smaller rivers (or tributaries) that ultimately combine to form the Yangtze that we know. For the purposes of this answer, I’ll use the Geladangdong source. This is the end point of a glacier, which feeds meltwater into a tributary which ultimately becomes the Yangtze River.
And why is the mouth of the river so far from the headwaters?
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