The source of the Rhein is a small lake. Even the Rhein has several small sources, can they supply such a big river only by rain and spring water? How big are those reservoirs?
In: Earth Science
You may want to play with [River Runner](https://river-runner.samlearner.com/), which is an app where you can trace the path of a raindrop downstream from any place in the continental U.S, including a satellite-view animation of its journey. Won’t help you answer for the Rhine, but it can be fun to click anywhere in the mid-US and watch it eventually end up in the Mississippi and Gulf of Mexico.
People take little buckets, fill them with ocean water, make a humain chain and pour it in the river. All during the night so you don’t see them.
Yup, it’s all rain. What you may be missing here is that rivers and lakes have things called catchment areas. When rain lands on the ground, it wants to continue to follow gravity. If the ground is impermeable, it’ll roll downhill until it reaches a point where it can’t go any further down. If the ground is permeable, the water will seep into the rock and keep going down through that until it hits an impermeable layer of rock. Then, it’ll follow that layer of rock down to the lowest point too. Over time, this can form caves as the water erodes the permeable layer its flowing through. If the water reaches a cliff of some kind, where the impermeable layer its following sticks out into the open air, it’ll continue to follow gravity, appearing as a spring, and eventually forming a lake if allowed to pool. If the layer of permeable rock is very deep, you can get a huge underground reservoir building up, the top of which is the water table – this is what wells are digging down to reach. Where this water table would rise above the surface of the rock, a lake forms. Here’s a [diagram](https://theneutralview.com/wp-content/uploads/2017/07/10-728-How-A-Watershed-Works-1400×681.jpg) of that. Notice that the surface of the river lines up with the water table.
Of course, this is way more complicated than I can fit into an ELI5, but the idea is there: It’s not just the rainwater that drops on the lakes and rivers themselves, but also all the rainwater that falls on the hills and mountainsides that follow the rock layers down to those lakes and rivers. That’s a hell of a lot of water, cos mountains are pretty big.
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