The underground river is flowing from the raining area of the mountain side of Ruler's Land under the mountains to the deserted area of Land of People. It is essential for both lands. The raining area and areas near it would suffer from devastating floods and landslides making the place inhabitable, if most of the rainwater would not be absorbed through the ground to the tunnel network that forms the underground river. The wells reaching the underground river are essential for the villages in the desert providing water for both the people, animals, and crops. Without them there was no way people could live in the desert.
The raining area
The dividing mountains prevent the clouds getting to the deserted side of the land. The area in Ruler’s Land where the clouds stop, and the rain falls down is called the raining area. In the raining area it rains year around, but during the over half year long raining season the rains are very intense. The raining season has periodical nature as it is formed of heavy rains that last for several days and the few rainless days between the rains. The rains during the raining season are so heavy that it feels like somebody is pouring the water from bucket. The rain obscures the view and sound, and makes it hard to work outside, so during the raining season people’s lives are focused to the rainless days and is sort of on pause during the rain. The rains outside the raining season are like regular rains and people of the area are not bothered by them. They actually call the season of regular rains to the rainless season.
The soil and the plantation of the raining area has evolved to survive the heavy rains and to let water go through it easily. The soil and plantation of the area forms sponge like structure above the cracked bedrock. The water pours to the cracks that forms the base of the underground river. There are also larger chambers under the ground that works as water containers that stores the water of the raining season. The water is then pouring out during the rainless season keeping the constant flow of the water in the underground river.
The towns in the raining area are build above visible cracks in the ground. Extensive sewer system runs the rainwater to the cracks in the towns. That way the towns hardly ever floods even if the buildings could easily cause the cumulation of rainwater to the streets.
Picture: Basic construction of towns in the raining area. The buildings are standing on pillars, so that rainwater gets under them. The sewer system runs the water from under the buildings to the cracks in the bedrock and to the underground river. The road is curved and covered with temporary roof made of leaves to keep it dry.
The wells of desert
It is said that the wells were gifts from the gods to the people when the desertification of the area started. The wells dig deep into the ground so that they reach the underground river. There are stories of wells drying out but nobody living remembers that ever to happen. People of the desert however respects greatly the wells and water they provide and wasting water is thought to be wrong. Because the wells are gifts from the gods to all people not just villagers, no person is prevented of using the wells. Of course there has been times in history where some group of people had decided to keep some well or wells only for themselves and prevent others of using them, but that has never ended well. That has strengthened the concept that the wells are meant to be free for all.
There are more wells in the desert than there are villages. The original population of the deserted area was lot higher than the modern population and many of the wells have been left to the area now uninhabited. Some of those wells has been destroyed in the geological changes or just by being neglected for so long. The excess of the functioning wells enables the variation of the desert’s population. Villages are formed and abandoned as the population of the desert keeps on changing.
There is not much life in the underground river as it is very harsh place for anything living with the lack of light and nutrients. At the beginning of the river the cave walls are inhabited by fluorescent mould that gets its nutrition from the nutrients and mote that is left to the water.
In the geothermal area of the river there are colonies of extremophiles. They get their energy via chemosynthesis from the hydrothermal vents. The people of the world will most likely never know about them, and they are rather irrelevant to the river ecosystem or the world, but they are still there.
Where there is an external access to the underground river, there can be several forms of life inhabiting the river or it's cave. Bats, amphibians, and insects are known to be found in these caves. Fishes can be also found from the areas close to accesses the sea. These fishes have evolved from saltwater species to be an estuary species. Because of the lack of the oxygen fishes do not survive deeper in the river even if they could swim deeper.
Unlike the rivers flowing on the ground the underground river does not just flow downwards. There are areas where the water is flowing upwards in the cavern because of the lower pressure in those areas or because the geothermal heat boils the water that either burst upward or evaporates.
There are also several places where the river splits as it fills all the cracks even those that are dead ends slowly enlarging and deepening them. When the cracking of the ground changes some of these caves can be left empty as the river founds new ways to flow. Other way for the empty caves to form is when an opening to top opens rising the pressure in the caves and thus preventing the water to rise upwards in the caves. The best-known cave formed this way is the cave of underground lake the shadow steppers use and the cave system it is connected with.
Picture: Underground lake
Underground / Subterranean
Map: Basic idea of how the underground river flows from the raining area under the mountains to the deserted area and to the ocean. Disclaimer: I have no idea of the final shape of the world map, so the shape of the continent, the mountain range, the raining area and the underground river are completely randomly drawn.
Picture: The rainwater runs easily through the sponge like soil that is formed from roots, slowly decaying plant parts and dirt. Then it goes to the cracks in the bedrock that leads to the underground chambers and from there to the underground river.