Lotic ecosystems comprise bodies of fresh water in constant movement, such as rivers, streams, and springs. These form the hydrographic basins, which play a fundamental role in the water cycle, as fresh water from the river systems circulates in them until it reaches the sea, where it flows into the sea. In addition, relief and climate are determining factors in the formation of rivers, but how? Do we know where the water for the rivers comes from? And what do we know about how rivers are formed?
If you want to learn more about the origin of these ecosystems and understand what rivers are, check out this article on how rivers are formed, where you can find out where rivers are born and what their main characteristics are.
What a River is and How Rivers are Formed
Rivers are bodies of surface water that flow thru channels from areas of higher altitudes to areas of lower altitudes by the effect of gravity, known as the course of the river.
These natural fresh water streams mostly originate in high mountain areas due to the accumulation of rainwater, springs, ice and snow. The water accumulates and concentrates in depressions, forming lakes that later give rise to the first riverbeds. These riverbeds are sculpted by the action of water erosion and, when they are worn away, reach depths that reach the saturated layer, thus allowing groundwater to rise to the surface, adding to the river’s flow.
In relation to its course or course, it is possible to differentiate 3 parts of the rivers:
High course: close to the source in mountainous areas, it is characterized by a steep slope over which the waters, with a low flow, flow at a high speed. For this reason, this part of the river has a very high erosive capacity.
The middle course: more typical of flat areas, it has areas of erosion and accumulation. In this section, the river has a greater flow and its waters circulate at a lower speed than in the upper course.
Lower course: it is located at the mouth of the river and, due to the low water velocity, sedimentation processes predominate, which can lead to the formation of deltas, estuaries, or estuaries.
Depending on the branching of the rivers, a classification system is established according to orders:
First order rivers are those that have no tributaries.
Second order rivers are formed by first order rivers (without tributaries).
Third order rivers are formed by the union of second order rivers and so on, thus giving rise to tributaries.
The territory thru which the waters flow into the same river and which may or may not include such branches is what we know as the hydrographic basin.
Where the Water for the Rivers comes From
The inflow of water to watersheds takes place mostly thru precipitation. This is a process in which all the moisture that is stored in clouds reaches the land surface in the form of rain, hail, snow, mist, or dew and becomes runoff or surface runoff, feeding rivers.
But rainwater can infiltrate into the ground and form groundwater. Groundwater is located in the saturation zone, where it is stored and completely saturates the pores or fissures of the subsoil. When rivers reach the depth of the saturated layer, the groundwater comes to the surface, forming part of the river flow.
Likewise, rivers can be directly from a lake in which water has accumulated from precipitation, located at a very high altitude where the water begins to overflow at one point and the water begins to form grooves in the ground, giving rise to a river.
However, rivers can also be born directly from groundwater located in high mountain areas. In these cases, a spring, which is a natural source of water, can be observed. A hole, of greater or lesser size, can be observed in the rock face thru which water emanates. As soon as enough water continues to flow out, the river begins to form. Of course, these rivers are later joined by water from rainfall, melting snow, etc.
Characteristics of Rivers
The following are some of the most important characteristics of rivers:
- The length of the river is the distance from its source to its mouth.
- “Flow” is understood as the amount of water that circulates thru the river.
- Rivers are agents that shape the relief and landscape.
- The erosive power of rivers is determined by their flow and velocity.
- The erosive capacity of the river can take place thru processes of abrasion, corrosion, wear and tear, and also by hydraulic action.
- The morphology and flow of rivers depend on the characteristics of the terrain, flow, vegetation, and sediment load.
- The course of the river varies in morphology and flow from its source to its mouth.
- The origin and evolution of the largest existing rivers are strongly influenced by plate tectonics, either directly or indirectly.
- River bends are called meanders.
- The physicochemical characteristics of rivers vary according to the nature of the basin thru which they flow. For example, in the case of granitic basins, the waters are soft, while in carbonate basins, the waters are hard.
Subway Rivers: What they are and How Subway Rivers are Formed
Rivers, lakes, and aquifers are all part of the drainage network that exists within a river basin. As long as we believe what our eyes perceive, we will continue to believe that the water flows that exist are merely superficial. However, we must remember that aquifers are geological formations that allow for the storage of fresh water as well as the circulation of that water. It is possible that we have been closer to these underground rivers than we realize since we were not aware of what we were seeing when exploring caves and other voids in the past.
What are Subway Rivers?
Subway rivers are bodies of water that flow through channels that are not visible from the surface of the earth, since they are located deep underground.
Natural subway rivers supplied by rainwater may flow thru underground cave or cavern systems. There are, however, manmade subway rivers that have been formed by humans as a result of urban growth and mining activities. Prospecting in the sima-mine region of Alzola (Guipuzkoa) in the previous century, for example, has given rise to an important network of caverns and galleries thru which the waters of the underground river mondmilch, or moon-milk river, circulate, since its water is white.
These underground rivers, whether natural or man-made, transport sediments (mostly calcareous, as this mineral is associated with their origin, as discussed in the following section) that end up replenishing the sandy strip of beaches where they run. However, the waters of these rivers do not always end up in the seas or oceans; they might also flow into springs or be pumped for extraction by drilling wells.
How Subway Rivers are formed
The geomorphological processes of karst that lead to the dissolving and collapse of limestone are critical in the construction of subway rivers and other subsurface water system structures such as cenotes and subway lakes. That is, the soluble rocks are diluted and shattered by the action of water, either from precipitation or snowmelt, giving birth to interconnecting caves and caverns that allow the fluvial current to flow.
When water erosion is exacerbated, the ceiling of these subterranean chambers may be undermined, favoring the creation of cenotes. These sinkholes or depressions in the earth that store water and are quite close to the surface and in touch with the outside might be regarded as subway lakes formed by dissolution processes (dissolution lakes).
Cenotes are linked together by tunnel rivers and differ from them in the arrangement of the impermeable layer of the aquifer systems. In cenotes and lakes, the impermeable layer develops a depression that enables water to accumulate, but in subway rivers, the impermeable layer stays inclined, allowing water to circulate via the hydraulic potential.
Finally, it is worth mentioning that cenotes are very abundant and important in the Yucatan Peninsula, where the Cenotes Ring stands out, formed 66 million years ago by the impact of a meteorite.