Landscapes are areas that are shaped by various forces, natural or artificial, that interact with each other, and rivers are important factors in their formation. The river water starts from the high peaks where the ice melts, and runs through the landscape, eroding the soil as it goes. It drags soil particles , thereby changing the geomorphology of the environment, and when the currents slow down, where the slope practically disappears, it deposits the sediments and nutrients that it has collected along its path, also modifying the structure and shaping the ecosystems of the coast there. where the river empties.
The tributaries and confluences of a river end up flowing into it and form a network of channels, which are kept isolated from each other by high ground or mountain ranges. In this way, a territory can be defined according to the river into which its surface waters flow. This unit of territory is called a hydrographic basin , and the edge of the basin, formed by the line of summits, is called the watershed .
In the same region, not all river basins carry the same amount of water. There are some that have large, long and mighty rivers, while others are very arid, and discharge an insignificant amount of water. In accordance with all of the above, a transfer is an infrastructure that allows the transport –or transfer– of water from a basin with a large amount of water to another much drier one. Its main objective, of a socioeconomic nature, is to provide water supply for consumption, irrigation or hydroelectric power generation.
However, the construction of infrastructure for transfers is not without problems; In some cases, the environmental impacts that they can generate make them unaffordable from an ecological point of view.
An immediate consequence of an infrastructure of the magnitude of an inter-basin transfer is the barrier effect . Although between the constructions of a transfer there are elevated aqueducts that, supported by pillars, facilitate the passage from one side to the other, in other cases artificial channels are built at ground level, which limit the movement of animals , unable to cross the water .
Plants whose fruits or seeds are carried by animals — zoochory — are also limited in their expansion and development. But in addition, the channeling of large volumes of moving water also produces transverse winds. This alteration in the natural movement of the winds also prevents the passage of fruits and seeds that are moved by the wind — anemochory —.
It also hinders pollination between populations on one side of the barrier and the other. Whether pollen is carried by the wind or by pollinating insects, these winds can partially impede passage.
In short, these constructions reduce gene flow between populations and fragment natural habitats. It is good to remember that habitat fragmentation is considered one of the five great drivers of anthropogenic global change.
But the barrier effect is not the only one, that same infrastructure can become a corridor that crosses natural barriers . In fact, that is, in principle, the function of a transfer: to move water from one basin to another through a biogeographical barrier, which is represented by the watershed.
In this way, animals, plants or microorganisms that previously only lived in one of the basins can move, using the transfer, and colonize the neighboring basin . Species that were not in contact before start to share the same environment, and new competitive relationships are created for which the populations are not previously adapted. In this sense, the hydrographic basins function as islands of biodiversity , and the transfer as a bridge that unites them.
This acts to the detriment of biodiversity. The species that are more suitable will be the dominant ones in the ecosystem, and the less suitable ones may disappear. This homogenizing effect converts two ecosystems that were previously different, into one, mixed between them, but with less richness of species than the one that both basins had separately.
If any of the basins is affected by an invasive alien species, a transfer is a new way to help it expand to new territories that it could not reach before, and generate new impacts where it arrives.
The impacts of a watershed transfer not only occur in the region where the infrastructure is built, but also have significant downstream effects .
The receiving basin of the transfer finds an unexpected volume of water to which its ecosystem is not adapted. The riparian vegetation, which is responsible for supporting the substrate against erosion, is not prepared to support the higher flow in a sustained manner, and will increase the erosion of the channel.
On the other hand, the donor basin loses flow, which translates into a lower contribution of water with respect to which the ecosystem is adapted, with possible effects associated with drought; and less sedimentation , which implies an impoverishment of the ecosystems near the mouth.
At the mouths of rivers there are particular conditions, in which seawater meets freshwater. The reduction in the flow translates into a gradual invasion of seawater in the surroundings of the mouth and the consequent salinization of the soil. If this environment is of a richness and biodiversity worthy of conservation, such as the Ebro river delta, carrying out a transfer of the Ebro can be disastrous .
But the impacts do not end there, they can reach the sea. Coastal ecosystems near the mouth of major rivers are adapted to a supply of nutrients and sediments that, in the event of transfer, will be decimated. This can lead to the destabilization of these ecosystems and even to the extinction of species, as is being observed with the vaquita marina in the Gulf of California, whose decline is caused, in part, by the impact of the transfers on the ecosystem. of the Colorado River.
Brusca, R. C. et al. 2017. Colorado River flow and biological productivity in the Northern Gulf of California, Mexico. Earth-Science Reviews, 164, 1-30. DOI:10.1016/j.earscirev.2016.10.012
Davies, B. R. et al. 1992. An assessment of the ecological impacts of inter-basin water transfers, and their threats to river basin integrity and conservation. Aquatic Conservation: Marine and Freshwater Ecosystems, 2(4), 325-349. DOI: 10.1002/aqc.3270020404
Ibáñez, C. et al. 2003. The environmental impact of the Spanish National Hydrological Plan on the lower Ebro River and delta. International Journal of Water Resources Development, 16, 485-500.