(CN) — Rivers are the lifeblood of many communities and host some of the world’s most diverse ecosystems. They have also become increasingly strained in recent years due to damming, diversion and sediment mining coupled with climate change.
Societies have always sprouted from major waterways because of the abundant resources they offer and the long-distance trade they enable. Those such as the Ganges, the Mekong and the Nile were the world’s first interstate freeways.
According to the authors of a new study published Friday in the journal One Earth, over three billion people worldwide currently live near a major river which provides the central source of resources, agriculture, trade and energy production for an increasing population.
Many communities near the poverty line depend almost entirely on rivers to earn a subsistence living or to earn cash through trading goods. These people are especially vulnerable to the increasing effects on rivers caused by climate change and unsustainable land use.
Rivers already stressed by climate change are being simultaneously bombarded with the task of supporting ever growing numbers nearby, and some are facing ecological collapse as a result. The authors say this is inhibiting efforts to meet the UN Sustainable Development Goals on schedule by 2030.
“Rivers respond to disturbances, such as changes in water or sediment ﬂux, through self-adjusting processes of erosion and sedimentation,” notes the study. “These responses typically involve feedbacks that impart some resilience, allowing rivers to absorb a degree of change. Climate change, as manifested through a complex global pattern of future ﬂoods and droughts, presents a background stress that is increasing through time and pushing this ﬂexibility to its limits.”
Every change made to the flow of a river has a knock-on effect down the line. In nature, these changes happen slowly – it can take millennia for a river to cut out a new pathway and alter its course.
That gradual rate of change allows for the surrounding banks to adjust slowly and for the sediments composing them to follow a natural course. The addition of a river channel quickly dug by people to divert a flow of water, particularly a hastily planned one, can lead to floods and droughts nearby as the banks struggle to cope.
Compounding water diversion with other stressors such as damming, sediment mining, groundwater extraction and others.
Stephen Darby, professor of geography at the University of Southampton and co-author of the study said in an email that “The scale and rate of change of other stressors — not just climate change, which receives most attention, is so large that often these are more immediate threats to system change.”
The authors worry the understandable concern about climate change is leading decision makers to overlook these more immediate threats.
Dams constructed for hydropower, flood control, irrigation and water supply are set to decrease the number of remaining free-flowing rivers by 21% — there are over 3,700 dams planned or under construction today. Damming a river alters its flow, traps sediment, leads to riverbank incision and instability. In tropical areas this causes vegetation decay, releasing methane, a potent greenhouse gas.
Dams may take decades to design and build, but their effects within a river’s ecosystem can be felt almost immediately.
One of the major rivers studied by the authors, the Mekong River in East Asia, home to 60 million people, exhibits a number of these issues. The intergovernmental Panel on Climate Change has designated the Mekong as one of the three most vulnerable deltas in the world.
Upstream damming and sediment mining, flow regulation and groundwater extraction are causing salt water to flow into the river channels. This kills nearby crops which depend on the river for fresh water, and heavily impacts agricultural production for millions of people. A lack of consensus from decision makers over water usage and which farming practices to pursue are compounding these issues and causing residents to flee.
According to Jim Best, professor of sedimentary geology at the University of Illinois and co-author of the study, our window for change is limited.
“The solution to many of these issues has to rely on improved governance — on all levels from local to international (as many large rivers cross national borders)," he said. "Of course, this isn't easy, but this will only come about with action on all levels — from local water advocacy to UN action, to better manage and sustain the world's great rivers. However, the timescale for action is short — with studies of some rivers indicating the possibility of riverine ecosystem collapse this century.”
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