When the river runs dry
New global-scale river research reveals a clear climate signature and challenges existing thinking. Ian Connellan reports.
Geoscientists have long believed that climate is the key sculptor of land surface topography – that landscape is an expression of the interaction between rainfall, runoff and erosion in drainage basins.
A river’s source-to-mouth elevation profile – its “long profile” – is a key indicator of a drainage basin’s evolution, so it ought to be possible to infer from it a regional expression of climate and its interaction with the land surface.
But researchers have struggled to detect climatic signatures in long profiles and to decipher the role of climate in their development because of a lack of relevant global data – in addition to the variable impacts of large and small geological processes, land surface properties and human activity.
Now, a new study led by Katerina Michaelides from the UK’s University of Bristol has discovered a clear climatic signature on rivers globally.
Key to the success of the research, published in the journal Nature, is a new, freely available, database of river long profiles generated from data originally collected by NASA's space shuttle.
The study team used specialist software developed by Stuart Grieve, at Queen Mary University of London, to develop the database, which includes more than 330,000 rivers across the globe.
The new research shows that while river long profiles tend to be “concave up” – think the inside of a bowl – in humid regions, they become progressively straighter in drier regions, and the reason for the differences is linked to aridity and streamflow.
“The long profile is formed gradually over tens of thousands to millions of years, so it tells a bigger story about the climate history of region,” says lead author Shiuan-An Chen.
“We would expect climate to affect the river long profile because it controls how much water flows in rivers and the associated force of water to move sediment along the riverbed.”
In humid regions, rivers tend to have flow in them all year round, which continually moves sediment and erodes the long profile into a concave-up shape.
As the climate becomes progressively arid (from semi-arid, to arid, to hyper-arid), rivers only flow a few times per year when it rains, moving sediment infrequently. Arid-zone rivers also tend to experience brief, intense rainstorms, which don’t create flow over the entire river length.
The study team used simple numerical modelling to explain the links between climate, streamflow and long-profile shape. They report that their results “illustrate that river topography expresses a signature of aridity, suggesting that climate is a first-order control on the evolution of the drainage basin”.
Michaelides says that long-held theories were “biased towards observations made in humid rivers, which are far better studied and more represented in published research than dryland rivers”.
“Our study shows that many river profiles around the world are not concave up and that straighter profiles tend to be more common in arid environments,” she says.
“I think dryland rivers have been understudied and under-appreciated… Their streamflow expression gives unique insights into the climatic influence on land surface topography.”
The study gives further proof – if any was needed – of the way satellite data and high-performance computing can combine to produce insights at previously unimaginable scale.
Grieve says this is revolutionising geosciences. “Our analysis of these data only scratches the surface of the potential that this fusion of data, computer power and geoscientific insight can offer us to understand our planet, as well as others in the solar system.”