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Strong connections between regional climate and the topography of rivers have been identified by researchers in the UK. Shiuan-An Chen and Katerina Michaelides at the University of Bristol and colleagues at Queen Mary University of London and Cardiff University discovered the relationship by combining an extensive study of satellite data with numerical modelling. Their analysis could offer important insights into how landscapes could evolve in the future, as climate change brings widespread changes to regional humidity levels.
For some time, geoscientists have understood that the topography of Earth’s land surface is strongly tied to climate through the processes of rainfall, runoff and erosion. The evolution of rivers over time is an important example of this effect.
The topography of a river can be characterized in terms of its longitudinal profile, which is plotted in terms of elevation versus downstream distance. Such profiles reveal two main ways that rivers can make their descent. One common profile resembles a linear ramp with the river falling in a straight line, while the other looks like a concave surface that is steep at the top and flattens out towards the bottom.
However, researchers have struggled to draw links between the longitudinal profiles and regional climates of rivers. One major difficulty is a lack of data on rivers in drylands, which comprise around 40% of the Earth’s land surface.
In their study, the team aimed to learn more about the connection between climate and profile using data gathered during NASA’s Space Shuttle program. These images contained the longitudinal profiles of over 330,000 rivers spanning the globe, allowing the researchers to explore how longitudinal profile concavity is affected by climate zones of all types.
The team’s analysis clearly shows that longitudinal profiles are more concave in humid environments and become more ramp-like as aridity increases. To explain this trend, they then employed a simple numerical model that accounted for effects including stream flow and erosion. It showed that the shapes of longitudinal profiles strongly depend on the rate of change of stream flow as downstream distance increases.
How do rivers alter sea-level?
In humid environments, which have significant rainfall and runoff throughout the year, rivers tend to flow constantly throughout their length, and so stream flow increases with downstream distance. This means that riverbed sediments are continually transported downstream, thereby carving out concave shapes over time. In arid regions, rivers only flow sporadically in localized regions. As a result, sediment transport is far less frequent, and longitudinal profiles remain straighter.
The team believes that with improvements in computing power, the techniques could also offer important insights into how the topography of drainage basins could be altered by climate change.
The research is described in Nature.