{"id":15592,"date":"2018-07-19T05:31:02","date_gmt":"2018-07-19T05:31:02","guid":{"rendered":"https:\/\/www.revoscience.com\/en\/?p=15592"},"modified":"2020-06-09T12:55:33","modified_gmt":"2020-06-09T12:55:33","slug":"study-finds-climate-determines-shapes-of-river-basins","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/study-finds-climate-determines-shapes-of-river-basins\/","title":{"rendered":"Study finds climate determines shapes of river basins"},"content":{"rendered":"

\"\"CAMBRIDGE, MA — There are more than 1 million river basins carved into the topography of the United States, each collecting rainwater to feed the rivers that cut through them. Some basins are as small as individual streams, while others span nearly half the continent, encompassing, for instance, the whole of the Mississippi river network.<\/span><\/p>\n

River basins also vary in shape, which, as MIT scientists now report, is heavily influenced by the climate in which they form. The team found that in dry regions of the country, river basins take on a long and thin contour, regardless of their size. In more humid environments, river basins vary: Larger basins, on the scale of hundreds of kilometers, are long and thin, while smaller basins, spanning a few kilometers, are noticeably short and squat.<\/span><\/p>\n

The difference, they found, boils down to the local availability of groundwater. In general, river basins are shaped by rainfall, which erodes the land as it drains down into a river or stream. In humid environments, a large fraction of rainfall seeps into the Earth, creating a water table, or a local reservoir of groundwater. When that groundwater seeps back out, it can also cut into a basin, further eroding and shifting its shape.<\/span><\/p>\n

The researchers found that smaller basins that are formed in humid climates are heavily shaped by the local groundwater, which acts to carve out shorter, wider basins. For much larger basins that cover a more expansive geographic area, the availability of groundwater may be less consistent, and therefore plays less of a role in a basin\u2019s shape.<\/span><\/p>\n

The results, published today in the\u00a0Proceedings of the Royal Society A<\/em>, may help researchers identify ancient climates in which basins originally formed, both on Earth and beyond.<\/span><\/p>\n

\u201cThis is the first time in which the shape of river networks has been related to climate,\u201d says Daniel Rothman, professor of geophysics in MIT\u2019s Department of Earth, Atmospheric, and Planetary Sciences, and co-director of MIT\u2019s Lorentz Center. \u201cWork like this may help scientists infer the kind of climate that was present when river networks were initially incised.\u201d<\/span><\/p>\n

Rothman\u2019s co-authors are first author and former graduate student Robert Yi, former visiting graduate student \u00c1lvaro Arredondo, graduate student Eric Stansifer, and former postdoc Hansj\u00f6rg Seybold of ETH Zurich.<\/span><\/p>\n

A climate connection<\/strong><\/span><\/p>\n

In\u00a0previous work<\/a>\u00a0published in 2012, Rothman and his colleagues identified a surprisingly universal connection between groundwater and the way in which rivers split, or branch. The team formulated a mathematical model to discover that, in regions where erosion is caused mainly by the seepage of groundwater, rivers branch at a common angle of 72 degrees. In follow-up work, they found that this common branching angle held up in humid environments, but in dryer regions, rivers tended to split at narrower angles of around 45 degrees.<\/span><\/p>\n

\u201cRiver networks form these beautiful branched structures, and previous work has helped explain the angles at which rivers join together to form these structures,\u201d Yi says. \u201cBut each river is also intimately connected to a basin, which is the area of land that it drains rainwater from. So we suspected that the shapes of bains could contain some similar geometric curiosities.\u201d<\/span><\/p>\n

The team set out to find a similar universal pattern in the shape of river basins. To do this, they accessed datasets containing detailed maps of all the rivers and basins in the contiguous United States \u2014 more than 1 million in total \u2014 along with datasets containing two climatic parameters for every region in the country: precipitation rate and potential evapotranspiration, or the rate at which surface water would evaporate if it were present.<\/span><\/p>\n

The datasets contained estimates of each river basin\u2019s area, which the researchers combined with the length of each basin\u2019s river to calculate a basin\u2019s width. They then noted for each basin, an aspect ratio \u2014 the ratio of a basin\u2019s length to width, which gives an idea of a basin\u2019s overall shape. They also calculated each basin\u2019s aridity index \u2014 the ratio between the regional precipitation rate and potential evapotranspiration \u2014 which indicates whether the basin resides in a humid or dry environment.<\/span><\/p>\n

When they plotted each basin\u2019s aspect ratio against the local aridity index, they found an interesting trend: Basins in dry climates, regardless of size, took on long, thin shapes, as did large basins in humid environments. However, smaller basins in similarly humid regions looked significantly wider and shorter.<\/span><\/p>\n

\u201cWe found that arid basins roughly kept their shape with size, but humid basins got narrower as they grew larger,\u201d Yi says. \u201cThat confused us for a long time.\u201d<\/span><\/p>\n

Answers in the ground<\/strong><\/span><\/p>\n

The researchers suspected that the dichotomy between dry- and humid-type shapes stemmed from their previous observations of branching rivers: In humid climates, groundwater plays an additional role to rainfall in creating wider branches of a rivers, compared with in drier climates. They reasoned that groundwater may play a similar role in widening a river\u2019s basin.<\/span><\/p>\n

To check their hypothesis, they looked at characteristics of each basin\u2019s geology, such as the types of rock and soil underlying the basin, and the depth to which groundwater might penetrate. In general, they found that in drier climates, any rainwater that seeped into the ground would dribble deep below the surface, like liquid running through a Brillo pad. Any resulting reservoir, or water table, would be too deep for groundwater to come back up to the surface.<\/span><\/p>\n

In contrast, in more humid environments, water is more likely to saturate the soil, like tap water soaking a damp sponge. In these climates, water would seep into the ground, creating large water tables close to the surface.<\/span><\/p>\n

The team then computed the extent to which stream locations corresponded to locations where groundwater emerged. They found a greater correspondence where there was more groundwater seeping out around river basins in humid climates, versus in drier climates. This suggests that groundwater plays a bigger role in carving out humid basins, creating wider, more squat shapes, in contrast to the longer, thinner shapes of dry-climate river basins.<\/span><\/p>\n

This groundwater effect may be especially pronounced at smaller, more local scales over several kilometers. At much larger scales, spanning nearly half the continent, the group found river basins, even in humid environments, took on long, thin contours, which may be attributed to the fact that, over such a vast area, the interaction between groundwater and the large-scale structure of river networks is relatively weak.<\/span><\/p>\n

\u201cOur paper establishes a new, large-scale connection between hydrogeology and geomorphology,\u201d Rothman says.\u00a0\u201cIt also represents an unusual application of the physics of pattern formation. \u2026 All this turns out to be connected with fractal geometry.\u00a0Thus in some sense we are finding a surprising connection between climate and the fractal geometry of river networks.\u201d<\/span><\/p>\n

This research was supported, in part, by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division.<\/span><\/p>\n

<\/p>\n","protected":false},"excerpt":{"rendered":"

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