Geomorphology on EARTH Magazine

Stronger monsoon drove ancient Indus civilization into the hills

Wed, 13 Feb 2019 15:57:41 -0500

Roughly 4,000 years ago, the Indus River Valley was home to the advanced and thriving Harappa culture. But by 1800 B.C., the civilization’s sophisticated cities along the river, which drains into the Arabian Sea on the coast of what is now Pakistan, were abandoned for smaller villages in the Himalayan foothills. A new study suggests that widespread changes in the Indian winter monsoon may have resulted in flooding that forced people to resettle farther from the Indus.

Humans accidentally created new rivers in Europe

Thu, 17 Jan 2019 13:26:26 -0500

The Lek River in the Netherlands formed as a result of avulsion triggered by human alteration of the surrounding landscape. Credit: Rijkswaterstaat/Joop van Houdt.

Meandering rivers that flow through and transport sediment to deltas often split off from their main courses and flow in different directions. This process, called avulsion, happens naturally when a river overflows its banks and the floodwaters carve out a new course for the river to follow. But humans can also trigger avulsions by changing the shape of the landscape, and in a new study, scientists report that people have been doing this for a very long time.

Mighty Mekong cut by monsoon, not tectonics

Wed, 16 Jan 2019 16:24:18 -0500

The Mekong River is the longest river system in Southeast Asia, flowing through six countries. Here, the river flows through Yunnan province in China, where it's known as the Lancang River. Credit: shutterstock.com/HelloRF Zcool.

The rivers draining the Tibetan Plateau are some of the largest, longest and most deeply incised waterways in the world. For decades, most geologists assumed that these river canyons were cut as the plateau was uplifted following the initial collision of the Indian subcontinent with Asia. However, recent studies have found that the plateau was already elevated by 40 million years ago — roughly 20 million years before the deep canyons formed. In a new study, researchers have identified a different force driving the Mekong’s incision: intensification of the Asian monsoon starting about 17 million years ago.

Down to Earth With: Matt Kondolf

Wed, 23 May 2018 12:07:51 -0400

Matt Kondolf. Credit: Courtesy of Matt Kondolf

As a fluvial geomorphologist teaching in the Landscape Architecture and Environmental Planning Department at the University of California at Berkeley, Matt Kondolf is an ambassador from the earth sciences to landscape designers and environmental planners. Kondolf’s research in river management ranges from the impacts of urbanization on runoff and sediment yield, to river restoration, to managing salmon populations and fishing. In classes like “Hydrology for Planners” and “Ecological Analysis in Urban Design,” he encourages up-and-coming environmental planners and designers to think carefully about the geologic processes that control river formation, as well as the roles that rivers play within ecosystems. He is also active in the policy discussions that are shaping California’s and the nation’s approaches to river management.

Spawning salmon engineer landscapes

Fri, 23 Feb 2018 11:16:59 -0500

Spawning sockeye salmon swim in a stream in Alaska. Credit: Katrina Mueller, U.S. Fish and Wildlife Service.

All animals depend on their ecosystems for habitat. And, in turn, many animals impact their ecosystems by engineering the landscape to suit their needs. Beavers provide an iconic example of ecosystem engineering when they build dams, which influence streams and wetlands. The engineering efforts of salmon, meanwhile, can even shape the bedrock of the watersheds in which they live, according to a recent study that modeled the evolution of those watersheds over several million years.

Searching for the ancestors of meandering rivers

Wed, 15 Jun 2016 12:16:31 -0400

In outcrops of the Tombador Formation in the Bahia state of Brazil, researchers found evidence of ancient channel-braided rivers — which combined traits of modern sheet-braided and meandering rivers — that existed before plants colonized land. Credit: Bruno Boito Turra.

It might be difficult to imagine Earth devoid of vegetation, but for billions of years the sun shone, winds blew and rivers flowed on a planet without any plants.

Wormholes may limit landslides

Fri, 06 Nov 2015 13:56:01 -0500

Puerto Rico's Luquillos Mountains appear in this photo taken from the International Space Station in 2007. (Note that north is down in the image.) Credit: NASA

Scientific investigations sometimes take unexpected twists and turns. When Emma Harrison, a graduate student at the University of Pennsylvania, began digging trenches in the mountains of Puerto Rico to study how the soil mantle responds to rapid changes in erosion, she couldn’t help but notice the worms.

Due diligence in river incision data

Mon, 02 Nov 2015 11:40:16 -0500

River terraces near Lees Ferry along the Colorado River between Arizona and Utah record how the river has cut and scoured the surrounding landscape. Credit: Joel Pederson.

As great equalizers of topography, rivers and streams whittle down landscapes by alternately scouring away broad flat swaths of sediment and rock, and incising deeply through them. If a landscape — a mountain range, for example — is being uplifted by tectonic forces, this whittling occurs even faster. River incision rates in particular are thus often used to infer past rates of rock uplift. But determining incision rates themselves is not clear-cut. In a new study in Geology, scientists look at one complicating factor in such calculations, what the authors call the “unappreciated effects of streambed elevation variability” on measuring river incision rates.

Sculpting the Alps

Fri, 04 Sep 2015 11:25:06 -0400

Above about 1,800 meters, the steepness of mountain slopes in the vicinity of the Mont Blanc Massif near Chamonix, France, tends to decrease with elevation. Credit: Lucas Joel.

Mountains typically get steeper the higher you climb. The European Alps are an exception: Beginning at altitudes between about 1,500 and 2,000 meters, most slopes in the range tend to become less steep with increased elevation. This is largely caused by ancient glaciers, which scoured away much of the rock from the tops of the Alps. However, the steepness of alpine slopes also decreases in areas beyond the reach of glaciers, although the reasons why have remained elusive. New research is revealing how tectonic and fluvial forces have also helped shape the Alps’ unusual topography.

Sediment load shapes rivers

Tue, 16 Jun 2015 11:43:57 -0400

Meanders of the Rio Beni in Bolivia. Credit: Landsat/U.S. Geological Survey.

The amount of sediment carried in meandering rivers influences how quickly the bends in those rivers migrate back and forth, according to recent research in Nature Geoscience addressing a longstanding question regarding river evolution. Meanders form when flowing water erodes one riverbank while simultaneously depositing sediment on the opposite bank, gradually creating more and more pronounced U-shaped bends. Sometimes, the rivers cut new channels across the narrow necks of such bends, isolating the abandoned meanders to form distinctive oxbow lakes.

Pre-settlement erosion rates illuminated

Tue, 12 May 2015 10:05:26 -0400

Researchers quantified background landscape erosion rates in the southeastern U.S. prior to European settlement by studying sediments along 10 major rivers, including the Dan River. Credit: ©MarmadukePercy, CC BY-SA 3.0.

Humans are one of the most powerful erosive agents on Earth, moving copious amounts of sediment to and fro, mainly through agriculture and development. But quantifying how much we actually move — often a necessary step for developing sustainable land management practices — hinges on determining erosion rates in an area before humans intervened. A new study using surface exposure dating to estimate pre-colonial erosion rates in the southeastern U.S. has now clarified the natural background rate in more detail than ever before, revealing the dramatic human impact on the regional landscape.

Down to Earth With: Geomorphologist Gregory Tucker

Tue, 13 Jan 2015 10:24:21 -0500

Gregory Tucker in the Grand Canyon. Credit: Courtesy of Gregory Tucker.

As an undergraduate anthropology student, Gregory Tucker thought math was a boring subject with abstract rules that didn’t relate to his life. Today, that “boring” subject provides the foundation for Tucker’s innovative research involving numerical modeling and unique field studies that recently earned him the European Geosciences Union’s 2012 Ralph Alger Bagnold Medal, one of the highest prizes in geomorphology.

Down to Earth With: David Montgomery

Mon, 15 Dec 2014 13:31:56 -0500

From the length and breadth of his body of work, you might assume that David Montgomery, a geomorphologist at the University of Washington in Seattle, is approaching the end of a highly successful career. After all, among other accomplishments, he pioneered our understanding of how river channels shape landscapes, explored how glaciers and climate determine the height of the world’s highest mountain ranges, and helped elucidate how erosion has shaped human civilizations through time. He has authored or co-authored more than 140 academic papers as well as three (going on four) books, and even snagged a MacArthur “genius grant” in 2008. However, Montgomery is only a youthful 51 and shows no signs of slowing down — he even manages to find time to write songs, play guitar and sing in two Seattle-based bands, Big Dirt, a folk rock group, and High Noon, a bluesy jazz rock ensemble. Recently, EARTH’s Julia Rosen caught up with Montgomery before he headed off to Africa for another season of fieldwork.