The Earth Story

River migration This photo from above catches several stages in river evolution. In a meandering river system like this one, river meanders grow outward by eroding away at a cutbank. Sediment moved away from the cutbank often is deposited on a point bar on the inside of a growing meander – the sand bars stand out clearly in this photo, showing meanders in various states of growth. River meanders grow outward through this process until eventually the meander gets so big that the river breaks through and establishes a new, shorter course. When that breakthrough happens, it often strands the old meander as an abandoned channel known as an oxbow lake. -JBB Image credit: https://www.flickr.com/photos/maitri/73035629/

A meandering river

Unless their banks are stabilized by humans…rivers don’t like to stay in the same place. As they flow from high grounds to the ocean, they pick up sediment and move it around in particular patterns. Here we see a lovely example of one river morphology; a meandering river entering the Arctic Ocean.

Meanders form because of how rivers move sediment. When a river begins to curve, the changing water direction causes a “helical flow” where water is moving downwards on the outer portion of the river and upwards on the inner portion. In this flow pattern, sediment from the outer side is stripped by the sinking waters, creating what we call a cut bank that erodes outwards, while sediment is deposited on the inner edge where the waters rise and slow, creating a point bar that fills in the area at the inside of the meander.

This process continues, widening meanders and making them ever more sinuous, until eventually the river ruptures through the extended meanders and finds a new, low-friction, quicker path through the area, stranding the meanders as old, unused channels that often will form small lakes.

Here, the river itself appears grey due to fog and clouds forming over the waters.

-JBB

Image credit: Mike Beauregard http://www.flickr.com/photos/31856336@N03/7831522814

Read more: http://www.indiana.edu/~g105lab/images/gaia_chapter_12/meander_formation.htm http://onlinelibrary.wiley.com/doi/10.1029/2001JC001082/abstract

Many Meanders In 2009, the state of Montana flew a series of LIDAR missions over the Flathead River Basin to improve maps of that area. LIDAR is literally described by the dictionary as being like RADAR, only using light. It is currently the most precise way to make maps of subtle variations in surface topography. In this photo, the LIDAR data collected along the river has been processed to show the river morphology. You can see a number of abandoned channels where the river once flowed and then moved away, a number of point bars where the river has gradually adjusted its current course by depositing sediment in one side, and an excellent oxbow lake forming. This could make an interesting piece of wall artwork. -JBB Image credit: Daniel Coe https://flic.kr/p/28Kiend Reference: http://geoinfo.msl.mt.gov/data/flathead_basin_mapping_project_-_2009/lidar_data/ground_returns.aspx http://www.oed.com/view/Entry/108026?redirectedFrom=lidar#eid

Avulsion

This beautiful gif was prepared using data from the long-lived Landsat series of earth orbiting satellites, You are looking at the course of the Padma River in Bangladesh and how its course has varied over the last 30 years. This is a textbook example of river erosion processes. When rivers begin to meander, they evolve by growing their meanders wider, until the meander finally gets too sinuous and the river finds another path. Pick a point where the river bends and watch what happens – the outer bank of the river erodes and sediment deposits on the inner side of the curve, making the river arc farther outwards. The outer side of the river that is eroding is called a cut bank, and sediment is deposited on the inside of the river building a point bar.

Once the river gets too sinuous or too bendy, the river will break its banks, straighten its course, and abandon the wide meander. This breakthrough into a straighter path is called an avulsion.

Bangladesh is one of the most densely populated countries on Earth. In places, this river has rapidly widened, eroding outwards through lands that were once farmland or homes. The flow rates here are high, there are occasional floods, and the sediment surrounding this portion of the river is loose sand that is easily mobilized as the river migrates. The big avulsion that cuts off 2 meanders near the center of the frame is likely associated with a 1998 flood that forced large releases of water from an upstream dam.

-JBB Gif credit: https://go.nasa.gov/2MgpiLh

This gif shows a series of Landsat photographs of the Ucayali River in Peru over several decades. Watch as the meanders of the river evolve and the bars shift, until there is a meander breakthrough and an oxbow lake forms. Gif by Zoltan Sylvester.

Bang Krachao

The largest city in Thailand, Bangkok, has an estimated 2015 population of about 8 million people. People flying over the cityscape see a dense, even polluted environment, like many modern cities. However, right in the middle of the city there is a single blob of green that has been described as “the lung of Bangkok” since its shape resembles an actual lung.

The lung of Bangkok is Bang Krachao (couple other spellings possible). Bang Krachao is surrounded and shaped by a large meander of the Chao Phraya River. Rivers moving over flat landscape naturally grow sinuous meanders by eroding material on one side, the cut bank, and depositing sand on the other side, the point bar. Meanders grow outwards until eventually they are completely cut off, leaving an oxbow lake.

Although cities are commonly built on rivers, natural river processes don’t commonly get along with cities. Buildings built on river shorelines don’t go well with rivers at flood stage and the natural process of meander growth and abandonment would undermine structures built along the river edge. Cities therefore often build river control systems to hold rivers in place.

Bang Krachao is somewhat locked in place by the surrounding city, but interestingly it has also been preserved in place. Today that part of the river is almost an island as a canal has been dug across the thin part of the meander. Much of the river water now flows through that canal, keeping the meander as a slowly-flowing but not stagnant lake.

Inside the meander, local development codes prevent the construction of large buildings and only a limited population lives there. A 100 acre park called Sri Nakhon Kuenkhan fills some of Bang Krachao, containing biking and hiking trails with shops and even a floating public market.

-JBB

Image credit: NASA/USGS/Landsat http://go.nasa.gov/1Ob1EHV

Read more: http://worldpopulationreview.com/countries/thailand-population/ http://bit.ly/1AOMGxX http://bit.ly/1RB2QGR

Songhua River, Manchurian Plains

Flowing lazily across the Manchurian Plain of northeast China, the Songhua River makes for an excellent case-study of a meandering river system. The various shades of brown in this photo highlight the system's bar and swale topography, a series of ridges and adjacent lows which form with progressive migration of point bars. Areas of piercing blue water are classic examples of ox-bow lakes, which form as a meander is cut off and abandoned by the main river channel.

This fluvial system is sourced from the astounding Changbai Mountains, and flows northward over 1,400 km to become the largest tributary of the Heilong River. Further downstream the Heilong River becomes known as the Amur River, as it breaches the Chinese border with Russia and continues to flow until reaching the Strait of Tartary.

With such wide reliance on its water supply, the Jilin chemical plant explosion of 2005 which released over 100 tonnes of toxins (dominantly benzene and nitrobenzene) into the Songhua River became both an environmental and political crisis. Notoriously poor management of the spillage by Chinese authorities led to chaos as the slick passed through Harbin, a city of nearly 10 million, and threats of lawsuits from Russian officials as the slick flowed down the Amur and into the Strait of Tartary.

The aftermath of the incident, however, has been somewhat positive. The long-term impact of the spill has been low, and the international attention the disaster received paved the way for several reforms in China's environmental protection policies. It also acted as an instigator for further accountability within the Chinese government, leading, apparently, to greater transparency.

-A.G.C.

Image: http://go.nasa.gov/1QVONvs Scale: 31.9 km x 41.0 km

Sources: http://bit.ly/1N0iENg http://bit.ly/1OkLlrz

Flooding on the Irawaddy

These 2 images from the Landsat 8 satellite show the Irawaddy River in Myanmar; the upper image was taken during a normal August and the lower was taken in August of 2015.

In the upper image, the Irawaddy is a classic braided stream, wandering back and forth between a number of channels with bars and islands of sediment in the gaps in-between. Heavy rains and Tropical Cylone Komen struck the country in July, causing significant flooding on this river and bringing it up to the level where it covers virtually all of the bars in the channel. You can also see that the smaller tributary channels are swollen as well.

More than 100 people reportedly died in the flooding and large amounts of farmland have been lost for the year.

Floods like these are natural features; in fact the bars probably testify to previous floods. When a river like this reaches flood stage, it submerges the piles of sediment in the channel and begins mobilizing that sediment. Floods help take sediment out to sea and if they are not anchored by vegetation, the bars and channels can be moved around and reconstructed in the process.

-JBB

Image credit: NASA/USGS/Landsat http://earthobservatory.nasa.gov/IOTD/view.php?id=86394

The kinds of neat things geologists can do in a laboratory. Here’s a flume tank, set up to run water down a slope, loaded with lots of sediment so that we can watch how it is redistributed. Various river processes like growth of bars, avulsions, channel migration, and channel expansion all make appearances.

Meandering

This lovely shot from Alaska’s Kanuti National Wildlife Refuge captures all sorts of processes that occur in meandering streams.

We’ve got the growth of several meanders, old meanders that have been cut off and formed oxbow lakes, the growth of several vegetation-free point bars and cut-banks on the opposite side as meanders are growing, natural levees along the river illustrated by the different types of vegetation and the appearance of fall colors, and even the cut-bank of one meander is starting to erode into an oxbow lake left behind when an old meander was abandoned.

-JBB

Image credit: US Fish & Wildlife service https://flic.kr/p/8NNeXm

Read more: http://bit.ly/1ll0ltD http://bit.ly/1C3ug3f http://pages.uoregon.edu/millerm/meander.html

An excellently described video of "Why rivers curve", for anyone who ever looked at a river and wondered why it was so curvy.