Over time and before humans contained them in canals and culverts, rivers would meander over their floodplains, regularly changing course and leaving behind abandoned channels.
These 'lost' river courses are some of the most enigmatic features in the landscape, with unique wildlife and human stories. The US's mighty Mississippi River offers some unparalleled insight into the dynamics of a river as it changes its course – and the habitats it leaves behind.
Rising in Lake Itasca in Minnesota, the Mississippi flows for 3780km and its entire watershed takes in 31 US states and 2 Canadian provinces. But it has never flowed in the same channel for long.
All along its impressive length, the river has changed course many times over the past 25000 years. Working for the US Army Corps of Engineers in 1944, geologist and cartographer Harold Fisk created an astonishing map of the Mississippi and represented its former channels in a rainbow of colours. The fantastic map resembles a writhing tangle of coloured ribbons.
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Slow, almost imperceptible redirections are called accretions, while sudden violent changes of course caused by a bank collapse or earthquake are called avulsions. The once great early 19th-century town of Rodney, which could have become the state capital of Mississippi, is now a ghost town because a sand bar upstream gradually, through accretion, pushed the river ever west and the town became unviable.
The Tennessee town of Reverie was the victim of an avulsion in 1876. Having been on the east bank of the river, it suddenly found itself on the west bank as the Mississippi suddenly abandoned its course for a new channel. Today Reverie is a part of Tennessee but within the boundaries of Arkansas.
Both avulsion and accretion can leave behind meander scars and cutoff channels, including oxbow lakes and very slow-moving rivers called bayous. These old abandoned river beds are known as paleo-channels and contain snapshots of primordial scenery and unique communities of fish. Later abandoned river courses caused by human action after the arrival of European settlers in the past 500 years are called neochannels but they can have similar impacts.
One of the most prominent old river channels was the Ohio River, which is estimated to have flowed 25,000 years ago over western Tennessee and Mississippi to join the Mississippi River north of Baton Rouge, Louisiana, 750–800km south of the present confluence. Over time, the Mississippi River migrated eastward and the Ohio River migrated northward, leaving behind hundreds of abandoned paleochannels. These include cutoff oxbows and long sections of abandoned rivers and even today they demonstrate fascinating transitions from river to wetland to dry land.
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A study between 2006-2012 published in the journal Fisheries in 2016 examined 20 lakes that arose from abandoned paleochannels, exploring the unique wildlife and especially the fish populations in each. It found that the most common fish were species such as orangespot sunfish, western mosquitofish and bluegill, all able to tolerate high levels of silt accumulation and low oxygen levels. Garfish species were also present and are known to be able to gulp air from the atmosphere when oxygen levels in the water are low.
The study found that many of the waterbodies in the lost river channels were struggling due to pollution from agricultural run-off, as well as silt build up. The precious wetland habitats of the paleo and neochannels risk being lost and with them their wildlife communities and their power to absorb greenhouse gases such as carbon and nitrogen.
Additionally, because of modern engineering controls of the Mississippi and its tributaries, no new abandoned channels were being created. The largest of these projects is the Old River Control Structure in Louisiana, a floodgate system built in 1963 to prevent the Mississippi jumping course into the Atchafalaya River something that would be a disaster for the communities, farmers and businesses of the lower Mississippi, including the city of New Orleans.
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Another study, by geologist Alex Kolker reported in the Journal of Hydrology 2013, found that some of the old river channels may still be flowing with water – but deep underground. Studying the overall flow of the Mississippi, Kolker was mystified that the output of water at the river's mouth was less than it should have been according to flow levels upstream.
Using sonar, the Kolker found evidence that the water may be traveling through old bayous, some of them prehistoric, others visible on early geological maps but buried now and wrongly presumed to be inactive. It is thought that as the bayous aged and filled in with organic material and fine-grained sediments, the buried sand deposits of their banks and bottoms became conduits for river water to enter the coastal ocean through underground paths. Some of the Mississippi's old river channels may not be so 'abandoned' after all.
top image: Scenic aerial view of the Mississippi River with forested islands wetlands and ridges in the Driftless Area. Credit: GummyBone/ Getty Images
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