History and Science; An Essential Duet for River Conservation
Featured Image: The Great Raft, Source: Noel Memorial Library LSUS
Featured Article: Wohl, E. (2019). Forgotten Legacies: Understanding and Mitigating Historical Human Alterations of River Corridors. 55(7), 5181–5201. https://doi.org/10.1029/2018WR024433
Recently I was down by the Mississippi River with a friend when he remarked, “Look at that tree!” A 30 foot log was barreling downstream in the middle of the 1 mile wide river channel bouncing along the many eddies created by the rushing, brown water. I had heard about large wood rafts that historically clogged up the Red River in Louisiana and many rivers around the world. I tried to imagine what thousands of 30 foot logs floating in the river together would look like and I didn’t get very far before I googled “The Great Raft.” The old images of a massive log jam displayed on my phone were beyond anything I could have imagined.
The Tale of the Great Raft
The Great Raft, as it was known, was a massive log jam along the Red River that started near Natchitoches, Louisiana and extended 160 miles northwest to Shreveport, Louisiana at the turn of the 19th century. It was estimated to be around 800 years old at the time of initial removal by Captain Shreve (namesake of Shreveport, Louisiana). It was so large that it not only could support the weight of humans, but many plants grew on the raft and it was thought to be a living thing in itself due to its dynamic nature. The raft was constantly changing, creating lakes that would abruptly drain as the raft moved and changed. The raft’s instability and the vastness of flooded area helped protect the native Caddo tribe from European invasion for about 150 years longer than other nearby tribes. The thick organic soil of previously flooded areas made pretty good farmland, too. Europeans, discouraged by the inaccessibility of the river, were determined to use the Red River for commerce, Captain Shreve included. It took him and his team 5 years to clear the jam only to have it clog up with wood again 70 years later. Upon the second removal, congress developed a plan to prevent log jams on the Red River and it has never been the same since.
The lack of wood in the modern day Red River is a legacy of Captain Shreve and those who came after him clearing the river for navigation. Today many people in Louisiana have no clue about this massive raft and assume the river in its current form is the result of natural processes. This misconception not only leads people to perceive natural, dynamic landscapes as abnormal, but can misguide restoration efforts and policy recommendations. A recent paper by fluvial geomorphologist, Ellen Wohl, reviews the importance of accounting for historical alterations of river corridors for river conservation and public understanding.
The holy trinity (nope – I am not talking about gumbo) of fluvial geomorphology, or the study of the physical features of the surface of the earth and their relation to its geological structures, is water, sediment, and large wood. All three parts interact with each other and the river valley geometry to influence the form of the river corridor. By form, I mean the shape of the river and its relationship to the floodplain. Rivers come in all different shapes and sizes. Some rivers have multiple channels with floodplain islands in-between (braided). Some rivers meander with cut banks on one side and sand bars on the other. Some rivers are full of sand; others have rocks. Desert rivers are flashy; completely drying up sometimes but with high flow in other times. And lastly, some rivers are lined with trees with highly erodible soils, such as the Red River.
Human intervention with the natural cycle of water, sediment, and large wood have altered the natural state of many river systems globally, mainly through changing patterns of connectivity. Understanding the concept of connectivity helps us see how our alterations to rivers can have long-term impacts.
Connectivity in River Corridors
Connectivity describes the transport of material and energy between the different parts of the landscape. In river corridors this is generally mediated by water. You can look at the connectivity of a river in three different ways: vertical connectivity describes the relationship between the surface of the river and sediments on the bottom. Longitudinal connectivity, in contrast, is the link between upstream and downstream waters, whereas lateral connectivity links the river and the adjacent floodplain. All three types of connectivity are important in shaping form and process of the river corridor.
When humans directly interfere with a river’s natural flow of water – as in the case of a dam – this arguably has the largest effect on connectivity. Dams alter these 3 dimensions of connectivity both upstream and downstream of the interference by generally moderating flows. The number of high flows downstream are reduced, thus decreasing the amount of time the river overflows it banks into the floodplain and the amount of water traveling downstream. This also affects the amount of sediment and large wood that flows downstream.
Levees are an example of another direct interference. Levees block transport of material and energy between the floodplain and river. Dams, levees, and direct removal of large wood or bottom sediment all act to stabilize the channel. Stable channels are desirable for ship navigation, private land boundaries, and permanent infrastructure along rivers. Removing the dynamic processes that naturally shape river corridors has been the desired goal of engineering interventions for the past 200 years.
River modifications can be discrete in time or persistent. Regulation of dams for water and energy, levees for flood protection, and dredging for navigation are examples of ongoing modifications. These structures and processes still function today and are generally visible to the public.
However, historical alterations to natural flow regimes are discrete changes that occurred in the past. Deforestation in the 19th century, for example, removed a huge amount of wood from all American watersheds. Land clearance also caused much higher rates of soil erosion, flushing unnaturally large amounts of sediment into rivers. This caused aggradation or sediment deposition in many rivers, destabilizing channels. In many cases, small scale channelization and damming occurred to control the increasingly unstable river channels post aggradation.
On large river channels like the Red River, removal of large wood on a massive scale for navigation contributed to stabilization. Today most of these isolated modifications have been forgotten just like the tale of the Great Raft. In addition to the invisibility of many of the forgotten changes that occurred hundreds of years ago, many people are not aware of the history of river engineering. The lack of historical context in relation to modern river systems has resulted in the misconception that present day river systems are naturally stable when in fact they extremely dynamic. This type of misunderstanding can result in misinformed environmental policy and misguided river restoration efforts. The public may wrongly view dynamic river corridors as unnatural or dangerous.
Incorporating historical context both in scientific understanding of present day natural systems and public outreach efforts can prevent misguided decision making. History and environmental science have not always been viewed as relevant to each other, however in today’s increasingly human modified world, few things remain untouched. Acknowledgement of the legacy effects of historical river alterations can help improve present day decision making and increase our understanding of the future trajectory of river systems.
Nanson, G.C. and J.C. Croke. (1992). A genetic classification of floodplains. Geomorphology 4(6), 459-486. https://ro.uow.edu.au/cgi/viewcontent.cgi?article=1989&context=smhpapers
Wohl, E. (2019). Forgotten Legacies: Understanding and Mitigating Historical Human Alterations of River Corridors. Water Resources Research 55(7), 5181–5201. https://doi.org/10.1029/2018WR024433