River islands are important landscape features for those who live on them or face them (Le Lay, Comby, 2012). Today, they are at the heart of river management issues. Their morphological evolution (whether they are increasing or decreasing) and their biological diversity (plant and animal) are good indicators of a river's ecological status. River islands are therefore ideal observatories for studying the changes affecting rivers today.

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Islands play an important role in the natural regulation of waterways. The flow of water and the amount of sediment transported (known as hydro-sediment dynamics) influence the shape of the waterways. When the flow of water is high, erosion increases and the islands may shrink or exist only as sandbanks. Conversely, a slower-flowing river encourages accumulation and sedimentation processes, which can lead to the formation of islands. These are then gradually colonised by vegetation and help to revegetate the river. Studies of the Loire, for example, have shown that since the 19th century, the gradual slowing of the river's flow has been accompanied by an increase in the number of fluvial islands and a marked revegetation of the riverbed (Grivel, Gautier, 2012).

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This vegetation, when it is spontaneous (without human action), is made up of specific species adapted to the presence of permanent water, which can also temporarily flood the lowest ground. This plant formation is called riparian (from the Latin ripa, "the bank" and silva, "the forest"). The riparian vegetation is not just made up of trees, but also includes bushes and grasses as you get closer to the water. This riparian vegetation is essential in many ways: it helps to maintain the banks thanks to its root system, and limits the damage caused by flooding. It purifies the water, particularly in small rivers, and is home to many endemic river species. However, the vegetation we see along rivers is no longer necessarily a riparian forest. In many cases, these forests have been cleared for agriculture. On the islands, there are also forestry activities (tree cultivation), most of which have now been abandoned. As a result, some islands may be heavily wooded but ecologically poor because they are covered by a single dominant species.  

 

On the Saône, the morphological and ecological evolution of the islands since the 19th century is essentially due to human development. Dykes were built, meanders were cut and locks were built to maintain a constant water level throughout the year. These developments, combined with the Saône's slow natural hydrological regime, have encouraged the maintenance and even the creation of new islands. At the same time, many islands that had previously been used for livestock farming or forestry have been abandoned, leaving the way open for a re-growth of vegetation based on the species present on the island. 

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Geographer Laurent Astrade's 1998 thesis on the Saône is the reference work for this study. He highlighted the complex hydro-sedimentary functioning of the Saône, marked by long periods of flooding and numerous human developments. In addition, his studies on a number of islands in the downstream Saône provide an interesting baseline for the study of riparian vegetation, enabling a comparison to be made twenty-five years later of their distribution and diversity.  

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The aim of this study is twofold :

• to determine the current morphological dynamics of the islands of the Saône (multiplication, narrowing, widening, etc.)

• to provide an overview of the riparian vegetation on these islands in order to better understand their dynamics and thus be able to better protect them