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Sediments transported by water: sand and silts

The dams had a major impact on sediment dynamics, leading in particular to the deposition of the finest sediments at the bottom of the lakes. As a result, the water downstream of the dams and as far as Ducey was artificially clearer and less turbid than upstream of the former reservoirs. This effect has disappeared with the removal of the dams: the concentrations of fine sediment in the water are identical upstream and downstream of the former lakes. The water downstream therefore appears more turbid without the dams, a phenomenon that is particularly noticeable when it rains, and the transport of fine sediment in the river naturally increases. The restoration of sediment continuity also concerns the coarsest elements. Previously stored in the Vezins reservoir, they are now dissipating downstream and gradually spreading out as the river floods.

Chemical elements of water: nitrates, phosphates and silica

In addition to sediments, the restoration of continuity has also modified the transfer of chemical elements downstream. In the presence of the dams, phosphorus and silica (important nutrients for the ecology of the watercourse and coastal marine areas) were trapped in the reservoirs where microscopic algae, or phytoplankton, developed. Without the dams, measurements taken upstream and downstream of the former lakes show that these elements are once again available to the ecosystems located downstream, to the river and to the Sélune estuary.

Nitrate concentrations are high in the water of the Sélune. This reflects the high level of agricultural activity in the catchment area. Unlike phosphorus and silica, the dams had no influence on the average nitrate values in the water of the Sélune downstream. The removal of the dams therefore neither increased nor decreased the nitrate concentrations measured downstream of the old dams. Only the variation in nitrate levels during flood periods has changed. Without the dams, floods are now associated with lower concentrations due to a dilution effect, as was already the case upstream.

What about the stock of sediment in the old lakes? What is happening to it?

The emptying and dismantling of La-Roche-Qui-Boit dam led to the formation of banks of sediment which had escaped from the old reservoir and deposited on and near the riverbanks downstream. This sediment stabilises and evacuates naturally, particularly when the river floods. In 2023, a year after the dismantling of La-Roche-Qui-Boit was completed, measurements of water turbidity at peak flood levels show a return to equilibrium between upstream and downstream of the old dam. This means that sediment deposits downstream have been reabsorbed or stabilised.

What does this mean for the Bay of Mont Saint-Michel ?

The quantity of sediment that arrived in the Bay of Mont Saint-Michel when the dams were emptied is small compared with what is stirred up every day by the tides in this area. The effect of the removal of the dams can therefore be considered negligible.

The increase in phosphate and silica inputs associated with the restoration of the continuity of the Sélune constitutes an additional nutrient input for aquatic organisms in the river and estuary. These elements are essential for the development of estuarine flora and fauna in particular. In addition, Mont Saint-Michel Bay is a nursery area for many species of commercial fish such as sea bass and flounder, which also frequent the Sélune estuary.