Understanding the effect of reservoirs on suspended sediments and biogeochemical fluxes over seasonal and storm scales
Résumé
Reservoirs such as dams modify the transport, the retention and the remobilization of sediments and nutrients
in rivers. Such modifications can lead to strong degradation of downstream ecosystems as consequence of
disturbances of biogeochemical cycles or habitat. When such negative impacts or cost of upgrading safety systems
outweigh the benefits of dams, removal operation may be considered, e.g. Service (2011) inventoried more than
500 removals in USA in 10 years. The modification on River fluxes induced by the presence of a reservoir as well
as the changes likely to occur in the case of its removal will depend on the fluxes from the catchment, on the functioning
of the reservoir itself, and on the linkage between both systems. Therefore, we argue that characterizing the
modification of hydrological, sediment and dissolved fluxes induced by dams should be achieved (i) on a variety
of time scales (storms, seasons, years), and (ii) within an integrative approach that combine sediments, associated
elements, and biological variables to enable understanding the biogeochemical processes impacted by the reservoir.
We conducted such an approach on the Sélune River, a 91 km river long, flowing to the Mont Saint-Michel Bay
(France), draining a catchment of 1 083 km2, and regulated by two hydroelectric dams:, La-Roche-qui-Boit and
Vézin (16 and 36 m respectively). A set of station has been equipped to monitor water, sediment and dissolved
elements fluxes from upstream to downstream the 2 dams since 2015. Sensors recorded every hour or less stream
flow and physical and chemical parameters such as turbidity, temperature and conductivity on 4 stations. Water
samples were collected every week and at higher frequency during storms on 2 stations to determine anions,
dissolved nitrogen and silica, dissolved and total phosphorus concentrations and Carbone and nitrogen content of
suspended solids. Results over 2 contrasted water years and more than 30 sampled storm events, showed that the
two dams have 2 main effects:
(i) They act as buffer regarding suspended sediment and phosphorus annual and storm fluxes by trapping
suspended solids. They also buffer concentration variations during storm events by mixing all dissolved elements
with the water they store.
(ii) They act as biogeochemical reactors seasonally: in summer when light and temperature conditions are
enhancing phytoplankton production into the reservoirs, the dissolved phosphorus and silica nutrients are
consumed in the reservoirs whereas lower C/N content of downstream suspended material suggest exportation of
autochthonous products from the reservoir.
No effect on stream flow dynamics is observed due to specific regulation strategy. Annual nitrate loads upstream
and downstream to the dams do not show significant difference.
Service, R.F. (2011) Will Busting Dams Boost Salmon? Science 334, 888–892.