Heterogeneous benthic methane (CH4) dynamics from river deltas with important organic matter accumulation have been recently reported in various aquatic and marine environments. The spatial heterogeneity of dissolved CH4 concentrations and associated production and diffusion rates were investigated in the Rhone River Delta of Lake Geneva (Switzerland/France) using sediment cores taken as part of the ELEMO Project. Benthic CH4 dynamics within the complex subaquatic canyon structure of the Rhone Delta were compared (1) between three canyons of different sedimentation regimes, (2) along a longitudinal transect of the ‘active' canyon most influenced by the Rhone River, and (3) laterally across a canyon. Results indicated higher CH4 diffusion and production rates in the ‘active' compared to the other canyons, explained by more allochthonous carbon deposition. Within the active canyon, the highest diffusion and production rates were found at intermediate sites further along the canyon. Stronger resuspension of sediments directly in front of the river inflow was likely the cause for the variable emission rates found there. Evidence also suggests more CH4 production occurs on the levees (shoulders) of canyons due to preferred sedimentation in those locations. Our results from the heterogeneous Rhone delta in Lake Geneva further support the concept that high sedimentary CH4 concentrations should be expected in depositional environments with high inputs of allochthonous organic carbon

Dams impact a significant portion of the world's rivers, and rivers downstream of the reservoirs created by large dams often emit significant amounts of methane (CH4). River CH4 emissions downstream of run‐of‐the‐river (ROR) dams are less well known. We investigated spatial and seasonal CH4 dynamics along a stretch of the Aare River (Switzerland) downstream from a bubbling ROR reservoir. We found that the CH4 horizontally accumulated in this vertically non‐stratifying ROR reservoir was consistently transported to the downstream river, but half was lost near a confluence with a tributary. We estimated CH4 diffusion using gas exchange coefficient (k) models that incorporate specific river characteristics and found CH4 emissions were threefold higher upstream of the confluence than downstream. We discuss the use of CO2‐derived k models in estimating k for CH4, and investigated it directly using a drifting chamber experiment. While chamber emissions only partly agreed with the k models, the models were robust enough to use in a CH4 mass balance along the river that indicated an overall CH4 loss from the study area. The loss predominantly occurred at the confluence and > 92% of it could not be accounted for by modelled or measured CH4 emissions. Ultimately, a ROR reservoir that does not form an anoxic, CH4‐rich hypolimnion enhanced downstream river CH4 emissions, but to a lesser extent than other systems (∼9% of total reservoir‐associated emissions). Regardless, small ROR dams and river features such as confluences should be considered when measuring, estimating or upscaling river CH4 emissions.

We monitored CH emissions during the ice-free period of an Alpine hydropower reservoir in the Swiss Alps, Lake Klöntal, to investigate mechanisms responsible for CH variability and to estimate overall emissions to the atmosphere. A floating eddy-covariance platform yielded total CH and CO emission rates at high temporal resolution, while hydroacoustic surveys provided no indication of CH ebullition. Higher CH fluxes (2.9 ± 0.1 mg CH per m per day) occurred during the day when surface water temperatures were warmer and wind speeds higher than at night. Piston velocity estimates (k) showed an upper limit at high wind speeds that may be more generally valid also for other lakes and reservoirs with limited CH dissolved in the water body: above 2.0 m s a further increase in wind speed did not lead to higher CH fluxes, because under such conditions it is not the turbulent mixing and transport that limits effluxes, but the resupply of CH to the lake surface. Increasing CH fluxes during the warm season showed a clear spatial gradient once the reservoir started to fill up and flood additional surface area. The warm period contributed 27% of the total CH emissions (2.6 t CH per year) estimated for the full year and CH accounted for 63% of carbonic greenhouse gas emissions. Overall, the average CH emissions (1.7 to 2.2 mg CH per m per day determined independently from surface water samplings and eddy covariance, respectively) were small compared to most tropical and some temperate reservoirs. The resulting greenhouse gas (GHG) emissions in CO-equivalents revealed that electricity produced in the Lake Klöntal power plant was relatively climate-friendly with a low GHG-to-power output ratio of 1.24 kg CO per MW h compared to 6.5 and 8.1 kg CO per MW h associated with the operation of solar photovoltaics and wind energy, respectively, or about 980 kg CO per MW h for coal-fired power plants.