Methane dynamics in the Willamette River, Oregon, U.S.A.

Weekly monitoring of dissolved methane (CH₄) at two sites along an ∼ 2-km stretch of the Willamette River (Oregon) between October 2008 and November 2010 revealed persistent supersaturation (24–170× atmospheric equilibrium). The CH₄ concentration difference (ΔCH₄: 0–200 nmol L⁻¹) between the two sites varied inversely with river flow, which ranged from 125 m³ s⁻¹ to 1500 m³ s⁻¹ over the time series. At the downstream site, an ‘excess’ of ≤ 125% was observed, with groundwater input being the likely CH₄ source. Quasi-synoptic studies of spatial trends in summer (2010, 2011) revealed steady CH₄ decrease along a 12-km river stretch downstream of the time-series sites. The estimated loss due to air–water exchange for this stretch was ∼ 9× greater than the perceived net loss, consistent with regionally widespread groundwater input of CH₄. Bi-weekly dissolved nutrient measurements indicated that a distinct nitrate (NO₃^-) source also existed between the upstream and downstream time-series sites. The excesses of and CH₄ were inversely correlated, with the greatest supply corresponding to periods of high rainfall in winter and highest river flow. Although groundwater input is also the probable source of , seasonal seepage of rainwater-saturated soils (shallow groundwater recharge) explains the input, while hyporheic exchange with a persistent deep aquifer best explains the CH₄ input. Improved understanding of groundwater input and exchange dynamics in the Willamette River will clarify the influence of human activities on river biogeochemistry and help to better constrain the magnitude of CH₄ and other greenhouse gas fluxes associated with inland waters.