This study was conducted in four lowland headwater streams in southeastern New Hampshire and northwestern Massachusetts, USA. The watersheds range in size from 2.3 to 4.1 km2. The watersheds of two streams, Sawmill Brook and College Brook, are characterized by a suburban landscape, while the other two, Cart Creek and Dube Brook, are predominantly forest- and wetland-covered. Mean discharge ranges from 25.5 L s-1 at College Brook to 36.7 L s-1 at Sawmill Brook. Mean reach depth at baseflow ranged from 7.2 cm at College Brook to 14.5 cm at Cart Creek, and mean reach width at baseflow ranged from 1.94 m at College Brook to 2.83 m at Sawmill Brook. All reaches exhibit shallow slopes of approximately 2 m km-1 or less. Benthic substrates included sand, silt, and fine organic matter, while rocks comprised less than 10% of benthic surface area across all sites. Macrophytes were absent from all reaches in this study.

Dissolved CH4 sampling and diffusive efflux estimation
Water samples for dissolved CH­4 analysis were collected at each stream using 60-mL syringes fitted with three-way stopcocks. Syringes were rinsed with stream water prior to sample collection. To collect water samples, syringes were filled with approximately 60 mL of stream water from 5–10 cm depth below the stream water surface. Syringes were cleared of air bubbles by inverting and expelling bubbles and water until 30 mL of sample water remained. Samples were stored on ice until returned to the laboratory within 6 h. In the laboratory, 30 mL of ambient air was added to each syringe to achieve a 1:1 ratio of sample water to air. Syringes were then shaken for 2 min to equilibrate gases between water and headspace (Magen et al., 2014). The water was then dispelled from the syringe, and the remaining headspace gas was saved for analysis. If the gas samples were not analyzed immediately, they were stored in evacuated glass vials sealed with a rubber septum until analyzed.  Samples were analyzed for CH4 concentration in the Trace Gas Biogeochemistry Laboratory at the University of New Hampshire. The CH4 concentration in parts per million by volume (ppmv) was determined using a Shimadzu Gas Chromatograph Flame Ionization Detector. Concentration was standardized using the average area response of ten injections of a standard CH4 mixture (Northeast Airgas, 2.006 ppmv or Maine Oxy, 1000 ppmv) to determine instrument precision (Frolking and Crill, 1994).