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Stream water was collected at weekly to monthly intervals at 29 stream sites in New Hampshire (USA). Ten of the stream sites were instrumented with high‐frequency sensors. Twenty-one of the stream sites (including 5 sensor sites) are in the Lamprey River Hydrologic Observatory (LRHO; Wymore et al 2021) and two stream sites were nearby the LRHO. Groundwater was collected from two riparian well fields (JF, 14 wells and WHB, 13 wells). Wells were installed in 2004 and sampled monthly through May 2007, then quarterly until December 2009, after which a subset (JF, 6 and WHB, 5) was generally sampled quarterly. Stream and groundwater samples span a 17-year collection period and were analyzed for sodium, chloride and specific conductance. Methods and findings are described in the associated Limnology and Oceanography Letters manuscript. 

Abstract Elevated salt concentrations in streams draining developed watersheds are well documented, but the effects of hydrologic variability and the role of groundwater in surface water salinization are poorly understood. To characterize these effects, we use long‐term data (12–19 yr) and high‐frequency specific conductance (SPC) data collected from 13 streams across New Hampshire, USA. Concentration–discharge (C–Q) relationships for chloride (Cl⁻) derived from high‐frequency SPC showed distinct seasonal variability. Diluting behavior was common, but flushing behavior occurred in autumn and winter, suggesting that both groundwater and surface runoff contribute salts to streams. Long‐term data show that although extreme flood events initially reduced salt concentrations in groundwater and rural streams, concentrations recovered to preflood conditions in about a decade. Chronic Cl⁻exceedances occurred in urban streams during all seasons. This research suggests that variation in stream flow, extreme events and application of deicing agents play a role in freshwater salinization.