A 30-year dataset of CO 2 in flowing freshwaters in the United States.

Increasing atmospheric carbon dioxide (CO 2 ) concentrations have been linked to effects in a wide range of ecosystems and organisms, with negative effects of elevated CO 2 documented for marine organisms. Less is known about the dynamics of CO 2 in freshwaters, but the potential exists for freshwater organisms to be challenged by elevated CO 2 . In flowing freshwaters CO 2 exhibits more variability than in lakes or the ocean, yet spatiotemporally extensive direct measures of CO 2 in freshwater are rare. However, CO 2 can be estimated from pH, temperature, and alkalinity-commonly collected water quality metrics. We used data from the National Water Quality Monitoring Council along with the program PHREEQC to estimate CO 2 in flowing freshwaters across 35,000 sites spanning the lower 48 US states from 1990 through 2020. Site data for water chemistry measurements were spatially joined with the National Hydrology Dataset. Our resulting dataset, CDFLOW, presents an opportunity for researchers to add CO 2 to their datasets for further investigation.