The partial pressure CO₂ (pCO₂) in streams and rivers reflects both internal carbon dynamics and external biogeochemical processes in terrestrial ecosystems. CO₂ is typically supersaturated in streams because rates of organic matter decomposition exceed photosynthetic uptake, and because of inflow of CO₂ supersaturated ground waters. Using the U.S. Geological Survey's NASQAN database, we calculated pCO₂ and dissolved inorganic carbon (DIC) concentration in 417 streams and rivers distributed across the contiguous United States over a 22-year period. In addition to demonstrating widespread supersaturation and distinct regional variation in pCO₂ across the U.S., our analysis revealed that pCO₂ significantly declined 78.4 ppmv y⁻¹ from 1973 to 1994. This decline is not attributable to changes in terrestrial weathering or in-stream processes, but instead suggests large-scale declines in terrestrial CO₂ production and import into aquatic ecosystems during this 22-year period.