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.