Streams and rivers play an important role in the global carbon cycle by moving carbon from the land to the oceans. However, we still do not fully understand how much carbon they transport, especially in northern regions where wetlands, complex soils, and seasonal changes in snow and rain create intricate water flow patterns. Understanding how water moves through these landscapes is key to predicting how much carbon reaches streams and how climate change or land use will affect this process.

In this study, we identified where stream water comes from, including rain, snowmelt, shallow soil water, and groundwater, across three forested watersheds in northern Wisconsin. We used stable water isotopes, which are forms of hydrogen and oxygen that act like fingerprints for water sources, along with measurements of electrical conductivity and naturally occurring radon gas to better understand water pathways.

We found that snowmelt and shallow soil water dominate early spring streamflow, creating sharp pulses of dissolved carbon in small headwater streams. In contrast, larger watersheds with deeper groundwater and wetlands slow this process, releasing carbon more steadily. These findings improve carbon budget estimates and help predict future changes from climate and land use.