Wildfires are increasing globally in frequency, severity, and extent, but their impact on aquatic ecosystems, and the resources they provide, remains unclear. Here, we combine remote sensing of burn perimeter and severity, in-situ water quality monitoring, and longitudinal modeling to create the first large-scale, long-term estimates of stream length impacted by wildfire for the western US. We found that wildfires directly impacted ~6% of the total stream length between 1984 and 2014, increasing at a rate of 342 km per year. When longitudinal propagation of water quality impacts was included, we estimated that wildfires affect 11% of total stream length. Our results indicate wildfire is one of the largest drivers of aquatic impairment, though it is not routinely reported by regulatory agencies, as fire impacts on aquatic systems remain poorly constrained. We identify three key actions to address this knowledge gap, and better understand the growing threat to stream ecosystems and associated water security/public health risks.