This post resonates with a theme that many readers of Hydrology Corner know I care about. This blog post makes a strong case for understanding headwater streams. However, it is at the intersection where sub-surface water becomes flowing surface water that we encounter the smallest possible flow volumes and ephemerality as small flows become zero flow, and then go back again. Conventional monitoring systems under-perform for measurement and monitoring of small and ephemeral flows and the uncertainties in this realm are practically unknowable. More needs to be done to improve source water protection by improving our systems for source water monitoring.