Diel variations in stream discharge have long been recognized, but are relatively little studied. Here we demonstrate that these diel fluctuations can be used to investigate both streamflow generation and network routing. We treat evapo-transpiration (ET) as a distributed impulse function in an advection model and analyze the effect of ET on diel fluctuations in discharge. We show that when flow velocity is high during high baseflow, discharge fluctuations tend to be in phase and constructive interference reinforces ET-generated signals resulting in strong diel fluctuations measured at a gauging station at the mouth of the watershed. As flow velocity slows with baseflow recession, ET-generated signals are increasingly out of phase so that fluctuations in discharge are masked by destructive interference. These results demonstrate that naturally produced fluctuations in discharge constitute discrete impulse functions that can be used to analyze eco-hydrologic behavior of whole-watersheds during baseflow periods.