We agree with Kanivetsky and Shmagin (Eos, 86(50) 2005) that better accounting of flows comprising the hydrologic cycle is needed and that better quantification of recharge (into groundwater) and discharge (i.e., streamflow) is important from human and environmental perspectives. However, because these authors promote their approach as being useful in assessing "sustainability," we feel compelled to offer words of caution about its applicability. The authors suggest that the ratio of renewable water supply to water use by humans and the environment is a "key indicator" of sustainability. We think that these authors got it half right. The ratio described above is a useful indicator because it quantifies how much strain a natural system may be experiencing. However, all ecosystem water is not equivalent. The river science literature strongly indicates the need for the full range of natural intra-annual and interannual variation of river flows, along with associated characteristics of timing, duration, frequency, and rate of change, to sustain the biotic integrity of aquatic ecosystems <Postel and Richter, 2003". In this context, only when an adequate assessment of ecosystem needs is performed and compared to net water availability after subtracting human uses will we know if we are truly managing for sustainability.