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Invasive riparian plants along arid and semi-arid rivers are thought to accelerate stream flow recession rates during drought. However, the fraction of river water used directly by plants is unknown. Riparian water sources for plants may also include shallow groundwater, which may differ from the river itself, or water from the unsaturated zone originating from localized rain events or prior floods. Floods that saturate the soil may persist as a lasting legacy to sustain plants over longer time scales. In this study, we examined potential sources of water used by Giant Reed (Arundo donax L.) following a flood and subsequent prolonged drought using natural-abundance stable isotopes of oxygen (delta O-18) and hydrogen (delta H-2). River and rain water, groundwater, soil, and rhizome samples were collected along four approximately 100-m transects perpendicular to the Rio Grande River in southwest Texas. Our observations coincided with a major flood that saturated soils in July of 2010 followed by extreme drought through fall of 2011. Our results showed that the isotope ratio of rhizome water was consistent with prolonged utilization of flood water, which persisted within flood-recharged surface soils throughout the following summer. Apparently, groundwater was not the dominant source for A. donax when soil moisture was sufficient for plant uptake following a flood. While hydrogeology of rivers vary, this finding highlights the potential for floodwater to sustain riparian vegetation for prolonged periods without rainfall and demonstrates that this may be an important alternative source of water that is not currently accounted for in hydrologic models. Copyright (C) 2015 John Wiley & Sons, Ltd.