The impact of invasive plants on stream baseflow has mostly focused on water-limited areas such as South Africa and the southwestern region of the United States. However, there is evidence suggesting that rivers in areas traditionally considered to be water-rich (e.g. the northeastern United States) will be facing water shortages in the foreseeable future. Consequently, it is imperative to understand what impact invasive plants in this region may have on river baseflows. In early June 2008, pressure sensors to monitor stream depth were placed upstream and within a stretch of river containing Japanese knotweed on the bank. Knotweed daily photosynthesis, stomatal conductance, and transpiration patterns were recorded during the course of two day-long periods at the end of July, 2008. At the beginning of August 2008, all of the knotweed plants in the study area were cut at the base and their leaves were harvested to determine total leaf area for the site. Pre-harvest stream water levels were found to be significantly lower than post-harvest stream levels within the stretch containing knotweed. In contrast, in the upstream location, baseflows decreased between pre-harvest and post-harvest. Furthermore, on the basis of the physiological measurements and total leaf area, it was estimated that the plants in the study area removed through transpiration about 1556 l or 10.4 mm of groundwater per day from the system. These findings suggest that the presence of knotweed within riparian habitats may have even more deleterious effects on water resources than previously considered and merit future research. Copyright © 2013 John Wiley & Sons, Ltd.