Dam operations commonly cause large, frequent fluctuations in river stage, which persist for long distances downstream. The stage fluctuations force river water into and out of the banks, defining lateral hyporheic exchange paths. To evaluate the penetration distance and rates of dam-induced hyporheic exchange, we monitored water-table elevation, temperature, and specific conductivity along a transect perpendicular to the Colorado River (Austin, Texas, USA), 15 km downstream of the Longhorn dam. Stage fluctuates daily by almost a metre. The daily hyporheic exchange volume per metre of bank is 1·0 m3. Dam-induced hyporheic exchange penetrates several metres into the riparian aquifer, while water-table fluctuations propagate 30 m into the riparian aquifer. Water chemistry and temperature fluctuate near the channel in response to the flow oscillations. In the absence of dam operations, groundwater would flow steadily through the riparian aquifer towards the river, laterally limiting hyporheic exchange and stabilizing temperatures and water chemistry near the channel. Therefore, dam operations fundamentally change the hydrological, thermal, and geochemical dynamics of riparian aquifers and their hyporheic zones. Copyright © 2009 John Wiley & Sons, Ltd.