Two examples of travertine bridges are observed at 8 to 15 in above stream level in the Lower Zamanti Basin, Eastern Taurids, Turkey. Yerkopu-1 and Yerkopru-2 bridges are currently being deposited front cool karstic groundwaters with log P-CO2 > 10(-2) atm. The surface area and the total volume of travertine in Yerkopru-1 bridge are 4350 m(2) and 40 000 m(3), whereas the values for Yerkopru-2 are 2250 m(2) and 20000 m(3), respectively. The interplay of hydrogeological Structure, local topography, calcite-saturated hanging springs, algal activity and rapid downcutting in the streambed appear to have led to the formation of travertine bridges. Aeration through cascades and algal uptake causes efficient carbon dioxide evasion that enhances travertine formation. Algal curtains aid lateral development of travertine rims across the stream. Model calculations based on a hypothetical deposit in the form of a half-pyramid implied that lateral development should have occurred from both banks of the stream in the Yerkopru-1 bridge, whereas one-sided growth has been sufficient for Yerkopru-2. The height difference between travertine springs and the main strearn appears to be a result of Pleistocene glaciation during which karstic base-level lowering was either stopped or slowed down while downcutting in the main strearn continued. Copyright (C) 2002 John Wiley Sons, Ltd.