The Mahras Dag complex landslide, upper Goksu River valley, Turkey

In 2005 and 2006, the geology and geomorphology of the Upper Goksu River Valley of south-central Turkey was studied as a part of the Goksu Archaeological Project. The project was undertaken in part to determine the archaeological impact from construction of a hydroelectric dam in Cogla Canyon, which separates the Upper Goksu Valley to the northwest from the Lower Goksu River Valley to the southeast. During the investigation, evidence was found indicating the presence of a mega-landslide across the Goksu River at Cogla Canyon. Analysis of sediments exposed in the upper valley indicates the presence of a paleolake within the valley, with a maximum elevation of 350 m amsl. Southeast of Cogla Canyon, there are very poorly sorted deposits characteristic of outburst flood deposits. This evidence indicated the presence of a large landslide that dammed the Goksu River, creating a large lake that later overtopped and/or breached the landslide dam. Further investigation using geographical information system and remote sensing techniques indicated the presence of a complex mega-landslide, averaging 2 km wide, with a 7.3-km-long run-out and a conservatively estimated volume of 1.4X10 (super 9) m (super 3) . This landslide dammed the Goksu River at Cogla Canyon and formed the paleolake. Geomorphic investigations indicate the slide underwent a series of movements, including a slump, a sturzstrom, and a secondary block slide, that opened the present-day Cogla Canyon. This is the region's largest recognized landslide deposit. The existence of this landslide, and of others in the area, has implications that relate to the construction of the hydroelectric dam.