Alluvial fans along the foothills of the Elburz Mountains,Iran

Alluvial fans are particularly well developed along the southern margins of the Elburz Mountains, Iran. In this work, 26 alluvial fans situated to the southeast of Tehran are examined. The fan of the River Jaj is extremely large and covers, with its drainage basin, an area of more that 2,500 km². The other 25 fans and their related drainage basin systems comprise all together an area of only 130 km². The fan/drainage basin systems show a direct relationship between the fan area and the drainage basin area and an inverse relationship between fan area and the mean slope of the fan. Hydrometeorological studies suggest that the severest storms under present environment conditions are possibly capable of moving many of the larger stones observed on one of the smaller fans. The poorly sorted sediments composing the fans suggest, however, that deposition occurred under debris-flow conditions. Fanhead trenching, the occurrence of desert varnish on stones over large areas of the fans and gully erosion of the lower portions of the fans indicate that alluviation is not of great importance at the present day. Archaeological evidence also suggests that sediment deposition on the fans has been of only minor importance during the last 750 years. Optimum conditions for fan formation are believed to have occurred during the glacial periods of the Quaternary era.