RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 107, NO. 4, 25 AUGUST 2014 675 *For correspondence. (e-mail: alpasridhar@gmail.com) Palaeoflood record of high-magnitude events during historical time in the Sabarmati River, Gujarat Alpa Sridhar*, L. S. Chamyal and Mansi Patel Department of Geology, Faculty of Science, The M.S. University of Baroda, Vadodara 390 002, India We report the occurrence of large floods in the Sabar- mati River Basin during cal AD 1400–1440. Slackwater palaeoflood deposits have been preserved in the highly dissected ravines along the middle reaches of the river, wherein 5–6 discrete flood events have been identified. The minimum discharge that emplaced the highest deposit has been estimated as ~15,680 m 3 s –1 and is higher than that recorded (3050 m 3 s –1 ) during the re- cent 2006 flood. The timing of these high-magnitude flood events in the Sabarmati and adjacent river basins suggests that extreme hydrological events have occurred in response to excess monsoon periods and are largely controlled by the regional climatic conditions. More- over, these flood events are seen to be broadly syn- chronous in the alluvial river basins of Gujarat across climatic zones, but are at variance in pattern from the records of the bedrock peninsular rivers. Keywords: Alluvial plains, flood events, monsoon, palaeofloods, slackwater deposits. PALAEOFLOOD records are significant for understanding the magnitude and frequency of large floods and related monsoon conditions 1 and also for a dependable prediction in the future. The sediment records of large-magnitude floods are selectively preserved, whereas deposits from smaller floods are more likely to be removed by subse- quent erosion due to their proximity to the active chan- nel 2 . The west-flowing rivers of India are predominantly fed by the southwest monsoon and therefore any avail- able palaeoflood record reveals the past monsoon condi- tions. The Narmada, Tapi, Mahi and Sabarmati are the four major west-flowing rivers with a long flood history. Whereas the Narmada and Tapi are sub-humid tropical rivers, the Mahi and the Sabarmati flow through semi- arid climatic zone. The flood history of Tapi (sub-humid), Narmada (sub-humid) and Mahi (semi-arid) has been found to be synchronous 3 and peak flood events from early to late Holocene have been recorded 4–6 . However, palaeoflood record from climatically sensitive Sabarmati River (semi-arid) Basin located at the southern margin of the Thar desert has not been investigated so far. The pre- sent communication provides fresh records of high- magnitude flood events during the last about 500 years in the alluvial plains of Gujarat from the Sabarmati Basin. An attempt has also been made to explore links between the identified events in the Sabarmati, analogous events from the adjacent river basins across climatic zones and long-term fluctuations in the monsoon rainfall. The Sabarmati River flowing through the semi-arid western India is a monsoon-dominated seasonal river and remains dry post-monsoon. At times, however, it pushes down very heavy floods that have caused devastation in the city of Ahmedabad, damaged crops in adjacent low- lying areas and filled the gulf with silt 7 . In the last about 100 years, the flood event that occurred in 1973 is consi- dered to be the highest in the basin, but there is no consen- sus on the actual peak discharge (according to the State Irrigation Department, probable peak discharge was 16,000 m 3 s –1 ). The more recent high-magnitude flood event that occurred in 2006 is better recorded with 8800 m 3 s –1 peak discharge 8 . According to the historical records 9,10 , high-magnitude floods are also known to have occurred earlier in 1683, 1714, 1739, 1755, 1868, 1927, 1941, 1950 and 1992 suggesting a clustering of flood events. An increase in frequency and magnitude of extreme rain events not only in the Sabarmati, but all over India, has been observed during the latter half of the 20th century 11–13 and this has raised serious questions on the future trends. The Sabarmati is a seasonal river receiving varied amount of rainfall (450–800 mm) and is located in the semi-arid climate zone. The Basin is bounded by Aravalli hills in the north and northeast; it is 371 km long with 21,674 sq. km drainage area and average discharge of 120 m 3 s –1 . The Sabarmati River with its origin in Rajast- han flows generally in the SW direction through the Gujarat Alluvial Plains (Figure 1 a) and joins the Gulf of Cambay. The Wakal, Sei, Majham, Harnav, Hathmati and Watrak are the main tributaries of the Sabarmati River (Figure 1 b). Palaeoflood slackwater deposit (SWD) is a major evi- dence used for inference about hydrological parameters of the past flood events 14,15 . Unconsolidated sands and silts that accumulate relatively rapidly from suspension during major floods, particularly where flow boundaries result in markedly reduced local flow velocities are described as SWD 1 . Although the potential for palaeo- flood studies in alluvial rivers is generally low 16 , the river channels in the Gujarat Alluvial Plains are bounded by indurated late Quaternary sediment cliffs and are highly incised providing scope for accumulation and preserva- tion of SWD 6 . With this understanding, the Sabarmati River Basin was explored for palaeoflood deposits in the alluvial reaches. During field investigations along the Sabarmati River, profiles with SWD beds were identified at Dedhrota, Derol and Juna Sangpur (Figure 1 c), occurring at differ- ent levels and with varying thickness. The palaeoflood events have been identified using sedimentological criteria based on abrupt vertical changes in the texture, structure and colour of the sediments, clay capping and deposi- tional discontinuity between two flood events in the form