HOLOCENE RIVER FLOODS IN THE UPPER GLOMMA CATCHMENT, SOUTHERN NORWAY: A HIGH-RESOLUTION MULTIPROXY RECORD FROM LACUSTRINE SEDIMENTS

Abstract

The Glomma catchment in east-central southern Norway has a discharge dominated by a general spring-flood regime with episodic large river floods In this study, the instrumental records (from AD 1871) and documentary evidence (back to the seventeenth century) have been extended for about 10 000 years by reconstructing episodes of palaeofloods as recorded by sedimentological depositional indicators. A record of Holocene flood events has been established based on a lake-fill sedimentary succession in a small basin in the upper Glomma catchment. The flood events as deposited in Lake Butjonna are discrete, sharp-bounded, normal graded units of silt-sized sediments characterized by low organic and water content. The reconstruction of individual palaeofloods is based on loss-on-ignition, dry bulk density, grain-size analyses and several mineral magnetic analyses: initial susceptibility, bulk susceptibility (X), anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM). Based on 13 14C AMS dates, the age-depth model covers the Holocene from c. 9800 cal. yr BP until present. About 115 discrete flood events (ranging from 1 to 620 mm in thickness) have been detected. The recurrence interval is about 90 years with an occurrence probability of about 1%. Important results include: (1) a weak increasing trend of higher river flood activity toward the end of the Holocene; (2) an early-Holocene calm (low or no river flood activity) period with subsequent onset of flood activity around 7600 cal. yr BP; (3) a pronounced and well-defined river flood event correlated to the ‘Stor-Ofsen’ disaster that occurred in July AD 1789; (4) enhanced southerly winds that bring moist air from the Atlantic Ocean apparently lead to periods of higher river flood frequency.