Holocene river floods in the upper Glomma catchment, southern Norway: a high-resolution multiproxy record from lacustrine sediments

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 C-14 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.