**4. Discussion**

The tolerance to waterlogging varies between different cereal crops. Cereals as barley and wheat may produce adventitious roots with about 20% of aerenchyma [12]. Photosynthesis may continue under waterlogged conditions. In the present study, waterlogging up to 5 days reduced the relative wheat yield to 60–82% of the not waterlogged reference. This is within the range of yield decrease reported in literature [13]. Flooding by cold running water may cause the soil not to be anoxic, and the yield loss could be decreased compared to situations with higher water temperature and stagnant water [14]. During the large flood in 1995, cold melt water combined with rain gave relatively low temperatures in the water. Therefore the crops could survive for some days under waterlogged conditions. In 1995, the mean day and night temperature was below 6°C until 21st May and during the flood, the maximum day and night temperature was 15–18°C in the lowland parts and 8–10°C in the mountain area [4].

As the cereal yields could be brought up to the level of areas without waterlogging by sufficient N-supply after the flood, it seems reasonable that the period of flooding had delayed the growth due to the cold water. Nitrogen had most likely been lost by leaching during the flood, while denitrification may have occurred at sites with stagnant water.

The flood in July 1789 occurred as a result of 3 days of continuous rain combined with snowmelt of snow and ice in the mountainous areas. This led to a combination of landslides from the hillslopes and enormous flood discharge in the large rivers Gudbrandsdalslågen and Glomma and also in smaller side rivers [15]. Prior to this catastrophic flood, there had not been any large snow melt flood for many years and very cold winters. This can be related to the eruption of the volcano Laki at Iceland in 1783–1784. The effects of the aerosols and extreme volcanic pollution (i.e., dry fog) that effected Europe and other regions in 1783 have been estimated to cause a drop in temperature of 1.3°C in Europe and North America, lasting for 2–3 years [16]. After the flood in July 1789, large amounts of sand (50–100 cm thick) had sedimented above fertile soil with sandy loam texture in several places in Gudbrandsdalen. These areas gave very low yields and were subjected to drought. A deep plowing project

showed promising results when the old plow layer was brought back to the soil surface. However, deep plowing gave only stripes with good effect [17], while use of excavator for bringing the old fertile soil to the surface fully restored the productivity of the soil. 14C dating of buried peat material below the sediments from the 1789 flood gave interesting results. In the peat layers it was found layers and lamina of silt up to 6–7 cm thickness between the organic layers. The difference in age between the bottom peat layer and the layer just below the thick mineral sediments was 4500– 5000 years [5]. This indicates that the 1789 flood was an exceptional event, which had not happened for several millennia.

The reconstructed soils at Øksna and other areas damaged by erosion and sedimentation by the 1995 flood have been protected by building higher levees and similar measures, which prevented damage by a smaller flood in 2013. When agricultural lands close to large rivers are subjected to flooding in spring almost every year, the flood situation will have large impact on the ground water table in the soils at fluvial plain. Periods with high groundwater level may have large impact on the yields both on cultivated Fluvisols and reconstructed soils along the rivers, as found in a study in Nedre Eiker along the River Drammenselva, north of the city of Drammen, Norway [18].

Outburst floods from glacial lakes about 10,000 years BP have significantly influenced the geomorphology of the main valleys in eastern Norway, Østerdalen and Gudbrandsdalen. Silty sediments of 0.5–1 m depth above marine clay deposits at Romerike, more than 200 km south of the start point of the outburst flood indicate flood of enormous dimensions [6, 19]. Similar to the soils at Øksna, the silty soils at Romerike were poor in nutrient content and cultivation of these silty soils took place in the 1920s and 1930s after trace elements were included for fertilization and appropriate drainage techniques were applied [20]. Documentation of such flood sediments is of importance for prediction of effects of flood caused by dam failure of a hydroelectric plant.
