Il record sedimentario di un evento estremo di piena in ambiente di barra di meandro fluviale: un esempio dal Powder River (Montana, US)

The effects of the discharge variability on point bar sedimentation are not well documented, although resulting changes in flow pattern are well known. The present work analyses a point bar of the Powder River (Montana, US) to reconstruct hydrology and development of an extreme flood event that affected the area of Powder River County in May 1978. A trench was excavated across the point bar to analyse the internal sedimentary features of deposits accumulated during the 1978 flood event. The section was analysed following a classical sedimentological approach. After detecting the main stratal pattern and sedimentary structures, 120 samples for grain size analyses were taken along the entire point bar. Grain size analysis allowed to calculate the bottom shear stress and the critical water depth when those sediments settled down. By comparing the estimated flood hydrograph of 1978 with the hydrographs at Moorhead and Broadus highly similar results were found. Merging together the grain size data and field observations was possible to reconstruct five flood stages and main changes in flow orientation. These changes caused development of particular features including downstream migration and coarsening upward grain size trend. We conclude that the available sedimentological models can still fail to describe the wide spectrum of deposits formed during extreme flood events.