Sedimentological interpretation of crevasse splays formed during the extreme 1997 flood in the upper Vistula river valley (South Poland)

The paper describes scours and crevasse splays formed at the breaks in embankments of the upper Vistula river valley during the summer 1997 flood. The scours cut into the floodplain composed of fining upward channel and overbank deposits. Erosional furrows have developed locally around the scours. In their vicinity, a thin layer of channel-ag gravel was locally laid down. Variously shaped crevasse splays were formed: finger-like, deltoidal and tongue-like. Surface relief, vege-tation and buildings controlled their geometry and sedimentary features. The lower parts of the deposits consist of fine and medium sands with horizontal and low-angle stratification. Higher in the sequences medium and coarse sands with pebbles display planar cross-stratification. Mud balls and black oak trunks redeposited from older alluvia are common. The whole succession represents sheet-flow sediments with partly channelized flow. Locally, at the top, coarse sands, pebbles, mud balls and boulders embedded in silty-sandy matrix occur, representing slurry-flow deposits. Also present were sediments composed of alternating sands and mud pebbles. The vanishing flow phase is marked by occasional ripple marks encountered in the top part of the sequence. Around the flow obstacles (plants, buildings) sand shadows were formed, composed of fine and medium sands with horizontal stratification in the lower parts and ripple cross-lamination along with climbing cross-lamination in the upper parts. The top part included medium and coarse sands with planar cross-stratification. Most of the studied sequences showed coarse-upward grading which is not the effect of changes in the energy of flood waters but originates from the supply of all the time coarser material from the successively deepening scours.