THE FORMATION OF A CREVASSE SPLAY AND ITS GEOLOGICAL AND ECOLOGICAL IMPLICATIONS AFTER A MAJOR FLOODING EVENT IN RIVER TISZA, HUNGARY

In the year of 2000 following the winter flooding of the river Tisza a huge crevasse splay developed behind a natural breech in the natural levee near the villages of Szajol and Tiszavárkony in Central Eastern Hungary. In a couple of hours following the retreat of the water from the floodplain huge canyon like depressions and troughs were formed with slumps on the softer sandy,loessy, muddy deposits exposing the underlying sediments. In historic times before the river controlling works in the 1800s these natural breaches served as a key hydrologic link between the floodplain and the river enhancing habitat qualities for aquatic and terrestrial riparian communities and supporting the development of human settlements and floodplain agricultural economy (fishing, collecting, grazing). During moderate flood flows, sediment and organic material were transported onto the floodplain. This material was accumulating to form a large "crevasse splay" where rapid deceleration of flows leads to deposition.

Hydraulic and sedimentologic conditions appear to favour the formation of small forests and bushy vegetation at breaches within a couple of days or weeks after the crevasse event. Protected native birds quickly inhabited the canyon walls made of soft sand and loess enhancing the remediation of the area.

Detailed sedimentological, geomorphological, and paleontological analysis of two profiles sampled at the two localities revealed the formation of two other crevasses during the Pleistocene and Holocene as well as the development of a point bar and infusional loess in the area. These data are very useful in understanding the prevailing conditions of the Tisza floodplain in historic times and help us to enhance natural conditions in the whole catchment of the river. They may also serve as modern analogies to depositional environment reconstructions via detailed macro and microstructural analysis of the sediments. From the upper soil layers bones of cattle, hog and and deer have come to light from historical times with signs of slaughtering and meet processing referring to the presence of a slaugtherhouse or a minor farm on the elevated part of the protected floodplain.