MORPHOLOGICAL CLASSIFICATION AND CHANGES OF ISLANDS ON THE DRÁVA RIVER, HUNGARY-CROATIA - Carpathian Journal of Earth And Environmental Sciences

In the 19^th and 20^th centuries channel regulation works (cut-offs, revetments and groins) and the hydroelectric power plants generated significant changes on the hydro-morphology of the Dráva River along its lower, 236 km-long reach. The aim of the present research is to study the long-term dynamism of islands on the Dráva and to evaluate the processes from the point of view of engineering works. Based on a series of maps and a satellite image (representing the period of 1882-2007) the number and area of islands were determined, and they were classified. During the last almost 130 years the number of real islands (formed of bars) increased (from 210 to 287), though their total area decreased (from 2381 ha to 1678 ha). Different tendency describes the evolution of floodplain islands (which were mostly created by cut-offs), as their number (32 to 8) and territory decreased (3797 ha to 976 ha). In the narrowing channel the islands became gradually more osculating and amalgamating into the bank or another island. Therefore, the frequency of non-amalgamated islands became lower, as it is indicated by the decrease of overlapping islands (from 52% to 30%) and the proportion of less frequent islands had increased (from 45% to 66%). During the studied period the different river engineering works altered considerably the island development. The floodplain islands almost disappeared and the channel became narrower, and more uniform. The originally anastomosing channel became straightened, therefore on the sediment-rich and steep-sloped Dráva intensive island formation started and its pattern became braided. However, due to the water-level drop caused by reservoir constructions and the blocking of side-channels by groins most of the islands osculated or merged to the banks. Thus, nowadays small islands develop behind the groins, and the main channel lost its diverse morphology.