SUBFOSSIL BRAIDED RIVER PATTERN AS AN INDICATOR OF EXTREME SHORT TERM EVENTS, SUDETES MOUNTAINS, CENTRAL EUROPE

The Sudetes are a mid-mountain range located in the temperate climatic zone of Central Europe. Deep valley sides are covered with thick mantles of periglacial regolith. Migrating cyclones cause intensive rainstorms and floods. Geomorphological and sedimentological traces of previous large floods have been found in the upper course of the Bila Opava valley floor, which drains the eastern slope of the Hruby Jesenik massif, 1000-1400 m a. s. l. Big boulders deposited here form a typical braided pattern overgrown with old spruce forest; the braided river pattern consist of a group of boulder palaeo-bars, te majority of which formed as longitudinal central bars with lee slopes running diagonally to the valley axis. Tree-ring increment analysis indicates that the oldest spruces growing on this braided river pattern may be more than 75 years old so that, allowing for the time required for the formation of the initial soil and the succession of pioneer vegetation, the spruces succeeded here around the turn of the 20th century. The majority of precipitation is linked here to synoptic situations, when cyclones from southern and western direction create favorable conditions for continuous heavy precipitation. Such heavy rainfalls trigger large floods, which occurred several times in the turn of the 20th century in the vicinity of the Hruby Jesenik massif. The largest flood, in July 1903, was caused by a precipitations amount not previously recorded; on 9 July 1903 the rain gauges in the northern part of the massif received 200-240 mm of precipitation which generated a flood wave sufficient to transport and deposits very coarse material, forming the braided river pattern. The periglacial regolith covering the steep, deforested slopes of the valley were the source of the coarse-grained clastic material supplied to the Bila Opava river bed.