river, gravel bar, vegetation cover, channel heterogeneity

The aim of our research is to evaluate the spatial and temporal changes of the gravel bars in two Czech Carpathian rivers: the Olše and the Ostravice in 20 years (2000–2020), with a focus on the progression of the succession of vegetation cover compared to hydrological events, and the possible influence of the heterogeneity of the river channel on river bar occurrence including relative channel width, weirs, and tributaries. We used 9 orthophotos and ArcGIS Pro software for mapping and spatial analysis. The reference year of 2000 was represented by a relatively high percentage of the total unvegetated bar area in both rivers (the Olše 58 %, the Ostravice 46 %), which shows the influence of major floods in 1997 and another flood event in 1999. Vegetation cover progressively increased from 2003 to 2009, when it reached 81,8 % in the Olše and 92 % in the Ostravice river. In 2012, vegetation cover decreased to 62,2 % in the Olše and 68 % in the Ostravice, as a result of major floods in 2010. Vegetation continued to decrease in 2014 (the Olše 53,2 %, the Ostravice 54,3 %) due to another flood event that occurred shortly before the aerial images were taken. We also observed the highest occurrence of unvegetated gravel bars (the Olše = 13,3 %, the Ostravice 14,1 %). The following years showed a significant increase in total vegetation cover in the bars (the Olše 87,1–93 %, the Ostravice 84,5–96,4 %) and the occurrence of completely vegetated bars (the Olše 23,5–36,1 %, the Ostravice 26,8–58,3 %). Only the relative width of the channel and, in some cases, weirs and tributary locations could be associated with fluctuations in the occurrence of the gravel bar. Our findings suggest the importance of major floods in the dynamics of the gravel bar vegetation, as well as significant progress in the vegetation cover on bars over the past few years. The locations with the largest gravel bars are associated with undammed, wide-channelled areas or tributary locations.