gravel bars, vegetation succession, gravel-bed rivers

We analysed spatio-temporal gravel bar dynamics, with a focus on vegetation succession, in two rivers of the Outer Western Carpathians for twenty years (2000–2020): the Czech part of the Olše River and the part of the Ostravice river. We used nine archive orthophotos (2000–2020) to analyse the effects of flood events, vegetation succession and river regulations on the gravel bar dynamics. Two major floods in 1997 and 2010 greatly affected the vegetation cover of gravel bars in both rivers. After the 1997 and 1999 moderate floods, the total vegetated area on gravel bars in 2000 was the lowest in all studied years in the Olše (42 %) and the second lowest in the Ostravice (54 %). Following years showed slow but progressive succession until the next major flood in 2010 and moderate floods in 2014. The total area of bar vegetation in the Olše decreased by 19,2 % in 2012 compared to the last studied year of 2009 (81,8 %), and by another 9 % in 2014 probably due to the time proximity of the flood event. The total vegetated bar area in the Ostravice decreased by 24 % in 2012 (from 92 % in 2009) and by another 13,7 % in 2014. Two years later, the vegetated bar area significantly increased in both rivers: compared to the year 2014, it increased by 39,8 % in the Olše and by 42,1 % in the Ostravice. The remaining studied years showed a similarly high percentage of vegetation cover (2018=87,1–93,6 %, 2020=84,5–91 %). Although the Ostravice is regulated by a valley dam, the floods had a very similar effect on both rivers. The main difference is in the rate of vegetation coverage in the years not affected by floods. In the Ostravice almost all the bars were vegetated completely or for the most part, while in the Olše, there was more variability in vegetation coverage on gravel bars. Our results suggest an ongoing succession of vegetation on most gravel bars in the last twenty years and show a significant effect of major flood events and river regulations on their morphodynamics.