Institute of Nature Conservation, Polish Academy of Sciences, al. Mickiewicza 33, 31-120 Kraków, Poland.
Institute of Nature Conservation, Polish Academy of Sciences, al. Mickiewicza 33, 31-120 Kraków, Poland.
Sci Total Environ. 2019 Apr 10;660:406-420. doi: 10.1016/j.scitotenv.2018.12.475. Epub 2019 Jan 4.
In the twentieth century the heavily channelized Raba River incised deeply in its mountain course. Abandonment of channelization structures in a 2.3-km-long reach within a forested corridor was followed by considerable channel widening during floods of 30- and 35-year recurrence interval, re-establishment of a multi-thread channel pattern and island development. Morphological and botanical surveys were conducted annually between 2011 and 2017 to determine the processes and patterns governing development of islands and their floristic complexity. Hydraulic conditions promoting establishment and persistence of islands were determined with one-dimensional hydraulic modelling of flood flows for 8 unmanaged river cross-sections with islands and 8 cross-sections in the adjacent channelized reaches. Average age, number of islands and their average and total area in the reach markedly increased over the study period. However, the increase was not steady but moderated by island erosion by flood flows, island establishment shortly after major floods and island coalescence in the years without such floods. Hydraulic modelling indicated that river cross-sections with islands are typified by significantly lower values of mean water depth, flow velocity, unit stream power and bed shear stress at flood flows than cross-sections in the adjacent, channelized reaches. Such conditions promote deposition of living driftwood on channel bars, initiating island development, and reduce the probability of erosion of existing islands. The total number of plant species on islands varied highly and either exceeded or was similar to that recorded on riparian forest plots in particular years. This study indicates that (i) island re-establishment in the river was initiated by substantial channel widening, (ii) variation in flood magnitudes exerts a considerable influence on the trajectory of island development, and (iii) the contribution of islands to the overall species richness of plant communities in the river corridor at early stages of island re-establishment may be highly varied.
在二十世纪，拉巴河（Raba River）被严重渠化，在其河道中深深地切入。在森林走廊内的 2.3 公里长的范围内，废弃了渠化结构，随后在 30 年和 35 年重现期洪水期间，河道明显变宽，重新建立了多流河道模式和岛屿发展。在 2011 年至 2017 年期间，每年进行形态学和植物学调查，以确定控制岛屿发展及其植物区系复杂性的过程和模式。通过对 8 个未管理的河道横断面和 8 个相邻渠化河段的洪水一维水力模型，确定了促进岛屿建立和维持的水力条件。在研究期间，该河段的岛屿平均年龄、岛屿数量以及其平均和总面积都明显增加。然而，这种增加并不稳定，而是受到洪水侵蚀、大洪水后岛屿建立以及无此类洪水年份岛屿合并的调节。水力模型表明，与相邻渠化河段相比，具有岛屿的河道横断面在洪水时具有明显更低的平均水深、流速、单位水流功率和床面剪切应力值。这些条件促进了活漂流木在河道沙洲上的沉积，从而启动了岛屿的发展，并降低了现有岛屿侵蚀的可能性。岛屿上的植物物种总数变化很大，在某些年份，岛屿上的物种总数要么超过，要么与河岸森林样地记录的物种总数相似。本研究表明：（i）河道岛屿的重新建立是由河道大幅度拓宽引发的；（ii）洪水大小的变化对岛屿发展轨迹有很大影响；（iii）在岛屿重新建立的早期阶段，岛屿对河道走廊内植物群落的整体物种丰富度的贡献可能是多种多样的。
