Ecological study
For each new process domain reach, we identified all vascular plants in the riparian zone between the summer low water mark and the edge of the riparian zone (as defined by the dominance of Vaccinium myrtillus(bilberries)). The identification of plant species followed the taxonomy in Krok and Almquist (2013). In the following cases, two or more species were treated as one taxon: Carex juncella + Carex nigra ,Callitriche spp. (difficult to identify in the absence of floral characters), Hieracium spp., Taraxacum spp. (apomictic),Sparganium spp. (common with hybrids). All vegetation inventories took place in June-August 2016. For large lakes, we only inventoried riparian plants found in the first and last 300 m along the shoreline. We calculated richness (number of species) and species density for each reach/lake and determined community composition. We used regression analyses to study the relationship between changes in species density and richness with geographical distance downstream. We also examined whether there were relationships between community similarity, using Jaccard’s index, and distance between reaches to determine whether hydrochoric dispersal is a potential control on riparian metacommunity organization. All statistical analyses were performed in R studio version 3.6.0 (R Core Team, 2021).