Despite the intrinsic potential for long-distance dispersal of many freshwater organisms, freshwater habitats may be effectively the most dispersal-limited of all major ecosystems. This is most likely due to the high degree of spatial isolation inherent in the structure of lakes, streams and rivers (Comte & Olden 2018) and the steep environmental gradients within them (e.g., of light, Stomp et al. 2007). Lakes are in many ways similar to oceanic islands, in that resident species are isolated by an uninhabitable terrestrial matrix (Kappes et al. 2014), though systems of ponds and lakes can be interconnected by a network of aquatic corridors (Baguette et al. 2013). Rivers experience isolation at two levels: first, because there is little exchange of organisms across drainage basins (Leuven et al. 2009) and second, because dispersal is constrained by the dendritic structure and directional flow of river networks (Hänfling & Weetman 2006; Campbell Grant et al. 2007; Carrara et al. 2012; Wubset al. 2016). Therefore, many riverine organisms have relatively small ranges and high levels of differentiation across river branches. Due to the many spatial limitations just described, freshwater communities are often not saturated (Shurin et al. 2000; Irzet al. 2004), suggesting species are limited in their ability to reach new sites via dispersal (Shurin & Smith 2006).