INTRODUCTION Freshwater ecosystems are collectively the most altered ecosystems in the world (Leidy and Moyle 1997; Dudgeon et al. 2006; Reid et al. 2019), with most river systems dammed, diverted, polluted, channelized, or otherwise permanently changed. They are also among the most invaded by alien species, especially in temperate regions (Moyle and Marchetti 2006; Light and Moyle 2015).