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The implementation of measures is affected by significant gaps in knowledge concerning barriers, their abundance, distribution in the European river networks (especially of small barriers) and their ecological effects. Recent AMBER study (see above) might have summarised many of the needed basic information. Also, knowledge still needs to be solidified on the effects of some of the key measures. For instance, barrier removal is increasingly viewed as a necessary management measure to reinstate natural connectivity. However, we so far have little knowledge to make predictions about the geomorphological and biological trajectory of a river system once a barrier has been removed (Birnie-Gauvin et al., 2017). Also, knowledge is lacking on measures to mitigate impacts on downstream migration of fish at hydropower turbines especially in large rivers.