Artificial barriers across rivers can reduce freshwater biodiversity by fragmenting habitats. According to Günther Grill and colleagues, dam construction has disrupted two-thirds of large rivers worldwide (Nature 569, 215–221; 2019). We have found that many more rivers have been disrupted by hundreds of thousands of smaller artificial structures such as weirs, ramps, culverts and fords.

With the help of volunteers, we discovered 23,618 artificial barriers to free-flowing rivers in Great Britain (J. Jones et al. Sci. Tot. Environ. 673, 756–762; 2019). Across Europe as a whole, we have so far recorded more than 415,000 barriers, 85% of which are small structures (see go.nature.com/2kc1orn). Many of those we surveyed are no longer in use and so could be removed.

Essentially uninterrupted rivers do still exist in Europe — in the Balkans, for example — so it is important to safeguard their biodiversity by keeping them free of barriers large and small.