Given the global importance and fragile status of wetlands and rivers, proposals for more dams must be carefully evaluated so as not to compromise human well-being and biodiversity. In his enthusiasm for using hydropower to generate renewable energy and cut greenhouse-gas emissions, Mike Muller overlooks the disruption by dam construction to the huge socio-economic and ecological value of wetlands (Nature 566, 315–317; 2019).

The extent of natural wetlands has decreased by more than 35% over the past 50 years or so, as a result of dam construction and land transformation (see go.nature.com/2uezpvu). This loss cannot be offset by an increase in artificial wetlands of lower biodiversity, such as rice paddies and reservoirs, as Muller suggests.

Wetlands are crucial for conserving the diversity of freshwater species, whose numbers have declined by more than 80% since 1970 (see go.nature.com/2tj2jhs). Dam projects already curtail fish catches that provide the main source of protein for millions of people in the floodplains of the Mekong River. And dams can do little to prevent methane emissions from the most important wetland sources, such as those in areas of permafrost thaw.

We must ensure that the collateral damage of hydropower programmes does not cancel out their benefits.