Amazonian run‐of‐river dam reservoir impacts underestimated: Evidence from a before–after control–impact study of freshwater turtle nesting areas.

Hydropower dams are associated with adverse impacts on biodiversity, yet there remains a lack of robust scientific evidence documenting the magnitude of these impacts, particularly across highly biodiverse tropical waterways. Aquatic and semi‐aquatic vertebrates are disproportionately affected by human changes to aquatic environments and hydropower expansion is an increasing threat to the Endangered yellow‐spotted river turtle (Podocnemis unifilis) across its tropical South American range.Yellow‐spotted river turtle nesting areas were monitored as an indicator of dry season river level changes following run‐of‐river dam reservoir filling. A before–after control–impact (BACI) study design was used with multi‐year field campaigns monitoring turtle nesting areas upstream of the dam.The cause and extent of changes in nesting areas were established using generalized additive models. Nesting area density was evaluated in relation to time (before vs. after), treatment (control vs. impact), time–treatment interaction (BACI), distance to the dam and precipitation. The extent of changes was examined by comparing the remaining proportion of nesting areas after reservoir filling.Dam construction generated an immediate and apparently permanent dry season river‐level rise that extended more than 20 km beyond impact assessment limits. On average the density of nesting areas declined by 69% (from 0.48 to 0.15 per km) across 33 km of river directly affected by the dam. This loss was reflected in a significant BACI interaction. Nesting area density was not explained by seasonal precipitation.Monitoring of freshwater turtle nesting areas provided an effective means to quantify hydropower dam impacts across biodiverse yet rapidly changing waterways. The adverse impacts documented in this study should be preventable by mitigation actions including habitat creation and dry season flow regulation. Such measures are also likely to benefit multiple sandbar‐nesting species (including turtles, crocodiles and birds) in tropical rivers increasingly influenced by run‐of‐river dams. [ABSTRACT FROM AUTHOR]