There are longstanding concerns about the environmental impacts of super-dams such as Iron Gate I, the Danube River's largest hydropower scheme. Iron Gate I is suspected of trapping up to 80% (∼590 000 tons per year) of dissolved silica in the form of sedimenting diatom frustules and 30 000 000 tons per year of suspended solids. This study, however, indicates that (i) conditions are unfavorable for primary production in Iron Gate I except for the small quiescent center of Orsova Bay, and the diatom production is much too low for the suspected silica uptake; (ii) Orsova Bay is the most important sediment trap as resuspension does not occur, with ∼1% (82 000 tons per year) suspended solids retention, and (iii) also the only significant silica trap, with ∼0.2% (1000 tons per year) retention. It is most conservatively estimated that no more than 5% of dissolved silica can be retained by the Iron Gate I reservoir, and therefore the earlier estimate of the huge retention can definitely be ruled out.