The increasing contribution of non-dispatchable renewable energy sources has changed the operating patterns of hydropower plants, especially in run-of-river schemes with limited storage capacity. Under intermittent dispatch, turbine shutdowns may occur while environmental-flow requirements must still be maintained. In these situations, spillway releases become more frequent, creating hydraulic conditions that can lead to total dissolved gas (TDG) supersaturation downstream. This study evaluates TDG supersaturation as an operational constraint in an Amazonian run-of-river hydropower plant. This assessment combines field measurements with controlled laboratory exposure tests using representative native fish species. Total gas pressure (TGP) was measured in both field and laboratory settings and converted into TDG saturation percentages. A laboratory-scale TDG generation system was developed to reproduce supersaturation conditions associated with spillway operation. Field measurements recorded TDG levels above 130% during spillway-activation events associated with dispatch decisions. In the laboratory, exposure tests showed rapid loss of viability in native Amazonian fish at TDG levels of 115% and above. These results indicate operational warning ranges for sustained spillway use. The findings suggest that TDG supersaturation should not be treated solely as an environmental issue. Rather, it represents a technical constraint on hydropower dispatch in systems with increasing renewable penetration. Incorporating TDG-related limits into operational planning and spillway management may help preserve hydropower flexibility while supporting environmental compliance.

As variable renewable generation grows, intermittent dispatch of run-of-river hydropower plants may transfer required environmental-flow releases from turbines to spillways, creating conditions that favor total dissolved gas (TDG) supersaturation and may constrain hydropower flexibility. This study presents a reproducible basin-scale screening assessment for 24 selected hydropower plants in the Brazilian Amazon and Tocantins–Araguaia basins. The analysis combines a plant inventory, spillway typology, standardized environmental-flow scenarios, and two configuration-specific linear relationships between unit discharge and the increase in TDG saturation (ΔTDG), derived from digitized Pubugou and Gongzui observations. Two release configurations were examined: flow distributed among all available bays and flow concentrated in a single bay as a theoretical hydraulic bounding case. The Pubugou-based relationship was applied only to broadly comparable ski-jump configurations within 9.1 ≤ UD ≤ 72.3 m³ m⁻¹ s⁻¹, whereas the Gongzui-based relationship was provisionally assigned, as an inventory-level first-order analog, to controlled spillways discharging into stilling basins within 34.6 ≤ UD ≤ 234.6 m³ m⁻¹ s⁻¹. Hydraulically dissimilar cases were classified as NE-H, and cases outside the applicable empirical domain as NE-UD. Digitization sensitivity, regression uncertainty, and model-form selection were explicitly evaluated and documented. None of the distributed-flow scenarios produced a numerical estimate: cases assigned to the Pubugou- or Gongzui-based relationships were classified as NE-UD, whereas hydraulically dissimilar free-surface cases were classified as NE-H. Four single-bay scenarios produced configuration-specific $\Delta \mathrm{T}\mathrm{D}\mathrm{G}$ increments: 22.5–33.4 percentage points for three Gongzui-based cases and 13.6 percentage points for one Pubugou-based case. Upstream TDG was not added, and no plant-specific final concentration or universal ranking was reported. Sinop and Colíder observations were retained as qualitative contextual evidence of limited transferability and the importance of site-specific hydraulics. The outputs support conditional monitoring prioritization under standardized assumptions, not compliance prediction, ecological-risk assessment, or gate-operation recommendations. Synchronized monitoring and plant-specific rating curves are required before TDG-related variables can be incorporated into operational planning.