Seasonal responses of chemical oxygen demand and dissolved oxygen to dam regulation in the Yangtze and Yellow Rivers during 1988–2018: Role of altered flow regimes

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The riverine chemical oxygen demand (COD) and dissolved oxygen (DO) dictate water quality and biogeochemical health. Mega-dams substantially alter the flow regimes in large river basins, yet their impacts on seasonal COD and DO dynamics in the long term remain unclear. This study examined the changes in annual and monthly COD and DO exports from the Yangtze and Yellow Rivers during 1988–2018, and quantified the influences of natural and anthropogenic factors in the pre- and post-dam periods using the machine learning (ML) approach and partial least squares-structural equation modeling (PLS-SEM). There was a pronounced decrease in annual average COD in Yellow (−35.0%) but an increase in Yangtze (3.7%), and DO increased substantially in Yellow (9.8%) but remained stable in Yangtze from pre- to post-dam period. The seasonal variation of COD substantially weakened in Yellow, and the peak of COD and DO flux occurred one month earlier in Yellow but later in Yangtze in the post-dam period. The relationships of COD and DO fluxes with discharge became stronger in the post-dam period, especially in the Yellow. The COD-discharge coupling shifted toward enrichment in the Yangtze River and weakened in the Yellow River during the post-dam period. From the pre- to post-dam period, the relative contribution of hydroclimatic and vegetation factors in the Yangtze increased to 72.62% for COD flux and dominated COD and DO concentration variability in the Yellow over the Yangtze (56.60% vs. 38.02%). Altered flow regimes under dam regulation changed the timing and magnitude of COD and DO flux responses, and amplified the role of sediment as a carrier of organic pollutants. Furthermore, vegetation greening improves water quality in the Yellow, whereas increasing precipitation and discharge enhance pollutant mobilization in the Yangtze. This study reveals that dam regulation is strengthening the flow-water quality connection and elevating the relative importance of hydroclimatic and vegetation factors in explaining water quality variation, with important implications for sustaining riverine health in heavily regulated basins including the Mekong and Amazon.