Reduced sediment load and vegetation restoration leading to clearer water color in the Yellow River: Evidence from 38 years of Landsat observations

The Yellow River (YR), the fifth largest river in the world, plays a crucial role in regional development, making water quality assessment essential. Remote sensing provides a rapid and convenient means of observation, but water quality inversion models are often limited by the complex optical properties of inland waters and the availability of limited in-situ samples. The Forel-Ule color index (FUI), combined with satellite data, is effective for large-scale, comprehensive water quality assessments. However, the long-term water color dynamics of YR and its response to environmental changes have not been systematically studied. This study developed an improved hue angle (alpha, FUI conversion parameter) inversion model using Landsat data and assessed YR's water color dynamics from 1985 to 2023. The results showed that YR's FUI was generally high (mean FUI: 16.84 +/- 1.85), with notable seasonal variation and spatial distribution patterns significantly influenced by dam construction and environmental features. Over the past 38 years, 86.47 % of river sections exhibited a declining alpha trend, particularly in the Loess Plateau. Reduced sediment load and increased vegetation, driven by engineering measures and the "Grain for Green" policy, were the primary factors behind long-term water color changes. Overall, the shift of YR water color towards greener hues was a positive signal, indicating improvements in water quality and ecosystem recovery. This study is significant for a deeper understanding of YR's water quality changes and for informing watershed ecological restoration and management strategies.