Numerical Simulation of Water Environment in the Mountainous River of the Upper Heihe River Based on MIKE21

• • The coupled MIKE21 hydrodynamic-water quality model achieved R2 = 0.89 (hydrodynamic) and average R2 = 0.86 (water quality), with MRE = 11.3% and 14.21%, respectively, demonstrating high predictive accuracy for mountainous river systems. • • Average flow velocities during wet, normal, and dry periods were 1.78, 0.72, and 0.36 m·s−1, respectively; natural sections reached up to 4.3 m·s−1, while reservoir sections approached stagnation, highlighting the hydraulic impact of cascaded dams. • • Spatially, reservoir sections exhibited significantly higher nutrient concentrations (TP: 0.12–0.17 mg·L−1; TN: 0.60–0.80 mg·L−1; NH3-N: 0.10–0.45 mg·L−1) compared to natural sections (TP: 0.07–0.10; TN: 0.25–0.50; NH3-N: 0.025–0.250 mg·L−1), indicating nutrient retention in reservoirs. • • Temporally, TN and NH3-N peaked during dry and normal periods (0.5 and 0.1 mg·L−1, respectively), while TP peaked during wet and normal periods (~0.1 mg·L−1), underscoring the need for seasonally adaptive pollution control strategies.