What causes "half a river is red"?

Hello, this poem is written about "half river rustling, half river red". The weather is clear and calm, the river flows slowly, and the river wrinkles into tiny ripples. The part that receives more light shows "red"; Where there is little light, there is dark blue. The poet grasped the two colors on the river, but showed the scene that the river was sparkling at dusk and the color changed rapidly in the sunset. The poet was intoxicated and put his happiness in the description of the scenery. Easy to adopt, thank you.