I say, and with God's success: Whoever performs ablution from a weak-flowing river must sit in a position where he does not use his washing water, or he should stay between the two rooms for the time it takes for his washing water to go away; so that he does not perform ablution with used water, as it is not permissible to do so. Unlike if the river has a strong flow, this condition is not required; because the water that has mixed with used water is judged by the predominance of weight, and here the used water is little compared to the remaining water. Refer to: Sharh Al-Wiqayah p. 97, Ghinayat Al-Mustamli 1: 819, Al-Bada'i 1: 71, and Fatwas of Qadi Khan 1: 4, and God knows best.