I say, and with God's success: If the river has a weak flow, one must sit in a position where they do not use their washing water, or remain between the two rooms for the time it takes for the washing water to go away; so that they do not perform ablution with used water, as it is not permissible to do so. However, 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 the weight, and here the used water is little compared to the rest of the water. Refer to: Sharh Al-Wiqayah p. 97, Ghinayat Al-Mustamli 1: 819, Al-Bada'i 1: 71, and Fatawa Qadi Khan 1: 4, and God knows best.