does anyone know what chapter he’ll be thrown into a river or something?

LMAO FOUND YOU. (Sorry bout that)

If you read the novel in archive of our own, it's chapter 2.12 (if you're asking for the novel chapter). However if in the manhwa, maybe 45 or 46. Taekjoo passed out right after Zhenya broke his ring finger (yes, that crazy horse didn't cut off his finger, he just broke his bone finger) and was thrown in the riverbank. He was then found and brought to the hospital where he met his supposed real partner. Zhenya had acted like he really was Taekjoo's partner (ew fake). It turns out he was the guy that Taekjoo fought underwater.