It was the river where Kenma and Kuroo trained in volleyball when they were college students. The Yujutanigawa River. It is seen in paragraph 73 of chapter 29 "Nou" of volume 4 of the manga. It is also here that Kuroo practiced the "Hitorijikansa". Paragraph 147 of chapter 32 "Otonaneko to Hinagarasu". This river is located near the author's family home.