Hi, I had a few minutes so I thought some (VERY :P) simple visuals might help. I don't know the whole situation, but I imagine a male is walking beside a river. Following the river path. Then he comes to some trees. Now it is too difficult to walk right beside the river. So, he goes around the trees. But the tree's are thick, and now he can't see the river. However, he thinks, "If I keep walking straight, I can rejoin the river when I get to a place where there are no trees." But he made an incorrect assumption. He assumed the river continued straight. In reality, not a long distance from his starting position, the river curved off to the left. So from his starting position: "...he didn't know the river made a very sharp curve away from him a little way beyond," [a little farther // a little ahead // a little past the part he could see] "and so as he walked straight ahead he was getting farther and farther away from the river." But it doesn't always relate to distance, sometimes it could be understanding (X is beyond my understanding), or physical ability, or financial affordability, etc. https://ludwig.guru/s/a+little+beyond I hope that helps!

beyond (where he could see, sense or predict)

Here, "just a little way beyond" means that the river is curving away from him a little bit in front of him/further ahead :) Hope this helps!