River networks affect fish diversity in the Upper Tocantins River. The study looked at fish in the main river and its branches. They found that different types of diversity exist between the main stem and branches. The turnover of fish species is higher in the branches, while the main stem shows a decrease in similarity as you move along it. Overall, the fish communities in this river have high functional similarity but different species turnover rates. This suggests that the river's structure influences how fish diversity is distributed.