In our Amazon baseline study we found that samples from consecutive sampling locations (c. 10 km apart from one another) were quite independent of one another even in a large river that is several hundred metres wide and flowing quickly. Shoaling species were useful here because you would see a large amount of DNA from them at one point and then no detection at the next location downriver. We did see some transfer of DNA where a natural barrier (a steep gorge) caused a significant change in species composition, and the sample taken just 1km or so downstream of the gorge still contained the DNA of the species in the upriver section. The river was flowing very fast here. In smaller, lowland rivers eDNA might integrate information over an area of up to around 1km upstream (there is some really nice work being carried out on this by researchers in Belgium at the moment using cage experiments).