I am not a hydrologist but am providing GIS infrastructure to our hydrology department which actually doesn't use GIS very much yet. Therefore I am asking for some hydrology related insights.
Btw: We are using mainly ESRI ArcGIS but also other OpenSource GIS Software is possible to perform any of the described tasks.
Given some waterbodies alongside a large drainage system, I was asked if it is possible to aggregate (sum up) the water flow amount at every basin for the rivers upside the stream. That sounds like a pretty common task for me in hydrologial GIS Software. As I come mostly from transport networks, I was thinking of this as a directed graph, where water amounts act as edge weight, and aggregation is a simple dijkstra algorithm which sums up the weights. So I was thinking of scripting this task. But I wonder if there aren't ready to use tools for that.