I'm working with ArcMap 10.2 and I'm pretty new in this area.. What I want to do is compute the river slope for a lot of rivers from a DTM...I know that there is a way to do it manually passing by excel..but I was wondering if there was an automatic way to do it..also maybe using other programs.. I have all the territory of Swizerland to analyse..
If you are new to hydrology/arc I'd suggest using arc hydro (for 10.2 try download and documentation). This will give you average slope and 10-85 using Watershed Processing - Flow Path Parameters - Flow Path Parameters from 2D Line.
elrobis's is correct that you may need to sample the slope more often depending on: (1) how the rivers were defined (think length, although it is usually accumulation based) (2) change in slope in the area (3) how you are using your results. However, if this is the case you can use the same slope determination method but on finer river segments/definitions.