def rivers_with_station(stations): #sam
    """
    Function that returns a set of names of rivers that have an associated monitoring station.
    """
    List_of_rivers =set()                                   #initialate a new empty set
    for s in stations:                                      #loop in side the station list 
        List_of_rivers.add(s.river)                         #add suitable elements into the set 
    return List_of_rivers