def rivers_with_station(stations):
    """Returns a set containing all rivers which have a monitoring station.

    Params:
        Stations - List of all stations. \n.
    Returns:
        {River 1, River 2, ... }
    """
    rivers = set()  # set
    for station in stations:  # iterate and add rivers to set
        rivers.add(station.river)
    return rivers