def rivers_with_station(stations):
    """Return a set with the names of the rivers with a monitoring station"""

    # make an empty list for the items to go into
    river_names = []

    for i in stations: 
        river_names.append(i.river)
    
    # convert to a set to remove duplicates
    river_names_set = set(river_names)

    return river_names_set