def stations_by_river(stations):
    """Find stations which are by the same river

    Args:
        stations (list): list of MonitoringStation objects
        
    Returns:
        dictionary: river name as key to a list of station names
    """
    dic_stations_river = {}
    for station in stations:
        key = station.river
        if key in dic_stations_river:
            dic_stations_river[key].append(station.name)
        else:
            dic_stations_river[key] = [station.name]
        
    return dic_stations_river