To start
                                                                                                                                            you need to invent gravity;
                                                                                                                                            and give it time to work its way with things.
                                                                                                                                            Then you find a slope and add some water,
                                                                                                                                            lots of it
                                                                                                                                            like from a dying glacier.
                                                                                                                                            Congratulations!
                                                                                                                                            You’ve just made a river.