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.