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.