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.