如图所示，质量为3×10⁴kg的鲸静止在海里，g取10N/kg．海水密度不变．

阅读下面这首诗，完成后面问题。
                                                                                      江亭夜月送别二首（其二）
                                                                                                    王勃
                                                                                               乱烟笼碧砌，
                                                                                               飞月向南端。
                                                                                               寂寞离亭掩，
                                                                                                江山此夜寒。

黄河下游形成“地上河”的主要原因是（  ）