直到20世纪40年代，由黑河、北大河贯注的居延海仍有300多平方公里的湖面，之后竟6次干涸。
O O B-Time I-Time I-Time I-Time I-Time I-Time I-Time I-Time O O B-Location I-Location O B-Location I-Location I-Location O O O B-Location I-Location I-Location O O B-Metric I-Metric I-Metric I-Metric I-Metric I-Metric I-Metric I-Metric I-Metric B-Thing I-Thing O O O O O O O O O