伏流也叫暗河，因岩溶作用在大面积石灰岩地区所形成的溶洞和地下通道中，是具有河流主要特征的水流。主要在喀斯特发育中期形成，发育及其形态常受地质构造及裂隙的控制。东兰国家地质公园东平河在约拉区域形成了地下河与地表河交替出现的“四暗五明”伏流景观。

Subterranean stream, also known as underground river, has the main features of rivers which formed in karst caves and underground channels due to karstification took place in a large scale of limestone area. The streams are formed in the middle stage of karst development, and its development and shape are usually controlled by geological structures and cracks. In the Yuela region of Dongping river in Donglan National Geopark, there is a subterranean stream landscape of underground river and surface river appearing alternately, which also called as four times dark and five times bright river landscape.