According to Spencer, the Natural Bridge was the remnant of an underground river tunnel that diverted water to the south. Initially, the underground river started capturing water from the Poague Run about to the north, contributing to enlarge the incipient Cedar Creek drainage basin. Over time, through weathering and erosion, the underground river collapsed and disappeared almost completely, the Natural Bridge being the only surviving relic of the roof of the ancient underground river. The creek's larger flow contributed to form the present-day gorge, significantly increasing the distance from the bottom of the water flow to the roof of the underground channel. Today, the "Lost River", about upstream from the Natural Bridge, is an example supporting the hypothesis of the underground river. Water from the side of the gorge flows into the Cedar Creek, similarly as the ancestral Cedar Creek underground river allegedly flowed into the Cascade Creek, presently entering the Cedar Creek approximately below the Natural Bridge.