Natural Bridge Formation
The three bridges in Natural Bridges National Monument, Sipapu, Kachina, and Owachomo, are among the ten largest in the world, and they are all developed in the Lower Permian Cedar Mesa Sandstone. Hopi names were assigned to the bridges because modern-day Hopis are descendents of the people who occupied these remote canyons in ancient times (Huntoon et al., 2000). Sipapu means �the place of emergence,� an entryway through which the Hopi believe their ancestors came into this world. Kachina Bridge is named for the rock art symbols on the bridge that resemble symbols commonly used on Kachina dolls. Owachomo means, �rock mound,� and was named in honor of a feature atop the bridge�s east abutment (Huntoon et al., 2000). Although the Cedar Mesa Sandstone was deposited about 270 Ma, the bridges are likely less than 30,000 years old (Huntoon et al., 2000).

When Eocene meandering rivers cut into the resistant Cedar Mesa Sandstone, the meandering patterns established in the overlying, non-resistant rocks were superimposed on the resistant sandstone. Because these rivers drained into the Colorado River and the Colorado River flowed to the sea, the Colorado River controlled the rate and degree of incision of these tributary streams. Until about 6 Ma, the Colorado River was about 1 km (0.6 mi) higher than it is today. There was no Grand Canyon. Then, for reasons related to tectonics (i.e., rearrangement of lithospheric plates) or climate or both, the baselevel of the Colorado River dropped. As the Colorado River began to cut the Grand Canyon, local baselevel for rivers throughout southeastern Utah also dropped. Rapid incision followed for all rivers that drained into the Colorado River. Vertical incision was more rapid than lateral erosion so that the rivers� channels entrenched into the underlying bedrock, preserving their meandering channel patterns (Huntoon et al., 2000).

The last ice age profoundly effected the formation of the natural bridges. During the Pleistocene Epoch of the Quaternary Period, the climate of Utah was wetter. Large floods were probably common in the wetter glacial period of the Pinedale Glacial, a period that lasted from about 30,000 to 12,000 years before present. Consistent with river dynamics, the cut-bank, or outer sides of the meander loops would erode until only thin canyon walls would separate one cut-bank from the next on the meander loop. Eventually, the river penetrates the canyon wall, shortens its course, and abandons the meander loop. The bridges in White Canyon and Armstrong Canyon are the remnants of thin canyon walls that were penetrated by the floods (Huntoon et al., 2000).

Figure 2: Interpreted evolution of the initial and final states of Sipapu Bridge. Initial stage corresponds to a time prior to bridge formation. Final stage corresponds to present-day conditions.

Figure 3: Interpreted evolution of the initial stage to final stage of Kachina Bridge development. Initial stage corresponds to a time prior to bridge formation. Final stage corresponds to present-day conditions.

Sipapu Bridge
At 67 m (220 ft) high and a span of 82 m (268 ft), Sipapu Bridge is the largest bridge in Natural Bridges National Monument and is second in size only to Rainbow Bridge located in Rainbow Bridge National Monument over Bridge Creek near Lake Powell (Huntoon et al., 2000). Sipapu is considered to be a �mature� bridge. Abutments that lie above the level of the present-day streambed edge the symmetrical shape with a smooth, rounded opening.

The bridge developed when the stream in White Canyon cut off a meander bend (figure 2). The abandoned meander is visible from the Sipapu Bridge Trail.

Kachina Bridge
Kachina Bridge is a massive, youthful bridge that is still growing in size. Located near the confluence of White and Armstrong Canyons, Kachina is 64 m (210 ft) high with a span of 62 m (204 ft) (Huntoon et al., 2000). The sandstone making up the span is 28 m (93 ft) thick. In a dramatic display of the impermanence of landforms, an estimated 4,000 tons (3.6 x 106 kg) of sandstone sloughed off the underside of the bridge on its west abutment in June 1992. Kachina Bridge formed when the stream in White Canyon broke through a thin canyon wall just upstream of its original junction with Armstrong Canyon (figure 3) (Huntoon et al., 2000).

Figure 4: Interpreted evolution of Owachomo Bridge. The initial stage corresponds to the time prior to bridge formation and the final stage is present-day condition.

Owachomo Bridge
The oldest bridge in Natural Bridges National Monument is Owachomo Bridge, which is nearing collapse in Armstrong Canyon. Standing 32 m (106 ft) high with a span of 55 m (180 ft), Owachomo is only 3 m (9 ft) thick at the crest of its span. The bridge lies above and parallel to the present-day streambed.

Owachomo Bridge formed when a stream in Tuwa Canyon eroded into Armstrong Canyon. The Tuwa stream twice cut through meander bends into Armstrong Canyon. A second cutting event resulted in abandonment of the part of Tuwa Canyon that passed under the bridge so that Owachomo Bridge is now isolated from the main channel (figure 4).

Stream Channel Morphology Change
Streams, especially in flash flood situations, are dynamic and can produce rapid changes in landforms. These landforms include channel shape, bedforms, stream banks, bar deposits, terraces and meander bends. The extreme channel sinuosity at Natural Bridges played a major role in creating the bridges and continues to have an effect on the landscape today.

Future bridges are in the process of being formed in White, Armstrong, and Tuwa Canyons. Channel dimensions and patterns are affected by changes in flow rate and sediment discharge, as well as the ratio of suspended sediment to bed load. These parameters are all pushed to extremes during the flash floods inherent to the desert landscape in southern Utah.

References:

Huntoon, J. E., Stanesco, J. D., Dubiel, R. F., and Dougan, J., 2000, Geology of Natural Bridges National Monument, Utah, in D.A. Sprinkel, T.C. Chidsey, Jr., and P.B. Anderson, eds., Geology of Utah�s Parks and Monuments: Utah Geological Association Publication 28, p. 233- 250.