Waterfall Natural Bridge

(Genetic type)

Examples: Springbrook Natural Bridge, Three Turkey Natural Bridge, Black Rock Natural Bridge, Death Valley Natural Bridge, Murl Emery Natural Bridge

This type of natural arch occurs when subterranean stream piracy undercuts, and eventually isolates, a portion of rock streambed, leaving a lintel supported by what was, and may still be, the two banks of the stream. This can happen at the lip of a waterfall when cracks and joints permit the water to shorten its course through the rock under the lip. It can also happen for long stretches of rock streambed where no waterfall is apparent. The shapes of the lintel and opening vary greatly depending on the type of rock, depositional mode of the rock (e.g., the extent and nature of cross-bedding), and stream characteristics. Therefore, the specific morphology of this type of natural arch is not usually helpful in its classification. Subterranean stream piracy must be deduced from contextual attributes. These in turn may be dependent on the relative age of the natural arch.

The contextual attribute that must be present is a stream. The natural arch must span either an active stream or an abandoned streambed. If the natural arch occurs through the neck of an incised meander, then it is a meander natural bridge rather than a waterfall natural bridge. The presence of a waterfall either at, or upstream of, the natural arch is also a clear indicator of a waterfall natural bridge. However, as stated above, a waterfall need not be apparent for the natural arch to be of this type. When a waterfall is apparent, its relative closeness to the natural arch is an indication of maturity, i.e., a tracer of subsequent development due to erosion. The waterfall retreats upstream from the natural arch as time passes. Thus, a waterfall natural bridge that is collocated with, i.e., still part of, a waterfall, is young. A waterfall natural bridge that is clearly separated from, but still within linear sight of, a waterfall should be considered an adult example. Once the waterfall has retreated from the natural arch beyond linear sight, e.g., around a bend in the stream, it should be considered old.

It is often the case that the waterfall is not apparent. This is usually due to the stream undercutting a long stretch of rock streambed. In this case, the precursor to the waterfall natural bridge is a sink and rise. The stream seems to disappear into a sinkhole and then rise from the rock down stream. The distance between sink and rise may be several miles. Vreeland labeled such a feature as a "siphon natural bridge." However, unless air is permanently present above the water as it flows through this underground stretch, this type of feature does not meet the requirements of our definition of a natural arch. Nevertheless, such a feature frequently evolves into a waterfall natural bridge. Erosion lowers the stream and isolates the undercut stretch of former streambed, creating a roof over an underground passageway. Almost all of the natural arches that have been labeled as "natural tunnels" in other taxonomies fall into this category.

It frequently happens that roof collapse occurs in one or more section of such an underground passageway. Roof collapse can cause multiple waterfall natural bridges to form in sequence along a single streambed. Also, if roof collapse occurs near one end of the underground passageway, a natural arch can form there even when the air gap between the streambed and roof has only formed over part of the distance between sink and rise. Such a natural arch would still be considered a waterfall natural bridge. Finally, the natural arch may be all that remains of a once extensive roof. In such cases there is no waterfall at all. This is not because it has retreated to a great distance, but because it was never apparent in the first place.

When a waterfall is not apparent, the best indication of maturity is the ratio of opening height to span. When the height is small compared to the span, this type of natural arch is young. When they are comparable it is an adult. When the height is large compared to the span, it is old.

A very special subclass of waterfall natural bridge occurs when a petrified log is found over a streambed. Here again, the flow of water has undercut the log, now rock, leaving it suspended above the streambed, with its two ends embedded in the banks of the streambed. A petrified log differs from other waterfall natural bridge lintels only in how the rock itself was originally formed, i.e., in the geologic attributes of the natural arch. There is no difference in the contextual or genetic attributes. Hence, these unusual features are considered waterfall natural bridges. They are very fragile and very rare.

The waterfall natural bridge is the only type of natural arch where flowing water is the primary cause of formation.