In early September, 2013, the rain started in Colorado. It didn’t relent for an incredible five days, until it had dropped about a year’s worth of water. Washed out roads dominated the news images, but there were also more than 1,100 landslides in the rugged Colorado Front Range terrain. It was unlike anything seen in 150 years of recorded history there.

University of Colorado Boulder’s Scott Anderson, Suzanne Anderson, and Robert Anderson (Suzanne and Robert are a married science duo, but Suzanne told Ars that “Scott is unrelated to us as far as we know”) saw an opportunity to learn something interesting from those landslides. Part of the area had been mapped two years prior by airborne LiDAR, which measures surface elevation very precisely. The researchers wanted to get a repeat survey funded right away to measure the changes, but Suzanne Anderson said their efforts were complicated by the sixteen-day shutdown of the federal government in early October.

In the end, the Federal Emergency Management Agency undertook its own LiDAR flights in November, though the data was slightly lower resolution. By calculating the differences in the 100 square kilometer overlap between the two LiDAR datasets, they could work out the volume of sediment that slid downslope and was washed downstream.

Everyday processes slowly shape Earth’s surface, requiring large spans of time to build to massive changes. But “everyday processes” include those that don’t happen every day. Infrequent, short-lived, and violent events like floods can have surprisingly large effects. (On the truly mega-scale, places like Washington’s Scablands and Tibet’s Tsangpo Gorge can testify to that.) These events are challenging for geologists to study, as they are more difficult to catch in the act. The Colorado storm presented an opportunity to do just that—measure the fresh consequences of a rare, extreme event.

The area of LiDAR overlap was just west of Boulder, Colorado. It extended from the serrated ridgeline that marks the edge of the Rocky Mountains, formed by layers of sedimentary rock that tilt up into the air. Several river canyons carved into igneous and metamorphic rocks that make up the core of the Rockies were included in the survey area.

Changes in surface elevation since the 2010 LiDAR mapping showed 120 new grooves formed by landslides, some of which broke apart and flowed hundreds of meters down gullies and into swift-moving rivers. In most cases, these grooves were about one-half meter deep, leaving only bare bedrock behind. In total, about 21,000 cubic meters of sediment came down in the landslides within this 100 square kilometer area.

If you compare the amount of material removed from the walls of the bedrock canyons to the area of each watershed, the average loss was equivalent to shaving the entire area down by 16 millimeters.

We can actually compare that to the long-term average. By measuring an isotope of beryllium that accumulates in quartz exposed to the effects of cosmic rays at the surface, geologists have worked out that those canyon walls erode away at a rate of 30 to 60 millimeters per thousand years. That means that, depending on the canyon, these landslides accomplished 150 to 2,200 years’ worth of erosion in just five days.

Now, these events weren’t so energetic that they carved out significant amounts of bedrock. The landslides mostly carried away loose sediment that had built up slowly as the bedrock weathered and crumbled. But swabbing the decks and exposing fresh bedrock also speeds up the weathering process, in addition to shipping out the products.

The Colorado rainstorm is another reminder that rare, potent events can be major contributors to the overall shaping of landscapes, as are the mild, everyday processes. Every once in a great while, the hare pulls up in a sports car and gives the tortoise a lift. Events that seem incredibly rare to us nevertheless happen again and again and again—if you wait long enough. And landscapes do.

Geology, 2014. DOI: 10.1130/G36507.1 (About DOIs).