One Flood Can Cause Major Change
Single large floods can change landscapes more than precipitation, a study in the Himalayas shows.
European geologists went to Tibet to figure out the role of large floods in the complex landscape of the Himalaya Mountains. They conclusions are stated in a preface:
Glacial lake outburst floods (GLOFs) are exactly what they sound like. The sudden emptying of a glacial lake in high-topography regions like the Himalaya can quickly destroy everything in its path. Cook et al. intercepted a GLOF in the Bhotekoshi and Sunkoshi river valleys in central Nepal as they were monitoring the region in the aftermath of the 2015 Gorkha earthquake. They found that a massive amount of erosion occurred during the outburst flood, which suggests that GLOFs may be the primary factor in landscape evolution for these regions.
The paper by Cook et al, “Glacial lake outburst floods as drivers of fluvial erosion in the Himalaya,” was published in Science. The team says that these large floods, which can transport boulders a meter or more in diameter, are not that rare.
The frequency of GLOFs in the central Himalaya is difficult to establish, because records are incomplete and recorded floods may not be correctly identified as GLOFs. Nevertheless, GLOFs are relatively common in the Himalaya, with a major flood occurring at least once every 2 years on average.
The site this team studied was instructive, suggesting that a GLOF can hit a particular site once every 30 years. The Abstract says,
Himalayan rivers are frequently hit by catastrophic floods that are caused by the failure of glacial lake and landslide dams; however, the dynamics and long-term impacts of such floods remain poorly understood. We present a comprehensive set of observations that capture the July 2016 glacial lake outburst flood (GLOF) in the Bhotekoshi/Sunkoshi River of Nepal. Seismic records of the flood provide new insights into GLOF mechanics and their ability to mobilize large boulders that otherwise prevent channel erosion. Because of this boulder mobilization, GLOF impacts far exceed those of the annual summer monsoon, and GLOFs may dominate fluvial erosion and channel-hillslope coupling many tens of kilometers downstream of glaciated areas. Long-term valley evolution in these regions may therefore be driven by GLOF frequency and magnitude, rather than by precipitation.
The classic GLOF in the western hemisphere was the one that caused the huge Lake Missoula Flood. That GLOF carved the extensive Channeled Scablands in eastern Washington State, cutting through solid lava rock in a matter of hours or days. The authors do not mention that case, but the evidence for a flood as the cause of the scablands catapulted J Harlan Bretz from scorn to fame. His flood theory went from maverick to mainstream quickly, after decades of ridicule by his peers. Other notable dam-breach events in the western hemisphere include Mt St Helens, Earthquake Lake, and Gros Ventre. Search “dam breach” for more examples.
Falling rocks can explode so hard that only nuclear weapons beat them (New Scientist). The power of falling rock is described as much greater than expected. “If falling rocks are big enough and hit the ground hard enough they can create a blast so intense that the rocks are pulverised into powder. Such extreme rockfalls are followed by a shockwave that can snap trees hundreds of metres away.”
Explosive lies: how volcanoes can lie about their age, and what it means for us (The Conversation). If volcanoes can lie about their ages, what else in geology is lying?
Looking at those GLOF floods, if we took just half the assumed age of the Earth, rounded down to 2 billion years, that would translate to 67 million GLOFs! Does that make any sense? Maybe the Earth is much younger, would you think?
The fact that this team states that “the dynamics and long-term impacts of such floods remain poorly understood” speaks poorly of open-mindedness in the geological community. When will they learn their lessons? Landscape changes don’t have to take millions of years. Forces of sufficient magnitude can accomplish a lot of change quickly.