And what of the opposite climate extreme—
the deluges and floods that ended the droughts? How often do these floods, with magnitudes similar to the 1861–62 event
{See here: Great Flood of 1862 and here} or even greater, recur? To further explore these questions, we turn next to the Little Ice Age. This cool period, which followed the Medieval Climate Anomaly and lasted until the nineteenth century,
brought several megafloods of a magnitude that no one alive today has yet experienced.
The deep droughts of the Medieval Climate Anomaly eventually drew to a close around AD 1400. For much of the 150 years prior to this date, extreme flooding was relatively rare. In California, people who lived in the broad Central Valley and around the San Francisco Bay would have seen irregular rainfall for hundreds of years, with the winters often failing to deliver the big storms that filled the lakes and fed the mountain snowpack. Around the bay, the mounded villages were empty most years; since the drought had begun, the creeks flowing to the marshlands dried early in the summer, forcing the people to make long treks for freshwater or, if their camps were inland near the larger creeks, long foraging expeditions to the mudflats in the bay to collect mussels and other shellfish. The native populations throughout the region suffered, some perished, and some managed to adapt to the dry conditions.
The pattern of weather began to change, however, and those who paid attention would have noticed the series of cold storms that now began the wet season, dumping rain on the coastal settlements and throughout the Central Valley and the lower mountain slopes. Winters were bringing more storms that were colder, and in the Sierra Nevada the snowpack grew thicker and lower on the mountain slopes than it had been in human memory {See this article from Nov 2015: Nearly 60 percent of Sierra Nevada covered with snow; 100 times greater for the same date in 2014}.
This period was known as the Little Ice Age, and, elsewhere around the globe, rivers and canals froze throughout Europe, and valley glaciers grew larger in northern and central Europe, New Zealand, and Alaska. The Thames River in London froze at least eleven times in the seventeenth century. Lower summer temperatures throughout Europe decreased the growing season by several weeks, causing crop failures in Scotland, Norway, and Switzerland and leading to widespread famine. In Norway, there is evidence for more frequent landslides, avalanches, and floods, and, in the Alps, expanding glaciers occasionally crossed valley floors, damming streams and forming glacial lakes. These lakes would on occasion break through the ice dams holding them back, flooding the valleys below.
In California, some of the storms originated in tropical regions of the Pacific, delivering copious amounts of warm rain that led to flooding. These storms would have swept over the Coast Ranges, across the Central Valley, and up into the Sierra Nevada, delivering heavy rain.
The rain and the rapidly melting snow would have caused small creeks and rivers to swell into raging torrents, ripping up vegetation and soils in the mountains, and turning California’s Central Valley into a vast inland sea.
Many proxy climate records from throughout California contain evidence that
such extreme winter floods occurred between 900 and 150 years ago. Tidal marshes around the San Francisco Bay contain buried evidence of these events in their sediments. Normally, the inflowing river waters spread across the marshes, depositing only a small amount of the finest sediments—clays and silts. Floodwaters, however, carry larger particles in their higher energy flows, depositing a layer of sand and silt on the marshes. Marsh sediment cores reveal that one such layer was deposited on the marshes around AD 1100, and two others around AD 1400 and AD 1650.
Sediment cores taken from beneath the San Francisco Bay itself provide more evidence of the flooding: a gap or hiatus in the sedimentary sequence suggests erosion of hundreds of years’ worth of accumulated sediments from the bay floor. The date for this hiatus matches the earliest of the sand layers in the marsh cores. That such an event is recorded in San Francisco Bay estuarine and marsh sediments
suggests a regionally significant flood, affecting almost half of the state—the drainage area of the bay.
Though considered “young” in geological terms,
these large floods were not documented by humans living at the time and left no evidence in the archaeological record. [...] Below, we describe several key examples of evidence for megafloods throughout the state of California.
Some of the most compelling evidence of large floods in California has been found off the southern coast, once again in the favorable environment of the Santa Barbara Basin. The location and conditions of that basin have combined to produce one of the most detailed and complete records of climate and environmental change anywhere in the world’s coastal oceans, including a record of runoff into this coastal environment during extreme storms.
[...] [Marine geochemist] Schimmelmann concluded that these sediment deposits must have resulted from enormous flood events—“mega floods,” as he called them.
[...] Ultimately, Schimmelmann found evidence for six megafloods over the past 2,000 years in Santa Barbara Basin sediments that would have affected the entire region. The flood layers could be precisely dated because the sediments are composed of annual layers, analogous to the growth rings in trees.
Based on the ages of these megafloods, they appear to have recurred on average every 200 years: AD 212, 440, 603, 1029, 1418, and 1605. During the period straddling the Medieval Climate Anomaly, two cycles were skipped, with 400 years after the floods that occurred in AD 603 and AD 1029.
[...] In its entire history as a state, California has yet to experience one of the mega-floods that Schimmelmann found to be a repeating part of the region’s natural climate. Schimmelmann points out that the periodicity reflected in his record
suggests the region is due for another one soon; it has been more than four hundred years since the last megaflood, which occurred in AD 1605.
[...] The evidence for a 200-year recurrence interval for floods as large or larger than the 1861–62 event in California is growing, bringing more urgency to flood policy and planning efforts in the state.
[...] Intrigued by the phenomenon of cataclysmic floods and their possible causes, Schimmelmann combed through the scientific literature and
compiled a list of other global events that occurred at the same time as these megafloods. He found that,
around AD 1605, at least two major volcanic eruptions occurred: the Huaynaputina volcano in Peru in 1600, and other volcanic evidence revealed in an Antarctic ice core. He also discovered numerous paleoclimate records worldwide showing that the years between 1600 and 1610 were unusually cold, particularly during the summer, with the summer of 1601 being the coldest in the Northern Hemisphere of the past millennium. These records were largely based on tree-ring records from California, Idaho, the Canadian Rockies, Subarctic Quebec, the Gulf of Alaska, New Zealand, Mongolia, and Norway. Ice-core records from both Greenland and Antarctica also suggest unusually cold conditions. But was there a connection between the volcanic eruptions, Northern Hemisphere cooling, and the California megafloods?