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Rounding the Four Corners By Larry Larason

Floods

Floods can happen almost anywhere. If they occur where humans have settled they can be disastrous. But people have always settled near water courses, so every ancient culture that had writing recorded a flood legend. Let’s look as some historic ones in the Four Corners and some prehistoric ones only recently discovered by geologists.

Bluff, Utah was founded by Mormons in 1880 on the banks of the San Juan River. Another Mormon settlement, Montezuma Creek, lay up stream, also along the river. The San Juan is the only perennial stream in the region, so the settlers were dependent upon it. But they soon learned that it is an undependable river.

The Mormons settled along the San Juan at the start of the American Southwest’s wettest decade on record. In 1883 Krakatau erupted explosively in Indonesia, sending ash into the atmosphere and disrupting weather patterns around the world. The effect may not have been as severe as that of the larger eruption of Tambora that caused “the year without a summer” in 1816, but 1884 was disastrous for the settlers in southeastern Utah.

Heavy rainfall in late winter and early spring raised the river level seven feet above normal. Rain continued, and the flood peaked on June 18, sweeping away the village of Montezuma Creek and other non-Mormon settlements nearby. At Bluff the irrigation works were destroyed and fields were covered by sand washed down from the heights above the town where cattle grazing had probably removed most of the grass that would have stabilized the soil. Montezuma Creek was resettled later by others. The Mormons who had lived there moved to Bluff or other communities in the region.

The wet decade of the 1880s was succeeded by drought and dust storms. Inhabitants said that you could walk across the San Juan without getting your feet wet. There was one respite: in September 1896 three days of continuous rain and snow flooded the river again. The settlers’ orchards were suffocated by mud, and the laboriously restored irrigation ditches filled with mud and debris. But the drought returned: in the summer of 1902 the San Juan River was only three feet wide and six inches deep at Farmington, New Mexico.

Alternating periods of floods and droughts continued on the San Juan. One flood spurred the residents of Bluff to build a riprap dam to shield their community from the river. That was a wise move because the big one was yet to come -- in October, 1911.

Freak weather conditions caused three straight days of rain in southern Utah and the San Juan mountains. Pagosa Springs, Colorado received nearly four inches of rain; Durango three and one half. One small community in Colorado saw more than eight inches. Normally in October this moisture would have fallen as snow on the mountains. Flooding began at Pagosa Springs and surged down river. At Shiprock W. T. Shelton, the Navajo Agent reported that his school was under six feet of water, and the entire valley was full “from hill to hill.” Rushing water ripped up the steel bridge at Shiprock and dropped it a quarter-mile downstream. Because of the riprap dam, Bluff was spared from the worst of the flood this time. But the flood destroyed a two-year-old steel bridge at Mexican Hat. A miner returned to his cabin downstream from Mexican Hat after the flood to find it buried in what he estimated to be seventeen feet of fresh mud.

The people of southeastern Utah mostly gave up on agriculture after seeing their fields and crops stripped away by floods again and again. They turned to raising cattle, instead. It was not until Navajo Dam was completed in 1962 that the San Juan River was finally regulated to some degree. * * * Last May, my wife, some friends, and I visited eastern Washington [state] to see firsthand the evidence of monster floods that occurred at the end of the Pleistocene [Ice Age] about 15,000 years ago. These floods are sometimes referred to as the “Bretz Floods” in honor of the geologist, J Harlan Bretz, who figured out what happened to create the unusual terrain called the Channeled Scablands. To understand them we need a short course in the geologic history of the region.

The West Coast of North America is on a plate boundary. Oceanic plates, the Farallon and Juan de Fuca, were subducted [carried under] by the North American Plate. What goes down usually comes back up as lava. And it did so mostly between 17 and 14 million years ago in about 300 flows of basalt, which covered almost half of Washington, a lot of northern Oregon and parts of Idaho. This lava erupted, not from volcanoes, but rather from long vents in a style of eruption called “flood basalts.” In places it is as much as 12,000 feet thick. The flows are stacked on top of each other. The eruptions were not continuous; sometimes a layer of soil formed after one before the next flow overrode it. After the lava ceased flowing, blowing sand and silt formed soil to cover it to a depth of 300-400 feet.

Harlan Bretz began studying and publishing papers on the channeled scablands in the 1920s and 1930s. When he proposed that they had been carved by a gigantic flood, he was ridiculed by more conventional geologists. His theory sounded too much like catastrophism; geologists preferred to think of slow changes brought about through everyday processes [uniformitarianism]. Also, Bretz could not say where all the water had come from. At one point he guessed that Lake Missoula was the source, but then decided that was wrong. It was known even then that Missoula, Montana is built in an old lake bed, but the mechanisms of the flood were not understood. Still, evidence continued to mount in support of his flood theory, and by the 1950s aerial photos of giant current ripples, barely noticeable from the surface, convinced most geologists that Bretz was right.

Other geologists refined Bretz’ theory: An arm of the continental glacier moving south blocked the flow of the Columbia River. Water pooled behind it in Lake Missoula until the 2,000 foot high ice dam broke. A flood crest 300 to 1000+ feet high rushed out at speeds of up to 50 miles an hour and scoured eastern Washington. The term “channeled scablands” refers to the channels, called coulees, where only the floodwater, but no river, ever flowed, and the surface of lava [the scabs] where all the soil was stripped away by the floods. The water finally rejoined the main channel of the Columbia, carving it deeper, and flowed on to the ocean.

So, what evidence did we see on our trip? Giant gravel bars along the Columbia River; huge ripple marks, some up to 30 feet high; hills with a “streamlined” shape from the flood erosion; and glacial erratics. Erratics are rocks ripped up by a glacier, incorporated into the ice, and then dropped where the ice melts – often far from where the rock formed. When the Lake Missoula floods poured across the landscape they floated icebergs broken off the glacier; eastern Washington is littered with erratics carried from mountains in Montana and Idaho. The most spectacular, Yeager Rock, is the size of a two-story house and sits in the middle of a flat field. Unfortunately, taggers have covered most of the lower portions of the rock with graffiti.

Geologists now believe that there was not a single flood, but a cycle of more than forty about 70 years apart. The floods occurred from about 15,000 to 12,000 years ago, so there may have been Native Americans in the region. If so, some of them must have perished in a flood, because by the time they heard the waters coming, it would have been too late to escape. We’ll never know.

Harlan Bretz outlived his critics, saw his theory accepted by a new generation of geologists, and in 1979, two years before he died, was awarded the Penrose Medal – the highest honor given to geologists.