Floods arising from heavy rainfall are regular events, but the greatest floods have involved collapses of natural dams and the draining of lakes in truly catastrophic floods unlike any witnessed in modern human history.
Natural dam failures:
Natural dams arise due to blockage of rivers by rock slides, glacial ice, and geomorphology. Accumulation of water behind the dam in lakes can lead to failure of the dam, resulting in sudden release of the water. The character of the resulting flood is quantified by the hydrograph of the flow (the plot of discharge as a function of time), just as for other river floods. The U.S. has been the site of several great flooding events in recent geological history (the last 20,000 years), involving the abrupt drainage of lakes.
The first case history we will consider is the Great Salt Lake. This lake is in a totally enclosed basin (all rivers lead in, none lead out), which is the primary reason for the high salinity. The regional climate has lead to substantial changes in lake level over the past 200 years. In particular, from 1860-1870 there was a 10 foot increase in lake level that coincided with the Mormon settlements in the area. This extended the lake significantly, as is part of the enduring myth of the westward expansion that "water follows the plot", which erroneously attributed cultivation to increases in water abundance. In fact, climate fluctuations can be influenced by agricultural practices, but there are many other causes of local variations. Recent wet years have led to repeated flooding of the lake, and plans to develop a drainage by tunneling through surrounding hillsides into lower valleys.
The Great Salt Lake is a small residual of a much larger lake that once covered about 1/3 of the state of Utah. That lake was called Lake Bonneville, and it resided in the eastern portion of the Great Basin. Lake Bonneville had a surface area of 51,000 square kilometers at its peak, 300 km by 165 km in size. It too was a trapped lake, with all drainages leading in. How do we know that this lake existed? The evidence is provided by the bathtub ring of shorelines that are beveled into the hillsides that once bounded the lake. Shoreline deposits and eroded terraces define the high water level of Lake Bonneville and the lower shoreline of a subsequent lake call Lake Provo. By dating the rock deposits around the lake, using fossils and other means, the time history of the lake elevation has been charted out over the past 30,000 years. The record is somewhat uncertain for times prior to the highest lake level, as those earlier deposits tend to be overridden by the lake maximum, but from 30,000-15,000 years ago there was a gradual 300 m increase in the lake level as Lake Bonneville grew. This corresponds to the time during which the North American ice pack was near its maximum (20,000) years ago, and then receded. The waters for Lake Bonneville were not glacial melt, as the Great Basin was too far south to be fed from the ice pack, but the microclimate was much wetter as a result of the regional weather patterns which were influenced by the ice pack.
Around 14,500 years ago the lake level was at its peak, at an altitude of 1550m, and then it abruptly dropped by 100m, moving from the Bonneville shoreline down to the Provo level, in an event so rapid that there is no intervening shoreline terraced into the hillsides. This event is now recognized as the Bonneville Flood, the result of catastrophic failure of one of the natural dams that had framed the maximum lake level, with the flood waters involving about 5000 cubic kilometers of water! (100m x 50,000 km2) It is estimated that it took about 20 days for most of the water to squirt out, indicating about 1,000,000 cubic m/sec as a flow rate. In contrast, floods in the Colorado river have involved about 1000 cubic m/sec, so this flood was about 1,000 time larger in terms of water flow. Subsequently, more arid climatic conditions have systematically lowered the lake levels, leading to today's residual in the Great Salt Lake.
The point at which the lake walls failed is at the north end of Lake Bonneville, where a saddleback called Red rock pass held in the lake water. This region was overtopped at the high point in the history of Lake Bonneville, and began to erode away, with the geometry allowing very rapid excavation of the retaining wall. About 100 m of elevation was swept away, allowing water levels to drop down to the level of Lake Provo, with a deep gash eroded that allowed water to continue to leak out of Provo until that lake lowered as well, much more gradually. When the wall collapsed, the water raced northward into the Snake River, which flooded throughout Idaho and up to the Washington boundary. All along the Snake River from where Marsh Creek drained into Porneuf River and then into the Snake River, there are geological markers of the massive flood, involving 100-200 m higher water levels in the river system. Large boulders, erosional features, and slosh deposits are traceable for over 1000 km from the rupture point. The flood hydrograph estimated from the various deposits suggests discharge rates of from 0.8 to 1.0 million cubic m/sec, with total flow duration at Red Rock Pass estimated at about 300 days. This remarkable event has parallels in the great lake history of the Rio Grande Rift area, where massive drainage events of much greater age emptied interior lakes into the Rio Grande system, but the Bonneville flood is very well documented due to the relatively young age of the flood.
There was substantial initial resistance by geologists to the evidence favoring a massive flood as Lake Bonneville dropped 100m, but eventually the weight of field evidence was compelling. A similar resistance to the nature of a catastrophic event was encountered in explaining the Channeled Scablands of eastern Washington. The work of J. Harlen Bretz in the 1920's and 1930's involved careful field mapping of the scablands, and he advanced the notion that massive floods of a repeated nature had scalloped the land surface much as a river plain is carved by more modest flows. Indeed along the Columbia river there are great gravel ripples, which appear to be scaled up versions of those normally seen at the bottom of streams. Bretz argued that these must be the product of massive flows, that could activate bedload deposits on a grand scale. This was viewed with great skepticism, as there was no readily apparent explanation for repeated great floods outside modern day experience, and the invocation of any catastrophic floods was deemed too biblical, and at odds with uniformitarianism. However, in the 1960s and 1970s, glaciologists began to understand the nature of floods in glaciated lands, and the specific case of jokulhlaups, or floods from floating of glaciers. In this case the waters are associated with former Lake Missoula, which formed at the southern end of large ice packs over westernmost Canada during the last ice age. The question that arose is why were there intermittent floods caused by spilling out of Lake Missoula?
In part the answer came from studying a very different environment, on the island of Iceland. Iceland has the unusual circumstances of being a glaciated region with active volcanism and hot crustal conditions. The glacier Vatna Jokul is a large ice pack east of the capital, along the southern margin of which there have been repeated massive floods. The explanation for the floods emerged in the 1960s, when it became clear that a subglacial lake (Grimsvotn) accumulates due to melting of the lower surface of the glacier. The water is normally trapped, and water pressure builds up until the glacier actually floats, upon which the water is able to squirt out in a glacial flood (jokulhlaup). This happens quite regularly, about every 20 years, because it takes that length of time for the water pressure to grow to equal the weight of the ice.
The situation in Lake Missoula was somewhat different, as in that case prongs of glacial ice formed boundaries on the glacial Lake Missoula. This was the only outlet of the lake, as on other sides the hills were never overtopped The water level accumulated against the ice dam, eventually leading to pressures sufficient to float the dam, which allowed floodwaters to squirt forth in large volumes, which swept down into the Columbia River drainage as massive floods. These floods formed the scabland deposits. In detail there were multiple sets of ice dams and several blocked lakes. The highest water levels of Lake Missoula reached 500 m against the lobe of ice. The flood deposits down the river drainage are from 150-250 m above the normal river level, indicating flow rates of 13-17 million cubic m/s, or about 10 times more than the huge Lake Bonneville flood. This is considered to be a minimum estimate. The floods moved 15 cubic miles of water/hour! The water levels through the current city of Portland would top the largest building in the city.
Like the repeated floods in Iceland, the lifting and settling of the ice dam at Lake Missoula allowed repeated events. The geological record of sedimentation in the Lake shows alternating cycles of silts and clays, suggesting varying water levels, while deposits in northern Washington and Idaho and in southern Washington show alternating patterns of silts and sands and gravels. The cycles suggest about 50 year intervals between flood events, with more than a dozen floods being recorded. Thus, these amazing floods happened again and again, until the ice receded and the lake levels dropped permanently.
While the outbreak of Lake Bonneville and the periodic floating of the ice dam of Lake Missoula were dramatic natural phenomena, human activity has also resulted in great floods, sometimes intentionally. An example is the history of the Yellow River, which has been used more than once as a massive weapon. The most recent case occurred in 1938, when the Japanese were advancing southwestward into China in their attempt to take over the country. They had overrun the city of Kaifeng, driving Chiang Kai-Shek before them. In an act of desperation, to slow the advancing Japanese, the Chinese leader mined the levees of the Yellow River, which build up high above the surrounding flood plain due to the high sediment load as the river drops out of the highlands to the west. The induced flood, released through a 1/4 mile wide gap in the levees created a massive bog, which actually mired the Japanese, who redirected their advance toward the south. The Yellow River actually changed its course, merging into a Yangtze tributary 400 miles south of its prior track, but near to historic drainages from 1494 and 1854. The stratagem was somewhat successful, but a huge price was paid by the Chinese. The massive flood, advancing at 5 mi/hr immersed 21000 square miles. Over 1 million Chinese perished in the flood, and massive crop losses caused famine. The river returned to its northeastward drainage in 1947. The river normally floods almost every other year, with 1500 floods in the last 2500 years due to natural causes. The previous human-induced flood was in 1642 when General Gao Mingheng also intentionally collapsed the levees in order to suppress a peasant rebellion.
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