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Paleoflood Analysis of the Colorado River Basin

January 13, 2017
by John Weisheit

Excavating slackwater deposits on the Colorado River above Moab, Utah. Red line is the elevation of Probable Maximum Flood (~350,000 cfs), which has occurred twice in the last 2,000 years.
Excavating slackwater deposits on the Colorado River above Moab, Utah. Red line is the elevation of Probable Maximum Flood (~350,000 cfs), which has occurred twice in the last 2,000 years.

While most water managers are currently focused on extreme drought in the Colorado River Basin (CRB), the other side of the hydrologic spectrum is the impact of extreme flooding. Drought has a slow impact on the human economy of the CRB and flooding has an immediate impact on the economy of the CRB.

Additionally, releases greater than 28,000 cfs from Hoover/Davis dams begins to damage private and commercial properties in Nevada, Arizona, California and Mexico. Maximum spillway releases to handle the magnitude of floods that have occured in the last 500-years is sufficient to fill the entire Salton Sink (Imperial Valley), which is a depression on the Earth's crust and below sea level. Not to mention the havoc these magnitudes would cause to pipelines, cables, and bridges that span the river corridor.

Dam managers are forced to confine large snowpack melts in the reservoirs and too avoid spillway releases as much as possible. However, dam managers also like to keep reservoirs as full as possible to avoid water shortages. 

This is a serious conundrum and this issue is getting no attention, despite the advances of paleoflood hydroogy research throughout the world. On your next river trip down the Colorado River, investigate for yourself the evidence in the form of perched sediment deposits known as a slack water deposit. VIDEO.

Water managers prefer to keep their reservoirs as full as possible, leaving no space to accomadate the unexpected flood events that periodically hit a watershed, whether a massive basin-wide snow melt or localized maximum precipitation event. Reservoirs cannot be evacuated quickly enough to make room for such flood volumes and forces managers to use spillways that may, or may not, have the engineered capacity to handle the event and avoid catatrophic dam failures. 

This article will explore the flood history of the CRB in the last 2,000 years, because enough data exists to reconstruct that history. Sediment deposits are a contribution to this data and another is the wood from trees (stabilized and mobilized).

  • What a "slackwater deposit (SWD)" of flood sediment looks like: PHOTO (Moab site)
  • Photos of Pits 1 - 14 (Moab site)
  • A slackwater deposit research PAPER from 2014 (Moab site). A discussion of this paper is provided below.
  • PHOTO in Cataract Canyon; the Powell Expedition of 1871; a cottonwood tree that germinated in the high water zone, which then survived another high water event, as evidenced by the driftwood snag wrapped around the trunk. Thus, the Powell photo records two high water events in the 19th century. This tree is still there, except it is now a fallen carcass laying on the floodplain (1999 repeat photo). Note the lack of non-native tamarisk trees in the 1999 photo, which did not dominate the floodplain until the snowmelt of 1941. Since 1871, no flood has reached the elevation of this tree carcass. This includes the large snowmelts of 1884 (~225,000 cfs) and of 1921 (~147,000 cfs). It is suspected the flood of 1862 may be responsible for the driftwood snag, and perhaps the flood of 1802 (or later) is responsible for the germination of this cottonwood tree. These two flood events of the 19th century will be discussed in this narrative, below. (See: Cataract Canyon: a human and environmental history of the rivers in Canyonlands. Webb, et al. 2004)
  • There is written evidence that indicates a major flood may have occurred between, or during, the two Powell Expeditions. Diaries from the Powell Expedition's of 1869 and 1871.

This article will also explore the impact and benefits of flooding on the natural environment. The benefits are related to the fact that the Colorado River, during its 6 million year history, has been molded by thousands of floods that humans today would label as catastrophic. Nature would disagree with that assessment, because Nature is doing what it has always done, and droughts and floods did not wreak havoc on the human economy until which time humans decided to build very big things in all the wrong places.

A discussion on the characteristics of floods mostly involve the transport of massive quantities of organic material (and seeds), nutrients, sediment, and large boulders; that the true purpose of water, is to quench, cleanse, renew and beautify the earth.

 The Colorado River has so far escaped massive flooding in the 21st and 2Oth centuries. Instrument gages to measure streamflow did not arrive on the scene until after the establishment of the US Geological Survey (1879) and after the construction of the continental railroads. The first major flood to show up with gages in place, occurred in 1884. Every one of those gages were damaged by the magnitude of this flood and the entrained driftwood. However, the administrative record does contain sufficient physical and historical data to tell the story of floods on the Colorado.

  • An archive of documents, photos and graphics related to Colorado River floods is located HERE

THE 1800s: THE BIG SNOWMELTS OF THE COLORADO RIVER BASIN

THE FLOOD OF 1802 - The evidence of this event is the result of a tree ring analysis of hackberry trees in Cataract Canyon (below the confluence of the Green River and the Upper Colorado River). This native elm grows along a distinct and consistent horizontal line in the high water zone (PHOTO). A sample from a hackberry tree at this line indicated its germination occurred in 1802. It should be noted that hackberry trees grow at higher elevations, and it is hard to determine if the germination is the result of animals foraging on seeds, from floods, or a combination of both.

THE FLOOD OF 1816 -  This event is related to the highly explosive eruption in 1815 at Mt. Tambora, Indonesia. Chronologers worldwide referred to the impact this eruption had on world climate as "the year of no summer." Airborne volcanic ash typically creates an ample snowpack in the headwaters of the Colorado River, but no scientist or chronologer was present to observe the magnitude of a flood event that probably visited the CRB. The snowmelt of 1884 was preceded by the eruption of Mt. Krakatoa, Sumatra.

THE FLOODS OF 1844 & 1864 - Evidence from the Rocky Mountain headwaters of the Mississippi River; the Front Range in the state of Colorado. Floods on the Western Slope of the Rockies were likely, but records from chronologers range from non-existant to scant. (See: Page 10 in Floods of Colorado. 1948, Follansbee).

THE FLOOD OF 1862 - this event is related to a phenomenon of climate now described as an "atmospheric river," or AR (See: Dettinger and Ingram). This event was chronicled as it happened by California newspapers, and in the diary of a scientist (William H. Brewer) working in the Central Valley of California, and by a Mormon pioneer (John D. Lee) in southwest Utah. Railroad engineers noted evidence of this flood in a schematic for building a bridge across the Colorado River above Topock Gorge near Needles, California (SCHEMATIC). Topock Gorge is 95 miles below Hoover Dam. The engineers took a guess that the flood occurred in 1857, but the evidence (described above) has led to a determination that this event actually occurred in 1862. There are two numbers for the flood magnitude at the Topock Bridge: the first analysis put the volume at 400,000 cfs and the second analysis put the number at 500,000 cfs (TABULATION).

If the flow at Topock, AZ was 400,000 cfs, than the corresponding flow at Cataract Canyon, UT, was ~300,000 cfs. And if 500,000 cfs at Topock, the flow at Cataract was ~375,000 cfs.

Why are these numbers important? if Lake Mead is full, Hoover Dam's maximum discharge capacity is only 322,200 cfs. If the reservoir is empty, this lessens the concern of dam failure. Unfortunately, water managers prefer their reservoirs to be as full as possible. Even if the reservoir was completely empty, Hoover Dam would still have to discharge half the total volume of the snow melt. In 1862 the April to August snowmelt was propbably around 50 million acre-feet; nearly twice the total volume of Lake Mead (I am deducting the sediment storage in the reservoir).

Hoover Dam will likely fail, mostly because Glen Canyon Dam upstream will fail first. Glen Canyon Dam only has a maximum discharge capacity of 253,000 cfs. If both dams fail, 100 million acre-feet are headed for the Salton Sink and the Gulf of California, taking every piece of infrastructure with it. 

THE RESULTS FROM THE PALEOFLOOD SITE NEAR MOAB, UTAH

The purpose of this research was to provide data to the Department of Energy (DOE) to support the removal of the second largest uranium waste pile in the nation from the floodplain of the Colorado River near Moab, which is indeed happening. So far, over 50% of the radioactive waste has been removed and taken by railroad to a burial site 30 miles north at a place between the Book Cliffs and Interstate 70 (Crescent Junction). The cost of removing the pile is projected to be $1 billion.

Funding for the paleoflood research was provided by The Citizen's Monitoring and Technical Assessment Fund (www.mtafund.org). The grant received was $40,000 and the work commenced in May of 2005. The preliminary report was presented to the DOE in 2006. Over 250 copies were mailed to agencies, stakeholders and tribes in the CRB. The peer-reviewed paper was released in 2014 and published by the American Geophysical Union.

Highlights of this report include:

  • River: Upper Colorado River (above the Green River confluence and below the Dolores River, the last major tributary).
  • Site location: 10.5 miles above the Moab Bridge.
  • Time frame: The last 2,140 years (+/- 220 years).
  • Number of floods: 44
  • Range of floods in cfs: 60,036 to 349,616
  • 34 to 40 floods have exceeded the magnitude of the USGS 100-year flood determination.
  • 20 - 25 floods have exceeded the magnitude of the USGS 500-year flood determination.
  • 5 floods have exceeded a peak discharge of 282,000 cfs.
  • The two largest floods were slightly over 349,000 cfs.
  • The 100-year flood would have a peak discharge ranging from 156,440 to 179,050 cfs.
  • The 500-year flood would have a peak discharge ranging from 224,780 to 265,570 cfs.
  • A 1000-year flood would have a peak discharge ranging from 256,740 to 310,770 cfs

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