Floods

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Overview

Figure 1. Inundation of buildings in Yuma, AZ, in the flood of late January 1916. The floodwaters came mainly from the Gila River, with some contribution from the Colorado River mainstem above Yuma. The Colorado River at Yuma gage peaked at 250,000 cfs on January 22nd. (Source: Yuma County Library)
Figure 2. Gaged daily streamflow, Colorado River at Lees Ferry, Arizona (USGS gage 09380000), 1921-2018 (thin black line); the stars and colored bars highlight seven historical peak-flow events that correspond to the series of strandlines of flood debris within the Grand Canyon. (Source: Figure 2 in Sabol et al. 2021)

The Colorado River Basin historically experienced periodic flood events, in which the river or its tributaries rose over their banks and inundated adjacent lands. These floods disproportionately contributed to the river’s overall sediment transport and deposition. Many native fish species and riparian plant and tree species depended on flooding for completion of their life cycle.

With increasing Euro-American settlement of the basin in the late 1800s, especially the agricultural development along the river in the Lower Basin, these periodic floods were turned into a destructive menace requiring control (Figure 1). Accordingly, Hoover Dam and many dams subsequently built on tributaries have flood control as a primary purpose.

Today, the natural flow regimes on the mainstem and most tributaries have been extensively altered by dams and diversions. These alterations have especially affected the periodic flood events, greatly reducing their frequency and magnitude compared to pre-management conditions. For the mainstem below Glen Canyon Dam (Lees Ferry, AZ gage; Figure 2), the average annual peak streamflow has been reduced by about two-thirds since the closing of the dam (1963), from 85,000 cfs to 30,000 cfs.

The flood of 1983 damaged the outlet works at both Glen Canyon Dam and Hoover Dam and inundated hundreds of homes and business along the river below Lake Mead, demonstrating that, despite the apparent control of the river’s flood regime, major impacts from large floods could still occur. Yet the high peak streamflows in 1983, and again in 1984, were not remarkable compared to historical floods in the 19th and 20th century, let alone the even larger paleofloods that have been reconstructed mainly from flood deposits stranded well above more recent high-water marks (Table 2).

The paleoflood records show that many events exceeding the largest historical floods (1862-present) have occurred on the mainstem and major tributaries over the last few thousand years. Accordingly, flood-frequency analyses that incorporate paleoflood events consistently result in larger flood magnitudes at a given frequency (e.g., 1-in-100 years) than analyses based on the gaged record alone.

The largest historical floods on the mainstem and major tributaries (Table 1) show seasonal and geographic clustering that reflects multiple meteorological and hydrologic mechanisms:

  • Most of the large floods on the Upper Basin tributaries and on the mainstem have occurred in May, June, and July as a result of the rapid melt of unusually large snowpacks in the headwaters, in some cases exacerbated by heavy rains.
  • In the Lower Basin tributaries and the southern Upper Basin tributaries (e.g., San Juan, Virgin, Escalante) and the downstream mainstem, large floods in September and October have been associated with low-pressure troughs and excess moisture from landfalling Pacific tropical storms.
  • Several of the largest floods on Lower Basin tributaries, and on the mainstem from Grand Canyon downstream, have occurred in winter (December-February), likely as the result of one or a series of Pacific extratropical cyclones accompanied by atmospheric rivers (e.g., Figure 1).

The flood events driven by snowmelt manifest not just in high peak discharges but also in very large monthly, seasonal, and water-year streamflow volumes. Rainfall-dominated flood events typically have much sharper peaks and shorter durations, and produce less anomalous streamflow volumes on monthly and longer timescales, given the same peak discharge.

While Colorado River basin floods have occurred during every phase of ENSO (El Nino, neutral, La Nina), there is an overall propensity for major floods during El Nino events. The floods in 1884, 1891, 1911, 1920, 1941, 1952, 1957, and 1983 coincided with El Nino events, which last for 1-2 years. This is consistent with a NOAA analysis that over the period 1895-2015, El Nino events have been associated with a greatly enhanced likelihood of very wet conditions (precipitation >80th percentile) in the Lower Basin in fall, winter, and spring (Oct-May), and also in the Upper Basin in late winter and spring (Feb-May).

The ongoing warming of the Colorado River basin is likely to increase the risk of extreme precipitation events in the basin, due mainly to warmer air having the capacity to hold more water vapor, all else being equal. Consequently, climate change may elevate the flood risk across the basin–including increased magnitudes of the largest floods–even as the warming is also causing annual average streamflows to decline.


Table 1. Historical large floods on the mainstem and large tributaries

Date and Year Gage or Location Gaged or estimated (*) discharge, cfs Reference or source Comment
January 1862 Colorado R. at Topock, AZ 400,000* Dickinson 1944; O’Connor et al. 1994 Winter rains; likely atmospheric rivers; same systems caused floods in CA
July 4, 1884 Colorado R. at Fruita, CO 125,000* Follansbee and Sawyer 1948 Snowmelt
July 1884 Colorado R. at Lees Ferry, AZ 210,000* Topping et al. 2003 Snowmelt; same event as above
Early 1888 (?) Little Colorado R. near Cameron, AZ 177,000-

194,000*

Unema et al. 2021 Date assumes that the high-water mark emplaced 1884-1891 was caused by the known flood in 1888
February 1891 Gila below Gillespie Dam, AZ 250,000* Smith and Heckler 1955 Main source of flood on mainstem at Yuma
February 26, 1891 Colorado R. at Yuma, AZ 140,000-

200,000*

USGS stage-height record; discharge estimated from height Winter rainstorm (atmospheric river(s)?); flood mainly from Gila
June 24, 1909 Colorado R. at Yuma, AZ 150,000 USGS gage record Snowmelt
October 6, 1911 San Juan near Bluff, UT 150,000* Webb et al. 2001 Tropical storm?
January 22, 1916 Gila R. near Dome, AZ 200,000 (230,000* at mouth) USGS gage record; Smith and Heckler 1955 Winter rainstorm (atmospheric river(s)?); main source of flood same day on mainstem
January 22, 1916 Colorado R. at Yuma, AZ 250,000 USGS gage record Flood mainly from Gila
June 17, 1917 Green at Green River, UT 68,100 USGS gage record Snowmelt
June 8, 1920 Colorado R. at Yuma, AZ 190,000 USGS gage record Snowmelt
June 16, 1921 Colorado R. at Fruita, CO 81,100* Follansbee and Sawyer 1948 Snowmelt
June 17, 1921 Green at Green River, UT 65,500 USGS gage record Snowmelt; contributed to the peak at Lees Ferry the following day
June 18, 1921 Colorado R. at Lees Ferry, AZ 170,000* Topping et al. 2003 Snowmelt; same event as above
June 1921 Colorado R. at Yuma, AZ 167,000-188,000* Topping et al. 2003 Snowmelt; same event as above
September 19, 1923 Little Colorado R. near Cameron, AZ 120,000 USGS gage record Rainstorm
September 19, 1923 Colorado R. near Grand Canyon, AZ 112,000 USGS gage record Same event as above, flood driven by Little Colorado inflows
July 1, 1927 Colorado R. at Lees Ferry, AZ 127,000 USGS gage record Snowmelt; heavy rains in SW CO
September 10, 1927 San Juan R. near Bluff, UT 70,000 USGS gage record Rainstorm
May 17, 1941 Colorado R. at Lees Ferry, AZ 120,000 USGS gage record Snowmelt
June 12, 1952 Colorado R. at Lees Ferry, AZ 123,000 USGS gage record Snowmelt
June 12, 1957 Colorado R. at Lees Ferry, AZ 126,000 USGS gage record Snowmelt
June 1983 Inflows to Lake Powell (Colorado R., San Juan R.) 116,000 USGS gage record Snowmelt plus rain-on-snow; major damage to Glen Canyon Dam outlet tubes
June 1983 Colorado R. at Lees Ferry, AZ 97,000 USGS gage record Same event as above; dam operations attenuated the peak by ~20,000 cfs
July 1984 Inflows to Lake Powell (Colorado R., San Juan R.) 125,600 USGS gage record Snowmelt
January 9, 1993 Gila R. below Gillespie Dam 130,000 USGS gage record Winter rainstorm; atmospheric river event
March 29, 1996 Colorado R. at Lees Ferry, AZ 45,900 USGS gage record Highest flow of the high-flow experiments (HFE; 1996-present)


Table 2. Largest paleofloods on the mainstem and major tributaries, as reconstructed from slack-water deposits and other evidence

Location Estimated streamflow, cfs Number of paleofloods of this magnitude Source Comments
Green R. above confluence with Colorado 265,000 1 (~700 BP) Liu et al. 2020 >13 other floods since ~700 BP exceeded 135,000 cfs
Colorado R. at Moab, UT 350,000 2 (since ~2100 BP) Greenbaum et al. 2014 3 other floods since ~2100 BP exceeded 282,000 cfs
Colorado R. at Lees Ferry, AZ 500,000 1 (~1200-1600 BP) O’Connor et al. 1994 9 other floods since ~2300 BP exceeded 240,000 cfs
Virgin R. at Littlefield, AZ 56,000-62,000 2 (since 1050 BP) Enzel et al. 1994 10 other floods since 1050 BP exceeded 35,000 cfs

Data and tools

USGS Peak streamflow records for key stream gages

The data inventory for most USGS stream gages includes a table of annual peak streamflows, which can also be viewed in graph form. Note that the values for extremely high flows may be outside of the calibration of the stage-discharge relationship for that gage, and likely have greater error than more typical streamflow levels.


Additional resources

Colorado River Basin probable maximum floods, Hoover and Glen Canyon Dams - Reclamation (1990)

After the 1983 flood, Reclamation undertook an effort to recalculate the probable maximum flood (PMF) for the two structures; PMF is “a flood that can be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in a region.”

References for Tables 1 and 2

Dickinson, W. E. (1944). Summary of records of surface waters at base stations in Colorado River Basin, 1891-1938 (Water-Supply Paper 918; p. 292). U.S. Geological Survey. https://doi.org/10.3133/wsp918

Enzel, Y., Ely, L. L., Martinez-Goytre, J., and Vivian, R. G. (1994). Paleofloods and a dam-failure flood on the Virgin River, Utah and Arizona. Journal of Hydrology, 153(1–4), 291–315. https://doi.org/10.1016/0022-1694(94)90196-1

Follansbee, R., and Sawyer, L. R. (1948). Floods in Colorado (Water-Supply Paper 997; p. 159). US Geological Survey. https://pubs.usgs.gov/wsp/0997/report.pdf

Godaire, J. (2019). Paleoflood Hydrology of the Colorado River System (Final Report ST-2019-1736-01; Research and Development Office Science and Technology Program, p. 121). Reclamation. https://www.usbr.gov/research/projects/download_product.cfm?id=2825

Greenbaum, N., Harden, T. M., Baker, V. R., et al. (2014). A 2000 year natural record of magnitudes and frequencies for the largest Upper Colorado River floods near Moab, Utah. Water Resources Research, 50(6), 5249–5269. https://doi.org/10.1002/2013WR014835

Liu, T., Greenbaum, N., Baker, V. R., et al. (2020). Paleoflood hydrology on the lower Green River, upper Colorado River Basin, USA: An example of a naturalist approach to flood-risk analysis. Journal of Hydrology, 580, 124337. https://doi.org/10.1016/j.jhydrol.2019.124337

O’Connor, J. E., Ely, L. L., Wohl, E. E., et al. (1994). A 4500-Year Record of Large Floods on the Colorado River in the Grand Canyon, Arizona. The Journal of Geology, 102(1), 1–9. https://doi.org/10.1086/629644

Sabol, T. A., Griffiths, R. E., Topping, D. J., et al. (2021). Strandlines from large floods on the Colorado River in Grand Canyon National Park, Arizona (Report 2021–5048; Scientific Investigations Report, p. 41). USGS Publications Warehouse. https://doi.org/10.3133/sir20215048

Smith, W., and Heckler, W. R. (1955). Compilation of Flood Data in Arizona, 1862-1953 (Open-File Report). US Geological Survey.

Topping, D. J., Schmidt, J. C., and Vierra Jr., L. E. (2003). Computation and analysis of the instantaneous-discharge record for the Colorado River at Lees Ferry, Arizona: May 8, 1921, through September 30, 2000 (USGS Numbered Series 1677; Professional Paper). U.S. Geological Survey. http://pubs.er.usgs.gov/publication/pp1677

Unema, J. A., Topping, D. J., Kohl, K. A., Pillow, M. J., and Caster, J. J. (2021). Historical floods and geomorphic change in the lower Little Colorado River during the late 19th to early 21st centuries (Report 2021–5049; Scientific Investigations Report, p. 34). USGS Publications Warehouse. https://doi.org/10.3133/sir20215049

Webb, R. H., Boyer, D. E., Orchard, L., and Baker, V. R. (2001). Floods and Riparian Vegetation on the San Juan River, Southeastern Utah (Open-File Report 01–314; p. 1). US Geological Survey. https://pubs.usgs.gov/of/2001/ofr01-314/pdf/OFR-01-314.pdf