Floods

Overview

The Colorado River is s...

File:FILENAME.png

Figure 1. CAPTION. SOURCE.)

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 cast as a destructive menace. 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. Annual peak streamflows greater than 100,000 cfs at the Lees Ferry gage occurred 12 times from 1921 to 1962, but have not occurred since (although Lake Powell inflows greater than 100,000 cfs occurred in both 1983 and 1984; Table 1).

The flood of 1983 damaged the outlet works at both Glen Canyon Dam and Hoover Dam and inundated hundreds of homes and business below Lake Mead, demonstrating that, despite the overall attenuation of the river’s flood regime, major impacts from large floods can still occur. infrastructure and property damage from large floods. Yet the peak streamflows in 1983, and 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.

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 much greater 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.

Additional resources

State of the Science Report

Chapter # of the State of the Science report, describes...