Although the Southwest has an overall arid climate, there are several large rivers that flow through the area, including the Colorado River and Rio Grande. Many of the Southwest’s largest floods have occurred along the Colorado River and its tributaries (Figure 9.26). Along floodplains, the soil is fertile thanks to nutrients deposited by the rivers, and nearby water allows for easy irrigation. These factors encourage development on flood-prone areas throughout the Southwest. In the Great Plains, a large proportion of farmland—a significant industry in the Southwest—is located on floodplains along rivers that flow through the region. Before humans settled along rivers, floods were often beneficial events: a flood would wash away nutrient-depleted soil and then deposit fresh minerals and other nutrients to help support future plants. People now face the dilemma of whether or not to build in areas that are potentially subject to flooding. Water control structures such as dams are engineered to protect infrastructure and lives, but nature is not always so easily controlled.

Figure 9.26: The Colorado River and its tributaries in the Southwestern states.

Floods are controlled by the rate of precipitation, run-off, stream flow, and shape of the land surface. They may occur as water overflows the banks of a standing water body (such as a lake) or flowing water (such as a stream), or when rainwater accumulates in an area that normally has neither standing nor flowing water. Areas near rivers, tributaries, creeks, and streams are likely to experience flooding during periods of heavy rainfall.

Flooding can occur at any time of the year and is caused when more water enters a stream/river channel than the channel can contain. This situation can develop when water is unable to soak into the ground and instead runs off into a river channel. Runoff can occur if the ground is already saturated (full of water) or if the ground is too dry, hard, or frozen. The slope of a river (i.e., the topography of the land) can also contribute to flooding. If rivers have a steep slope, water can quickly move through the channel and continue downstream. If rivers have a shallow slope, water moves slowly through the river channel and remains in the area instead of moving downstream. Large floods typically result from unusually rapid regional melting of snow in the spring or from major weather systems that bring heavy rainfall over a large region. Flash floods—rapid flooding of low-lying areas—are often associated with heavy rain, which can quickly waterlog soil and lead to mudslides on steep terrain, resulting in damage to roads and property. In areas of lower elevation, flash floods can be produced when slow-moving or multiple thunderstorms occur over the same area. When storms move more quickly through an area, flash flooding is less likely. Although flash floods may be of only a short duration, they can cause major damage—they have been known to wash coffins out of graveyards, destroy structures, and demolish manmade dams.

In the Southwest, arid air travelling from the western mountains draws in moisture from the south where there are no mountains to block the moisture, a phenomenon known as a monsoon climate. Warm, moist air has a concentration of energy that may be released in sudden, violent thunderstorms, generating downpours that lead to flash floods. Monsoon floods occur in every Southwestern state, and can reach heights of 9 meters (30 feet) or more, moving rocks and trees, sweeping away vehicles, and destroying buildings (Figure 9.27). Flash floods in the Southwest also tend to be especially deadly and destructive due to the area’s many canyons, which funnel water to great speeds and depths. In September 2015, extreme rainfall generated by Pacific Hurricane Linda flooded Keyhole Canyon in Zion National Park, Utah. In only 15 minutes, the Virgin River’s flow increased from 1.5 cubic meters (55 cubic feet) per second to 74.5 cubic meters (2630 cubic feet) per second. Seven hikers were swept away and killed. Near Hildale, Utah, rainfall from the same event caused major flash floods that swept away vehicles, killing 13 people, as well as destroying water lines, bridges, and power infrastructure for the town (Figure 9.28).

Floodplains are areas adjacent to rivers and streams that occasionally flood but are normally dry, sometimes for many years. When storms produce more runoff than a stream can carry in its channel, waters rise and inundate adjacent lowlands, leaving behind layers of settled sediment. Significant damage and sometimes loss of human life can occur when buildings and other human infrastructure are built on floodplains, under the assumption that future floods may never occur or will only occur in the distant future. Floods can occur at any time, but major floods are more frequent in spring and fall after periods of heavy or sustained rains when stream levels rise rapidly. For example, rapid runoff from distant storms in the Rocky Mountains has had devastating effects, both in the mountains and where streams spread over broad areas of more open land. These floods have damaged structures, property, and put lives in peril. For example, in September 2013, torrential rains over Colorado’s Front Range resulted in catastrophic flooding along the South Platte River and related tributaries. Up to 510 millimeters (20 inches) of rain fell over a three-day period; water levels of the river reached as high as 2.7 meters (8.8 feet) above flood level and affected 17 counties (Figure 9.29).

Figure 9.27: In August 2006, runoff from heavy rains sent a wall of water into the town of Hatch, New Mexico. No one was injured, but damages exceeded $4 million. The summer of 2006 was a record monsoon season in New Mexico, with a total of 91 flash flood events.

Figure 9.28: The remains of a vehicle swept away by the Hildale flash flood in September 2015. Ten of the vehicle’s eleven occupants were killed.

Figure 9.29: Before (top) and after (bottom) images of the South Platte River flood near Greely, Colorado, in September 2013.

While floods are always considered a hazard to life and property, they present a compound threat when they trigger mudslides or contribute to the conditions that cause expansive soils and karst topography. While there is no way to completely avoid the destructive impacts of flooding, good community planning and informed decision-making can greatly reduce the safety concerns and economic impacts of these events. Flood control is part of the mission statements of many government agencies, including the National Resource Conservation Service (NRCS), US Corps of Engineers (USCE), and US Geological Survey (USGS). These agencies and others maintain gauges on most large rivers and streams in the Southwest from which flow data are gathered. Using historical records and flow data collected over a long period of time, hydrogeologists can apply statistics to calculate the frequency and recurrence intervals of flows of different magnitude. These data have been used by the USGS to produce special topographic maps showing flood-prone areas. The Federal Emergency Management Agency (FEMA) provides guidelines for communities that are planning mitigation strategies designed to minimize the impacts of natural hazards such as flooding.