Floods
Floods are controlled by the rate of precipitation, run-off, stream flow, and shape of the land surface. They may occur when 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 contains 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. 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.
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. Major floods in the Northwest Central generally occur along the Missouri River or its tributaries (Figure 10.23), and these events are more frequent in spring and fall after periods of heavy or sustained rains when stream levels rise rapidly.
Figure 10.23: The Missouri River and its tributaries.
Flooding in the Northwest Central generally occurs through flash floods, periods of long-term rainfall, spring snowmelt, or some combination of these factors. While flash floods tend to impact a smaller area than do long-term rainfall and snowmelt, they can be especially dangerous because they arise so suddenly. Famous flash floods include the Republican River Flood of 1935 in Nebraska, when 4661 centimeters (1824 inches) of rain fell on May 30th that year; the Cheyenne Flood of 1985, when 18 centimeters (7 inches) of rain fell in three hours on August 1st in Cheyenne, Wyoming; and the Black Hills Flood of 1972 on Rapid Creek in Rapid City, North Dakota, when 38 centimeters (15 inches) of rain—approximately one million metric tons overall—fell over six hours from June 910, 1972. The Black Hills Flood is considered to be one of the most significant floods in US history: a surge caused a breach in the Canyon Lake Dam, releasing water into Rapid City and killing 238 people, destroying 1335 homes, and causing over $900 million (adjusted) in damage (Figure 10.24).
Figure 10.24: A pile of cars swept away by the 1972 Black Hills Flood. This event destroyed over 5000 vehicles.
There are numerous recorded instances of flooding on the Missouri River due to long-term rainfall, contributing to subsequent flooding downstream in St. Louis and into the Mississippi. The Great Flood of 1993, when floodwaters traveled down the Missouri River from South Dakota and Nebraska into Iowa, Kansas, and Missouri, flooded over 4 million hectares (11 million acres) and caused at least 50 deaths and over $24 billion (adjusted) in damage. The 2011 Missouri River Flood, caused by high winter snowfall in Montana and Wyoming followed by large spring rainfall on the plains of Montana, inundated roads and buildings (Figure 10.25) and threatened towns and cities along the river from Montana to Missouri. The Great Flood of 1881 in South Dakota and Nebraska (notably including Omaha) was caused by ice jams along the Missouri River, and the April 1997 Red River Flood of Grand Forks, North Dakota was due to abundant snowfall combined with heavy rain during the previous fall (Figure 10.26).
Figure 10.25: The Fort Calhoun Nuclear Reactor and surrounding areas of Nebraska were inundated by floodwater during the 2011 Missouri River Flood.
Figure 10.26: Wreckage in Grand Forks, North Dakota, after the 1997 Red River Flood.
Devils Lake in North Dakota is known for dramatic annual changes in water level depending on local precipitation, and has gained a net increase of about 40 meters (130 feet) in water depth since 1940. The lake has quadrupled in size over the last two decades, growing from 18,000 hectares (44,000 acres) in 1994 to about 82,000 hectares (202,000 acres) today (Figure 10.27). Devils Lake is a closed drainage basin with no natural outlets, and water can therefore leave its confines only through evaporation, ground infiltration, or overflow. During one period of especially rapid increase, rainfall between 1993 and 1999 caused the lake’s water level to rise about 20 meters (66 feet), flooding 28,000 hectares (70,000 acres) of farmland, displacing 300 homes, and costing about $450 million to mitigate the flooding. There has been substantial controversy about the ecological impacts of proposed mechanisms to create an outlet that would offset further lake rise, partially focused on where to divert the water and the consequences of potentially moving invasive species into other basins. Flooding from the lake today continues to affect agriculture and infrastructure in the surrounding area.
Figure 10.27: The extent of Devils Lake at different water level elevations.
