Rivers and Floods

This present flood of 1882 will doubtless be celebrated in the river's history for several generations before a deluge of like magnitude shall be seen. It put all the unprotected lowlands under water, from Cairo to the mouth; it broke down the levees in a great many places, on both sides of the river; and in some regions south, when the flood was at its highest, the Mississippi was seventy miles wide! a number of lives were lost, and the destruction of property was fearful. The crops were destroyed, houses washed away, and shelterless men and cattle forced to take refuge on scattering elevations here and there in field and forest, and wait in peril and suffering until the boats put in commission by the national and local governments and by newspaper enterprise could come and rescue them.

--Mark Twain, Life on the Mississippi, 1883.

scanned image from: Academic Affairs Library,
University of North Carolina at Chapel Hill,
Documenting the American South

Floods are a natural process of rivers. During periods of high-discharge, usually caused by increased rainfall or snowmelt, water fills to the banks of the river and spills over into the surrounding floodplain. During a flood, sediment is carried out of the channel and onto the floodplain, providing a mechanism for the slow accumulation of sedimentary layers away from the river channel. However, in many places these floodplains are occupied by towns, farms, roads etc. Therefore, both the floodwaters and the sediment carried by the floodwaters can damage structures on floodplains. One method of zoning the region adjacent to many large rivers is to map out areas that lie within the "100-year-floodplain." These 100-year floodplains are important not only for zoning but also for homeowners’ insurance and other property-rights issues. You will explore how the 100-year floodplain is determined in the lab assignment.

Some flood events are extremely catastrophic in terms of human loss. In 1887 the Huang He River (Yellow River) in China flooded and killed over 900,000 people. It flooded again in 1931 and killed an estimated 3.5 MILLION people. By comparison, the Mississippi River flood around the St. Louis area in 1993 killed 50 people, but did cause damage estimated at between $10 and $15 billion.

In terms of geologic time, these floods are, although not normal, short of "geologically catastrophic." In other words, these floods are part of the normal processes of Earth. However, there has been flooding events on Earth's surface that truly must have been catastrophic. These floods must have truly dwarfed the flood events described above, however, they occurred when no one was around to write about them. Therefore geologists have had to interpret the size and scale of these floods from the deposits they left behind. In this prelab exercise you will read and learn about one of these catastrophic floods.

Procedure:

Read through the links on the sidebar to learn about one catastrophic flood event.