What is a Flood?
Why is the hydrograph (flood pattern) of a river much like a signature?
The hydrograph of each river is unique, because it reflects the region and climate.
Are small rivers flashier than large ones? Why or why not?
Yes, in general small rivers are flashier because their catchments are smaller and a big rainstorm in the catchment will cause a flood fairly quickly. The floodwaters likely will recede quickly also. Large catchments integrate the water events over a large area as well as a long time period. So floods usually build slowly, and last a long time.
Are seasonal floods regular events and do they occur on a schedule?
Yes, they are regular events, and the life of the river and floodplain are adapted to them. But they do not occur on a schedule!
What might happen if the Amazon failed to flood for one year? What about for 10 years?
One year would probably not pose a problem. But if the extensive flooding failed to occur for a decade, you would expect to see big changes in the plants of the floodplain, in the fish and other organisms that take advantage of the flood, and in the main stem of the river itself, which would no longer be interacting with the floodplain on a regular basis.
What is a Great Flood?
How is a great flood different from a seasonal flood?
A great flood is a very large flood that occurs “off-season,” when the river is not expected to be flooding.
Are great floods predictable?
Yes and no. It is not possible to predict great floods with any accuracy beyond a few months. Rain and snowfall in the catchment can give scientists the ability to forecast a great flood within a year, however between years it becomes problematic.
How Are Floods Good?
Where are most of the cities?
Along the river. Where there is water, there are people. There is a high concentration of cities near the mouth of the river.
How many major cities are in the watershed?
There are about two dozen major cities in the watershed.
What proportion of the watershed is forest, grassland, cropland, or barren?
The proportion of forest is 2%; grassland is 52%; cropland is 10%; and barren is 30%.
Is this an arid watershed?
You bet. 67% of the watershed is counted as arid.
How might these factors influence flooding in the watershed?
On the barren land, 30% of the watershed, precipitation would be very likely to run off quickly. This could contribute to flashy hydrographs, especially in the upstream tributaries of the river. The major cities near the river would likely be affected by floods, especially great floods.
How Are Floods Bad?
Can a great river like the Mississippi be controlled by humans?
We used to think so, and we invested millions of dollars in so-called flood control structures such as levees. Unfortunately, the built structures usually just moved the flood problems from one place to another, and sometimes they failed altogether.
Can humans work hand-in-hand with natural flood-dissipation processes to protect their towns and property?
We think so. Floodplains are enormously important to dissipating the enormous energy of a river in flood. Allowing the river to go out of its banks might be a good solution. It is also important to minimize the settlement of the floodplain.
What are some of the causes of catastrophic floods?
Weather, of course, can dump a huge amount of precipitation in a catchment, causing a flood. Humans also hold a lot of the responsibility, by clearing forests, paving portions of the catchment, and trying to keep the river off the floodplain. Sometimes dams give way, causing downstream floods.
What are some of the effects of catastrophic floods on humans?
Loss of life and property. Increased spread of disease. Loss of cropland. Huge investments in infrastructure to prevent future catastrophic floods.