Thousands of years ago before people built towns and planted crops, rivers cut deep canyons and molded the continents. Often these rivers overflowed their banks and flooded the surrounding areas, depositing mineral rich silt and soil in the surrounding plains and valleys. Because of the way floods enrich soil some of the first cities were built along rivers. The most important ones grew along the Indus River in Pakistan; the Nile in Egypt; the Yellow River in China; and the Tigris and Euphrates in the Middle East. These rivers floodplains are called the cradles of civilization.
Even though floods happen everywhere, there are only four types of floods. River floods occur along rivers and usually happen because of heavy rain over a large area. This can also be caused by melting snow and ice jams and can last for weeks. In most parts of the world dams and levees help keep rivers from flooding by blocking off water and storing it in lakes and reservoirs. Sometimes a dam will break causing another kind of flood. Dam failure floods happen when dams or other manmade structures are neglected, poorly designed, or damaged.
These floods are very dangerous because a huge amount of water is released suddenly and with no warning. Flash floods happen when a large amount of rain falls very quickly. Flash floods generally occur in mountainous areas and are so deadly because they happen with little or no warning. Coastal floods occur along the shores of oceans and seas and are often caused by hurricanes and strong storms. Earthquakes that happen under the ocean or on a coast also cause them the winds produced by these winds whip up huge ocean waves that are together known as a storm surge.
These earthquakes cause tsunamis, which are huge waves. Tsunamis travel long distances across the ocean before reaching land where they flood the coasts, drown people, and smash buildings. (Dennis Fradin) Volcano eruptions can also cause tsunamis like in 1883 when Krakatoa erupted killing 37,000 people making it the worst tsunami in history. China is the largest and most populated country on the earth. It is also where the worst flood in history occurred. In 1887 the Yellow River toped its seventy-foot-tall levees and flooded eleven cities and hundreds of surrounding villages.
This flood became the deadliest flood ever by killing at least 900,000 people. Thousands of square miles of farmland was covered in water that sometimes stood twenty feet deep over farm houses. It took two years for this much water to recede. With crops washed away there was little to eat; it is not known how many starved to death. In 1889 Johnstown Pennsylvania flooded. Fourteen miles away from Johnstown the Little Conemaugh River had been dammed to form Lake Conemaugh. The dam was owned by a country club and was known to be in poor condition. The members sailed and fished on the lake but did little to repair the dam.
May 30, 1889 was Memorial Day and it rained hard. The river and lake rose quickly and the next day the water topped the dam and spilled over the top. By three in the afternoon the dam had broken. The water smashed through several towns including Johnstown killing over 2,200 people. Flooding also causes other natural disasters. (Michael Burgan &Robin Doak) The heavy rains that often cause flooding can also cause mudslides that destroy anything in their path. Floods can destroy crops causing famine or taint water supplies and cause an outbreak of disease. But not all flooding is bad.
Every year the Amazon River in Brazil floods its banks. The forests fill with water and the land animals head for higher ground. Anyone who lives in the Amazon River basin builds their homes on high ground or on stilts. Even family gardens are planted in raised beds several feet of the ground to keep them safe from the rising water. The flooded forests are foraged by fish who disburse the seeds of the trees and other plants. ( Goulding, Smith, & Mahar) Floods can be very deadly and cause millions of dollars of damage every year. But a flood is only dangerous if your in front of it.