OF ALL THE NATURAL hazards in the world, floods are the most widespread. Historically, floods have killed more people and caused more damage than any other type of natural disaster. A flood is defined as an overflowing of water on an area that is normally dry. There are several types of flooding, including river flooding, coastal flooding, urban flooding, inland flooding from hurricanes, and flash flooding. Each has its own causes and characteristics, but all can be deadly and floods can happen anywhere. Throughout history, people have always built their towns and cities near water, both along the seacoast and along rivers further inland. The water serves many purposes. It is used for drinking, cooking, and bathing. Water in many places has been used as a source of power, to turn wheels that grind grain. Running water is similarly used to create hydroelectric power. Another reason for settling close to water is that it has always been an easy way to travel and to transport goods. The disadvantage of living near water is the possibility of flooding. The amount of water on Earth is fairly constant all the time. Each day, some of the water on the Earth is lost into the atmosphere through the process of evaporation. Water in rivers, lakes, and oceans is always evaporating at a slow rate. Most water on the Earth is in the form of liquid, but at the poles we find water in its solid or ice form. Water in glaciers is also in the form of ice. At some places, water is in the form of water vapor in the air. This is the water that has evaporated from the surface of a body of water. A water molecule remains in the atmosphere for approximately 9 or 10 days before it falls as rain or snow. This happens when the amount of water in a cloud becomes heavy enough to form drops of water, which then fall to Earth. Rain that goes into the bodies of water begins the cycle again. Some rain will soak into the ground, where it will spend some time as groundwater. River flooding is 1 of the most common types of flooding. It occurs when rainfall throughout the year is unevenly distributed. Too much rain goes into the river at 1 time and the water exceeds the capacity of the channel. When that happens, the stream will overflow its banks. Another cause of river flooding is melting snow in the spring. In areas with hills or mountains, the snow in the higher elevations melts and the water runs down into the streams. If the snowfall was heavier than usual, floods may occur. Also, spring rains often occur at the same time as the snow is melting, contributing more water to the streams and rivers. The torrential rains that accompany a hurricane or other tropical storm can also cause flooding in rivers. When heavy rains fall on bare soil, it is churned into mud. This keeps the water from soaking into the ground, and most of it will flow across the surface of the ground, creating gullies leading to rivers and streams. Coastal flooding occurs when ocean water is driven inland by winds from tropical storms or hurricanes. Sometimes escape routes are blocked by this high water and people cannot escape. Coastal flooding can also be caused by a tsunami, which is a sea wave sometimes referred to as a tidal wave. These huge waves are caused by volcanic activity or an earthquake. When urban flooding occurs, the streets in a town become swift rivers and basements fill with water. This is partly due to the fact that land that used to be fields or woodlands has been converted to roads and parking lots. The concrete or asphalt cannot absorb any rainfall and it runs off the surfaces of the parking lots and roads.