To understand how floods work, you have to know something about how water behaves on our planet. The total amount of water on Earth has remained fairly constant for millions of years (though its distribution has varied considerably in that time). Every day, a very small amount of water is lost high in the atmosphere, where intense ultraviolent rays can break a water molecule apart, but new water is also emitted from the inner part of the Earth, by volcanic activity. The amount of water that is created and the amount that is lost are pretty much equal.
At any one time, this volume of water is in many different forms. It can be liquid, as in oceans, rivers and rain; solid, as in the glaciers of the North and South Poles; or gaseous, as in the invisible water vapor in the air. Water changes from state to state as it is moved around the planet by wind currents. Wind currents are generated by the heating activity of the sun. The sun shines more on the area around Earth's equator than it does on areas farther north and south, causing a heat discrepancy over the surface of the globe. In warmer regions, hot air rises up into the atmosphere, pulling cooler air into the vacated space. In cooler regions, cold air sinks, pulling warmer air into the vacated space. The rotation of the Earth breaks this cycle up, so there are several, smaller air-current cycles all along the globe.
Driven by these air-current cycles, Earth's water supply moves in a cycle of its own. When the sun heats the oceans, liquid water from the ocean's surface evaporates into water vapor in the air. The sun heats this air (water vapor and all) so that it rises through the atmosphere and is carried along by wind currents. As this water vapor rises, it cools down again, condensing into droplets of liquid water (or crystals of solid ice). Collections of these droplets are called clouds. If a cloud moves into a cooler environment, more water may condense onto these droplets. If enough water accumulates in this way, the droplets become heavy enough that they fall through the air as precipitation (rain, snow, sleet or hail). Some of this water collects in large, underground reservoirs, but most of it forms rivers and streams that flow into the oceans, bringing the water back to its starting point.
Overall, wind currents in the atmosphere are fairly consistent. At any particular time of year, currents tend to move in a certain way across the globe. Consequently, specific locations generally experience the same sort of weather conditions year to year. But on a day-to-day basis, the weather is not so predictable. Wind currents and precipitation are affected by many factors, chiefly geography and neighboring weather conditions. A huge number of factors combine in an infinite variety of ways, producing all sorts of weather. Occasionally, these factors interact in such a way that an atypical volume of liquid water collects in one area. For example, conditions occasionally cause the formation of a hurricane, which dumps a large quantity of rain wherever it goes. If a hurricane lingers over a region, or multiple hurricanes happen to move through the area, the land receives much more precipitation than normal.
Since waterways are formed slowly over time, their size is proportionate to the amount of water that normally accumulates in that area. When there is suddenly a much greater volume of water, the normal waterways overflow, and the water spreads out over the surrounding land. At its most basic level, this is what a flood is -- an anomalous accumulation of water in an area of land.
A series of storms bringing massive amounts of rain is the most common cause of flooding, but there are others. In the next section, we'll look at some of the ways floods start, as well as some of the factors that determine their magnitude.