floods can form after heavy rain over a long period or when a large snowfall melts there are many different types of flooding which fall under three main categories the first results from overflow after heavy rain a river can burst its banks the water can engulf entire valleys far from the precipitation zone other tributaries add to the flow of water worsening the deluge further downstream the second type of flooding involves the accumulation of water in lowlands or basins in normal weather conditions water is absorbed into the ground and merges with the water table when the ground is saturated water builds up in low-lying areas runoff then quickly fills the valley and its waterways the third type is caused by urbanization in flood prone areas excess water passing through drainage systems can accumulate in low-lying zones this effect is worsened by the coverage of land with buildings and impermeable surfaces such as concrete less water can be absorbed into the ground overloading sewers