Water is an absolute necessity for all living organisms, especially for humans, animals and plants, along with increasing the number of population, the activity of the use of natural resources, particularly water resources is also increasing, the water resources needs to be improved preservation by balancing the water cycle on earth known as the hydrological cycle. The process of the hydrological cycle in nature is useful as a renewable resource, the global quantity of water resources on earth are relatively fixed, while the quality is increasingly declining.
In addition to the needs of living things, water can also be used for irrigation, power generation, industry, agriculture, fisheries and raw source of drinking water, associated with diverse needs, the availability of water that meets both the quantity and quality to the needs of very limited availability of water, especially surface water management relies heavily on the origin of the water, the river is one of the surface water that needs to be managed, the rivers are joined in a watershed (DAS).
In general the watershed can be defined as a region, which is bounded by natural boundaries, such as a ridge or a mountain, or limit aid such as roads or embankments, where the rainwater that fell in the region contributes to the flow control point (outlet). The existence of different water can be utilized by humans to preserve and promote life. The most important thing is how people can manage water properly to be material / capital live better and sustainable.
Figure 1. Flooding
Precipitation that falls to the surface soil granules can bring soil from a higher place to place lesser known (erosion). Arsyad Sitanala (1989) states, the movement of soil erosion as events or parts of one tenpat elsewhere by natural media. The natural medium is water and wind. The grains of soil carried by water or wind on the surface of the soil will be deposited somewhere in the form of sediment.
Average deposition process itself is called sedimentation. Average factors affecting erosion is the climate (especially rainfall), topography, soil, plants and humans. Means in relation to rainfall intensity and distribution. Specific events such as flash floods in Wasior (Papua Barat) and Bahorok (North Sumatra) occurs due to natural factors. In both instances caused landslides (either because the conditions / texture of soil and vegetation) on the outskirts of the river / valley and formed a dam.
Dams newly formed low strength so that when the stored water is pretty much it did not hold and collapsed cause flash floods.