From begins its journey to the oceans, either by

From the moment water falls to the ground as rain it starts making its way back to the oceans, regardless of it falling over flatlands close to the river’s mouth, or high in the mountains near the drainage divide. A drainage divide, also called a watershed, is a line that separates two drainage basins so that any water that falls on one side goes to one river and any water that falls on the other side even just a metre away will make its way to a different river system. A drainage basin is an area of land where precipitation collects and drains off into a common outlet, such as into a river or other body of water. The drainage basins water system not only includes all of the surface water, such as from rain and snowmelt but also the groundwater underneath the earth’s surface as well.As soon as the rainwater falls it begins its journey to the oceans, either by percolating into the soil or porous rocks if possible, or if it lands on a surface it can’t absorb into, for example concrete in urban and developed land, then it will run-off the surface due to gravity. Either way gravity moves the water downhill towards the lowest possible point, most often the oceans; however around 18% of the Earths surface drainage basins are endorheic meaning that they do not drain into the ocean, an example of this would be inner Asia that drains into the Caspian Sea or the Aral Sea. Drainage basins always drain toward the lowest point meaning that drainage divides usually follow ridges or mountain ranges because as the name suggests they are basins, so large lowland areas may only have a few very large drainage basins, while mountainous areas will usually have lots of smaller basins.In terms of flooding risks there are 5 main factors around the catchment zone that will affect the likelihood or amount of flooding. Topography, the shape and features of the Earth’s surface, affects water runoff speed, for example a rocky steep environment will allow the water to reach the river faster increasing the risk and severity of flooding. The basin shape also affects how long it will take water to drain into the river for example a circular basin will take less time to drain than a long thin one. Size is also a factor as larger drainage basins will generally have more water moving through them increasing the potential for flooding. Land use contributes to the volume of water reaching the river, water that falls onto urban areas will runoff into the river with almost no absorption into the groundwater. Possibly the single most significant factor to flooding and water drainage would be the type of soil in the basin. Sandy soils are very permeable, and so rainfall on sandy soil is likely to be absorbed by the ground. However, clay soils are impermeable and therefore rainfall on clay soils will run off rather than being absorbed. Given enough rainfall even free-draining soils can become saturated, meaning that any additional rainfall will run off rather than being absorbed into the ground.The amount of time it takes for water to reach the river from when it rains down is the main factor for flooding because if all of the water gets into the river very quickly, for example in an urban area with very high run off, then all of the water would be discharged from the river at the same time increasing the likelihood of flooding. However if water is absorbed into the ground it will enter the river at a slower more steady rate so the rivers peak discharge would be significantly less. The difference in time between the peak rainfall and the peak discharge is called the lag time of the river, and in general the longer the lag time the less severe any potential floods will be. A rivers runoff/ discharge is measures in cumecs which stands for cubic metres per second.The absorption of water into the soil is called infiltration, and the downward movement of water within the soil is called percolation. The pore spaces within a soil are the conduit that allows water to infiltrate and percolate through it.