From endorheic meaning that they do not drain

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

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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.