In wet seasons it overflows its banks and becomes greatly extended in area, discharging its surplus waters into the San Joaquin; but in dry seasons the evaporation is so great that there is no such discharge.
In Alemtejo the climate is very unfavourable, and, though the heat is not so great as in Algarve (where Lagos has a mean of 63°), the country has a more deserted appearance; while in winter when the Tagus overflows, unhealthy swamps are left.
As the low-lying plains are altogether an alluvial deposit, the coarser sediments accumulate in the regions where the river first overflows its banks to spread out over the plains.
With the exception of a few flat ridges running from north to south, it is so low that it requires, to protect it from overflows, an unbroken line of levees averaging 15 ft.
The most fertile soil is the alluvium of the' Delta, deposited during the overflows of the Mississippi.