In the period of floods, for a natural process, these mineral and organic debris if deposit on the subject to flooding edges, giving to them to a good fertility and value as areas for the agricultural exploration. In the hydrography of next fertile valleys to the coast the tides constitute the dominant element. They exert influences in the direction of the rapids, in the intensity of the sedimentation, the quality of the water, the transport of mineral and organic debris and in the oscillation of the level of floodings. The tides when overflowing in the maritime edge or the low course of the rivers, effect a bigger deposition of sediments in the stretches next to the edges. For the action of the gravity force it is there that the particles of bigger diameter are deposited.
Following the edges of the rivers or the coast, it more has a band of raised level. It is what we call of high fertile valley. In each high water, the waters that cover the high fertile valley do not remain what two hours more than on the ground, returning soon to the riverbed, in the receding tide. This dry stretch completely during messes less rainy Later to this band delinquent comes another one that receives the sediments lesser. She is of quota lower and it can be called of fertile valley low. In this, flooding if processes for more time because being the inferior level to the one of the edge, part of the water that overflows on the high fertile valley, cannot return in refluxo of the tide and is dammed. The fertile valley low is invaded or humidified partially during almost all the year, for waters of full moon and new moon.