The humid geomorphological system

The humid geomorphic system is characterized by an annual surplus of precipitation in relation to evaporation and, consequently, a dense and thick vegetation cover. The characteristic mechanisms are as follows:

• ·         Total cover of the surface by vegetation, often with several superimposed strata.
• ·         Little or no surface runoff.
• ·         Little or no torrential runoff.
• ·         Absence of superficial and torrential ablation.
• ·         Very slow erosive evolution of the slopes and even total stabilization. Very low density of secondary thalwegs, due to the absence of torrential erosion.
• ·         The contribution of lateral materials to the main thalwegs is also very scarce.
• ·         Therefore, there is high competition from the river waters, very poor in sediment.
• ·         This determines the incision or notch of the bed bottom.
• ·         Chemical weathering predominates; hydrolysis and neoformation, transformation of alterable minerals solubilization of various salts.
• ·         In general, evacuation of cations (Na, K, Mg, Ca, Si). Residual accumulations rich in Al and Fe.
• ·         Gradual development of a mantle of alteration of considerable thickness.
  

The existence of a dense vegetation cover and the presence of a mantle of alteration of a certain permeability, makes possible the wide predominance of infiltration processes versus those of surface runoff.

This mantle of alteration constitutes the place of transit of the "hypodermic" runoff towards the main thalwegs.

A part of the infiltration can traverse formations of the substrate and reappear in the thalwegs giving rise to the phreatic discharge, which differs from the hypodermic by the type of formations traversed during the march. Hypodermic runoff occurs in surface formations, while the water table takes place in the subfloor. It should be noted that there are a multitude of intermediate cases where categorization is not easy.

For all this, the lateral contributions to the fluvial course depend much more on the characteristics of the hypodermic and phreatic layers than on the instantaneous rhythm of the rains. These layers act like a reservoir that is slowly drained when it passes a certain time without raining, preventing too great drainages at the level of the channels. When it rains, however, the infiltrated water is used first to charge the layer, and it is not if not several days after it reaches the main thalweg, slowly and homogeneously distributed over time. All this causes that the flow of the rivers or streams are relatively independent of the recent rains, and of a great regularity.

Due to the presence of a thick mantle of alteration the substrate only emerges on the slopes of stronger slopes, although it is frequently seen in the main thalwegs.

In wetland valleys the longitudinal profiles show frequent irregularities as a result of the lithological heterogeneities of the substrate.

Structural control of interfluve forms is weak.

The final result of a wet morphogenetic evolution is the development of convex and valley valleys with transverse V-profiles. The slopes of moderate to soft slopes have few secondary thalwegs and are generally devoid of outcrops. These are much more frequent when the slopes are abrupt.

In certain zones of energetic tectonic style, the river valleys can deepen sufficiently to give rise to very strong slopes in the slopes. Coming to a certain threshold of instability, depending on the density of vegetation, the type of materials, soil moisture and slope, mass movements are usually produced, which are the main modeling agents of wet systems.

Some regions with this type of climate and morphogenesis are the northwest coast of North America, the east coast of South America from Rio Grande do Sul to Espiritu Santo. and the southeastern coast of Africa.

Extracted from chapter 3 of "Sequía en un Mundo de Agua" (Drought in a Water World), Danilo Antón, Piriguazú Ediciones and CIRA. Mexico