Freshwater Systems: Rivers, Lakes & Aquifers

Rivers and River Basins

Rivers (Ríos): Rivers originate directly from precipitation falling from clouds or from deposits formed by precipitation. Rivers can begin as springs where underground water comes to the surface or at places where glaciers melt. A river with its tributaries drains an area called a river basin. The river where it empties is called an emissary. If there is no emissary, then both the lake and its basin are recognized as endorheic.

Lakes and Types of Lakes

Lakes: Lakes are landforms that depend on the region in which they appear and often support important flora and fauna. Lakes can be connected by a river system that provides water. If they occupy large tracts of land, they are defined as inland lakes. Lakes can be fed by one or more rivers called inlets (Spanish: inmisarios).

Types of Lakes

• Tectonic: These are lakes that fill depressions caused by faults and folds. Lakes are formed by ground motion that prevents the free flow of a river.
• Barrier (Moraines): Moraines are formed when glaciers or other materials plug valleys and allow the accumulation of water, preventing drainage; the lake forms behind the barrier.
• Glacial: Glaciers dig broad basins, polish the bedrock, and redistribute removed materials. These lakes form when water fills the space eroded by ice masses.
• Crater: Lakes can form in volcanic craters or calderas after an eruption; the circular sink may flood and form a crater lake.
• Endorheic: These are depressions in the earth's crust that have no outlet to the sea. Their waters are usually somewhat saline.
• Pelagic: Often remnants of ancient seas that later became surrounded by land.

Aquifers and Groundwater

Aquifers (Acuíferos): The difference between the amount of rainfall and the amount of water carried away by rivers seeps into the soil and forms aquifers (Spanish: acuíferos). Filtration depends on the physical characteristics and porosity of the rocks. Porosity is not synonymous with permeability: certain rocks, such as clay, may have high porosity but are virtually impermeable because their pores do not interconnect. Types of aquifers include free (unconfined), semi-confined, and confined.

When a permeable layer is arranged between two impermeable layers, it forms what is called a confined or captured aquifer. Under these conditions the water is subject to considerable pressure. If a breach occurs in the impermeable layer, the water may rise quickly toward the water table to balance pressure differences. If the permeable layer is not limited at depth, it is called a free or unconfined aquifer.