What is an alluvial aquifer?

An alluvial aquifer is characterized by sediments that have been deposited by river activity. These sediments, typically composed of sand, gravel, silt, and clay, form in river valleys and floodplains, where water from rivers slows down and deposits its load. The key feature of alluvial aquifers is their high permeability, which allows groundwater to flow easily through the sediment. This makes them vital sources of freshwater in many regions, as they can store large quantities of water and provide significant yields when tapped by wells.

In contrast, an aquifer composed of volcanic rock would involve different geological processes and characteristics, primarily linked to igneous formations and their ability to hold or transmit groundwater. An aquifer found exclusively in mountainous regions suggests a limitation to topography rather than the sedimentary processes associated with alluvial aquifers. Lastly, an aquifer containing only saline groundwater is unrelated to the definition of an alluvial aquifer, which is commonly associated with freshwater resources deposited by rivers. Thus, option C accurately reflects the nature and characteristics of an alluvial aquifer.