Factors Influencing Spanish River Systems and Watersheds

Vegetation and Water Resources

Vegetation retains rainwater, favors soil moisture, and prevents subsoil erosion, resulting in a greater wealth of water resources in areas with dense plant cover:

• Protects the river basin area from soil evaporation.
• Contributes to slowing the progression of floods.
• Diminishes sediment drag to curb erosion.
• Reduces irregular and seasonal variations by delaying the incorporation of rainwater into streams and slowing surface movement.

Human Action and Waterworks

Through waterworks, humans amend river systems. For example, reservoirs store water during times of abundance to release during shortages, reducing flow irregularity. Dams are used to generate electricity, while transfers modify channel paths and reduce river water volume.

Factors Influencing Spanish Rivers

Spanish rivers are conditioned by various factors that influence their flow, organization, and watershed management:

Climate and Rainfall

Climate is the primary factor responsible for river regimes. Rainfall determines flow and its variations throughout the year. From this, we distinguish:

• Wet Spain: Regular and abundant river flow in areas with an oceanic climate.
• Dry Spain: Less voluminous and irregular rivers in regions with a Mediterranean climate.
• Arid Southeast: Mediterranean regions with poor flow and frequent droughts.

Temperatures influence water evaporation rates. Evaporation is highest during summer in inland areas and the south of the peninsula, coinciding with the period of insufficient rainfall.

Relief and Topography

The terrain determines the organization and layout of the river network. For example, the slope of the plateau toward the west, established at the end of the Tertiary period, created the asymmetry between the Atlantic and Mediterranean slopes. The watershed between these two areas is defined by the peaks of the Iberian and Betic systems.

Topography influences the erosive capacity of rivers, as well as the speed and volume of floods. Higher slopes increase the erosion capacity and water speed. Conversely, flat areas with little movement may exhibit endorheic (landlocked) circulation. Finally, altitude increases precipitation, which often falls as snow in high mountain areas.

Lithology and Soil Characteristics

Lithology determines the characteristics of the river system through the permeability and erosion resistance of the underlying rock:

• Limestone: Permeable, defined by low and irregular surface runoff and dominated by groundwater runoff.
• Siliceous Lithology: Predominant in the western peninsula; it is impermeable, which promotes superficial circulation.
• Clay: Typical of large sedimentary basins; these rocks are very tight and characterized by high surface runoff.