Understanding Spanish Hydrology: Rivers, Flow, and Basins

Factors Influencing the Spanish Fluvial System

Spanish hydrology is characterized by both surface water and groundwater. These may manifest as flowing rivers and streams or stagnant lakes and wetlands. A river is a continuous stream of water that flows into the sea, another river, or a lake. Peninsular rivers are conditioned by several factors:

• Climate: Rainfall significantly influences river flow and its annual variations, as do temperature and evaporation rates.
• Relief and Topography: The organization of rivers and watersheds is influenced by topography, with river erosion increasing with slope.
• Lithology: This determines the dominance of surface runoff versus underground water flow.
• Vegetation: Vegetation slows evaporation and reduces erosion and flood risk.
• Human Action: Humans create infrastructure for water supply and to regulate rivers.

Flow and Flow Regime

Flow is the amount of water flowing past a point in the river in one second. The specific flow is the ratio between the average annual flow and the basin's surface area. Flow variations occur:

• Along the River's Course: The most voluminous section is usually the mouth, where tributaries' contributions accumulate, such as in the Duero and Ebro rivers.
• Over Time: Flow can experience annual variations, with more or less pronounced droughts, particularly on the Mediterranean side.

Flow regime is the seasonal variation in river flow. It depends on rainfall distribution and the importance of snow precipitation. Based on these factors, rivers are classified as having a nival, rainfall, or mixed regime.

• Nival Regime Rivers: These originate in high mountains, with peak flow in late spring or summer due to frequent snowfalls and low water in winter during the thaw.
• Rainfall Regime Rivers: These depend solely on rainfall, with maximum flow reflecting maximum precipitation.
• Mixed Regime Rivers: These exhibit characteristics of both nival and rainfall regimes.

Catchment Area and Watersheds

A catchment area is the territory whose waters flow into a main river and its tributaries. Catchment areas are separated by watershed summits formed in the surrounding mountainous landscape, such as the Minho, Douro, and Guadalquivir. Within a basin, rivers flow through a channel and form a network.

A watershed basin is the set of basins whose waters flow into the same sea.

• Cantabrian Watershed: Rivers here are short because they originate in mountains near the coast. They have a high erosive force and an abundant, regular flow due to consistent rainfall.
• Atlantic Watershed: Rivers here originate near the Mediterranean, are long, and empty into the Atlantic. They have a low erosive force and an abundant but irregular flow.
• Mediterranean Watershed: Except for the Ebro, rivers here are short and are often gullied due to erosion of deforested hillsides. They have a low flow due to reduced rainfall.
• Balearic and Canary Islands: These islands lack true rivers and have only small streams fed by springs.