Freshwater Ecosystems: Rivers and Lakes in Spain

Freshwater: Rivers and Lakes

Physical Elements of the River Regime

1. Climate

The greatest precipitation occurs in the North, i.e., the oceanic climate of Spain. This results in the largest relative flow and the greater regularity of the rivers located in humid Spain, which occupies only 20% of the mainland.

2. Relief

Relief influences the size of the watersheds. In mountainous areas, watersheds are small and compartmentalized. Consequently, rivers in mountainous areas have little absolute flow. The relief influences the erosive capacity of rivers, which is related to the slope. It also influences decisions on whether or not to build hydraulic structures.

3. Ground

If the bedrock is waterproof (granite, clay, shale), much of the precipitation joins the surface circulation. In permeable rocks (limestone), the water passes into underground circulation.

4. Vegetation

Greater or lesser vegetation coverage influences the extent of erosion and the degree of runoff. It also reduces evaporation and flood peaks.

Human Impact

Humans have transformed the banks of rivers, making them suitable for the economic activities developed in them or for human habitat. Early river civilizations like Egypt and Mesopotamia undertook this work. Currently, this transformation has been much more profound, as seen in specific actions such as the use of rivers for irrigation, urban supply, and energy production. This has resulted in the construction of levees, canals, and reservoirs that interrupt or alter the course and regime of rivers, and water transfers from one watershed to another. The consequences are the degradation of the environment of the banks, artificialization of the channel, and destruction of riparian vegetation.

Elements of Flow

• Absolute Flow: Measured in m³/sec and depends on the surface of the basin and precipitation.
• Relative Flow: The relation between absolute flow and the area of the basin in km². It indicates if a river is torrential or not and if it belongs to humid or dry Spain.

Irregularity

The relationship between floods and droughts.

• Flood: A sudden increase in flow.
• Low Flow: The minimum flow period.

2. The Principal Hydrographic Regimes and Watersheds

North Slope

This basin covers an area north of 53,800 km² (5% of the peninsular surface). The rivers are influenced by humid Spain, so they are numerous, copious, regular, and short. Their hydrographic regime is oceanic storm, some have pluvio-nival influences. All have a flow of more than 20 l/sec/km² and a small absolute flow (around 100-200 m³/sec maximum) due to their short length and small basin. They have maximum flow in autumn-winter and minimum in summer. An example is the Miño River.

Atlantic Watershed

This watershed occupies 65% of the peninsular territory. Most of their courses are very long, and the river basins are very extensive and irregular. The maximum flow occurs in February-March and the minimum in summer. They have a Mediterranean subtropical rainfall regime. The river flow may be important because all major watersheds have significant tributaries. The major rivers of this watershed are:

• Duero River: The largest hydrographic basin. It originates in the Sierra de Urbión and flows into Oporto. Its regime is nivo-pluvial. Its maximum flow is in February and March, and low water is in summer. It is the mightiest river on the peninsula. Its flow is around 7 l/sec/km². Its tributaries are flowing outside the river.