Flow and Floods
The Ebro is the most important river in Spain in terms of length (928 km) and area of drainage basin (85,550 km²). However, the mean annual flow decreased by approximately 29% during the 20th century due to many causes: the construction of dams, the increasing demands for irrigation and the evaporation (higher than the rainfall, due to low rainfall, high sunshine and strong and dry winds) from reservoirs in the river basins. This situation has a direct impact on the deltaic system at the mouth of the river because its hydrological dynamics are mainly controlled by the river discharge.
The decrease in river discharge has meant introduction of the salt wedge further into the river. The mean annual river flow is approximately the critical flow which determines the formation and the break-up of the salt wedge. Thus, when the river discharge is between 300 and 400 m³, the salt wedge can occupy the last 5 km of the estuary, but when the discharge is between 100 and 300 m³, the salt wedge can advance up to 18 km from the mouth. For less than 100 m³, the salt wedge quickly advances to its maximum extent, reaching 32 km from the mouth. In addition to decreased mean annual flow, the increased river regulation in the Ebro basin has produced daily and seasonal changes in the flow pattern.
With regards to the sediment load, several authors conclude that the sediment load was reduced by more than 99% during the last century. The drastic reduction in sediment transport implies a sediment deficit in the Delta, which is causing the erosion of the coastline. This erosion, together with the sinking of the Delta produced by soil compaction and tectonic subsidence, cannot be balanced by the deposition of fluvial sediments, nearly all of which are retained behind the dams.
River flow in Zaragoza during floods, from the end of the nineteenth century:
The Ebro poured 1,874 hm³ in the river delta from 2007-03-27 to 04-11, with an average of 117 hm³/day .
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