The lack of water flow and sediments

Before the mass construction of reservoirs along the Ebro basin, the irrigation season for the rice fields was completed with a process called filling, which used the canals to carry sediment from the river to the rice fields—a physical and natural protective measure against subsidence and regression—. Now, with the sediment being retained in the reservoirs and less waterflow in the river, the mid- and long-term solution to correct the vulnerability of the Ebro Delta rests on once again enabling the river to carry water and sediment to the Delta. It is possible and absolutely necessary

The lack of water flow

Water flow and annual water contribution from the Ebro river show a significant decrease –near 30%- when comparing figures from before the Tortosa plant began operating (1912-2013) with those from after the large dams were built throughout the river basin (1980-2013).

The statistical analyses in the report from the 2015-2021 Water Plan for the Ebro hydrographic region show:

Decreased precipitation, with an estimated annual average in the Ebro basin falling from 641.2 mm/year (1940-2006) to 617.8 mm/year (1980-2006), with a -3.6% variation;

Decreased replenishment of aquifers and the water generated in the basin:

  • 18,217 hm3/year from years 1940-1986
  • 16,448.1 hm3/year from years 1940-2006
  • 14,623.3 hm3/year from years 1980-2006
Period 1912-2013 Period 1980-2013 Reduction
Average annual flow
423,93 m³/s
289,40 m³/s
Daily average minimum discharge (annual Qe)
80,24 m³/s
79,65 m³/s
Daily average maximum discharge (annual Qc)
2.024,58 m³/s
1.379,17 m³/s
Annual contribution
13.369,16 hm³/year
9.132,29 hm³/year

The higher temperatures and longer dry period in summer lead to an increase in demand for water for agriculture.


Over the past five decades, throughout the entire Iberian Peninsula, an increase has been detected in the severity and frequency of droughts, which is attributed to the increase in atmospheric evaporative demand in response to the higher temperatures. It is seasonal process, particularly important in summer.

At the Ebro Delta, there is a delicate balance between the amount fresh water and saltwater, due to its geography, and most importantly, because the water balance is controlled by the agricultural activity. The Ebro is the paradigm of a costal water system that suffers the effects of climate change both from the continent-side and from the sea. In addition, the impact humans have had on the region has altered this natural system, in such a way that taking on climate change becomes an issue that involves the entire water basin area and local communities.

The lack of sediment

The retention of sediment by dams and reservoirs in the Ebro basin causes the coastline to recede and speeds up the sinking of the deltaic marshes. These effects are intensified by the rise in sea level. Given this scenario, it is essential that the flow of sediment from the river to the sea and to the Delta gets restored.

Consequently, the current 3,000 hm3/year of minimum contribution flow should reach up to 7,000 hm3/year in wet periods, which would guarantee the arrival of sediments and allow the generation of soil to maintain the elevation of the deltaic plain and stop, as far as possible, the erosion of the coast.

Currently, there are 100,000 tons of sediment that reaches the mouth of the river each year, which is less than 1% of the volume there used to be before the dams were built.

Research carried out by the UPC and the IRTA shows that at least 1.2 million tons of sediment will be needed each year until 2100, in order to combat the combined threat of rising sea level and subsidence. If the sea level rises more than 53 cm, the amount of sediment needed to maintain the elevation would need to be between 2.5 and 3.5 million tons each year.

As part of the Life Ebro Admiclim project, several pilot projects were carried out that injected sediment into the final section of the Ebro river and into the Delta’s canal system.

In the first, two sections of the river were treated (at Móra d’Ebre and from Benifallet to Assut de Xerta) to determine the Ebro River’s ability to transport sediment (sand and clay) to the delta plain and out to the sea.

In the second one, specific injections of sediment from the river were made, after being treated previously at a CAT plant to ensure potability.

The objectives behind these pilot projects was to evaluate the feasibility of reinjecting sediment into the irrigation network in order to, on one hand, release the sludge generated at the plant and convert it into a resource that contributes to the adaptation to climate change and, on the other hand, to evaluate the possibility for sediment to be transported and distributed through the network of irrigation channels, and the rates at which it gets deposited in the rice fields.

It was shown that injecting the sediment into the canals is viable and efficient, and has no effect on rice production.

Consequently, a system to reinject 1,000 t/year of inert sediment from the facilities of the Tarragona Water Consortium (CAT) has been designed.

Flood simulations of the Ebro Delta. On the left, evolution with the current contribution of sediment, almost nothing; on the right, evolution with an expected sediment contribution of about 2M tons per year. The areas in red are areas that are already below sea level due to subsidence, but are not submerged yet since they are not connected to the sea.

The Secretariat for the Environment and Sustainability, in collaboration with the Platform for the Defense of the Ebro river, is promoting this exhibition which aims to show the impact of climate change on the evolution of one of the richest in biodiversity and, at the same time, most vulnerable deltaic systems in Europe: The Ebro Delta.

Curated by:

Secretaria de Medi Ambient
i Sostenibilitat

Conceptual design and content coordination:

In collaboration with:

Design and production: