The Ebro Delta, Spain: water and sediment management in the context of relative sea level rise  <B>The Ebro Delta, Spain: water and sediment management in the context of relative sea level rise</B><P>C. Ibanez1, J.W. Day2, A. Canicio3, N. Prat1 and A. Curco4</B><I><P> 1 Departament d'Ecologia, Facultat de Biologia, Universitat de Barcelona, Barcelona, Catalonia, Spain <BR> 2 Department of Oceanography and Coastal sciences, Louisiana State University, Baton Rouge, USA <BR> 3 Consultat Geologist, Deltebre, Catalonia, Spain 4 Depatament de Botanica, Facultat de Biologia, Universitat de Barcelona, Barcelona, Catalonia, Spain</I><P> Abstract<P> Deltas are particularly sensitive to sea level rise because most are subsiding and this, in addition to eustatic sea level rise, leads to a relative sea level rise (RSLR) rate which is often much greater than eustatic rise. If wetlands and agricultural low lands in deltas do not accrete vertically at a rate equal to the rate of RSLR, they will become stressed due to waterlogging and salinity increase, and ultimately disappear. The Ebro Delta is one of the most important wetland areas in the western Mediterranean. Between 40 and 50% of the delta is below 0.5 m and part of the southern margin of the delta is at mean sea level in an area protected by dikes. Preliminary estimates of mean rates of subsidence in the Ebro Delta ranges between 2 and 5 mm yr1, whereas eustatic sea level rise has been evaluated at 12 mm yr1. Thus, the mean range of RSLR rate in the Ebro Delta must range between 3 and 6 mm yr1. Measured accretion rates in the delta range from 4 mm yr1 in the wetlands surrounding the river mouth to less than 0.1 mm yr1 in impounded salt marshes and rice fields. The annual sediment deficit in the deltaic plain to offset RSLR is close to 1 million m3 yr1. Accretion rates in the rice fields prior to the construction of large dams in the Ebro watershed where higher than RSLR rates, ranging between 3 and 15 mm yr 1. Presently, more than 99 % of the riverine sediments are retained in the reservoirs and rice fields are losing about 0.2 mm yr1. Future management plans in the Ebro Delta should take into account the problem of RSLR, and include a comprehensive management of freshwater and sediment from the river in order to offset negative effects such as waterlogging and salt intrusion. This is a sustainable way to maintain land elevation in a RSLR scenario, simulating the supply of sediment to the deltaic plain produced by river floods before the construction of dams and dikes. In the case of the Ebro Delta, a management approach of diversions of river water to the delta plain must include a partial removal of sediments trapped behind the dams of Ribarroja and Mequinenca. The removal of sediments from these reservoirs is feasible for several reasons. These dams which retain most of the sediment transported by the lower reach of the river, are the last ones within the watershed, and its distance to the sea is not large (about 100 km). Stocks and inputs of sediment within both reservoirs are large enough to produce significant accretion rates in the whole deltaic plain. Present stocks are equivalent to a sediment thickness over the deltaic plain (excluding spits and lagoons) of about 50 cm. Technology to bypass and transport sediments is available. Advantages of this solution include: new sediments to the delta to offset subsidence (via rice fields) and coastal retreat, enhanced delta functioning (productivity and nutrient processing), avoids the accumulation of sediments within the reservoirs, and is a solution to offset negative environmental impacts due to the suppression of solid inputs. From this point of view, it is very important to make a careful management of river discharges at the dams, because nowadays only the hydropower and agricultural requirements are considered. It is also crucial to maintain periods with high discharge, to have enough river energy to transport as much sediments as possible. This issues should be considered in future hydrological plans of the Spanish government.<P>