Alternative strategies for the restoration of floodplains in the middle Ebro river (NE Spain)

The floodplain of the middle Ebro River (NE Spain) was submitted to intense degradation for the last decades because of: decreased river discharge, regular groundwater discharge from irrigated land, extensive agricultural use. This study defines patterns of land use cover change experimented by the floodplains after observations of aerial photographs of four different decades since 1927 and to evaluate alternative strategies for the restoration of the floodplains including alternative zero (doing nothing), passive restoration (increasing river discharge); active restoration (increasing river discharge and flooding of degraded zones and manual plant recovery). A decreased landscape diversity and evenness with increasing fragmentation took place associated to increasing agricultural activities. Lost of naturalness is associated to decreased water discharge and dikes constructed in the banks of the river during the last three decades. The present river conditions have not capacity for maintaining floodplain functions. Passive restoration may be a good strategy at long term. A recurrent flood event of 500 m³/s may initiate the recovery of 4% of the present floodplain area, while a recurrent 1,000 m³/s per second would recover 14% of the present floodplain area. A recurrent flood event of 2,000 m³/s may be required to re-establish the geomorphologic dynamics of the study floodplain area (1041 ha) at long term. Passive restoration of zones with high river connectivity is a good strategy at long-term because it combines restoration at landscape scale with site by site selected objectives. However, active restoration is required for short-term restoration of floodplain functions and structure.