10 CARmu. MAR. Snm., SPECIAL EDITION. 2007 Box 2.1: (Coutiuuetf) 80 Land Use Change in The Magdalena River Basin GO "' liB 1970s D 1990s :t'" .5 40 c: "' .c: (.) tf!. 20 SouttcE: Restrepo and During the past 30 years, the Magdalena basin hus been under increasing cnvironmC"ntal stress. Recent lund usc analysis for the 1970-1990 year period indicates that forest cover was to 54 11/i) within the river's watersheds. The reduced fhml only remaining rainforest area is located in the lower Magdalena valley, and most of the land on the lower and middle slopes is under cultivution. In uddition. this period witnessed an increase in the proportion of soils and habitats converted to agricultural usc. from 25% to 40 1Vo of the basin area. 2006, Water Discharge into the Caribbean Sea The Magdalcnu River discharges 22S kmJ of water annually, or on H\'Cragc 7.200 m3 per into the \\'t.-stcnt Caribbean, based on 70 years of daily data between 1942 and 2002. Analysis has shown thnt 69% of the variation of the Magdalena streamllow is explained by the multi-mmual cycle of the Southern Oscillation (the El Nilio etlcct), with high disclmrge occun·ing during La Nifia phase and low discharge during El Nino phase. Snut<cE: Restrepo et al. 2005. Sediment Load into the Caribbean Sea The mean sediment load for the Magdulena is 144 million tonnes per year, coJTc!'iponding: to u sediment yield of 560 t km :! yr 1 lor the 257A38 basin. This corresponds to H6% of the total sediment load of all Colombian rivers draining into the Caribbean. The nmin tributary, the Cauca River. contribute!i 31% of the Magdalena total. HI ......... ........ u -:. -t 0 -'-,-,.-,-,'1'1""•'1-.,--1'I'lrrl'1-.,--.,-1'II 1975 SOURCE: IIJ79 19/B I!NI ftJ95 Tht! sediment loud also shows an oscillation \Veil corrclutcd with the El Nino/Southern Oscillation (ENSO) cycle which explains 54'}'o of the variation. TI1e La Nhia high Oow in 1988-89 caused a marked increase in sediment load from the Mngc.h1lcna, with one prominent peak of J.6 111 tonncs per day. Other llood events urc ckar but less pronounced. e.g .. in 1975-76, 1981-82, and 1995. Low sediment loads occurred during El Nino events in 1977-78, 1982-83, and 1991-92. The mean daily sediment loads during El Niiio and La Niiia ycnrs arc 25(> tonncs per day and 511 tonnes per day, respectively. Restrepo and Kjerfve 2000b. - 31-