Examinando por Materia "erosion rate"
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Ítem Fission-track datings and geomorphic evidences for long-term stability in the Central Cordillera highlands, Colombia(Gebruder Borntraeger Verlagsbuchhandlung, 2006-01-01) Toro, G.; Hermelin, M.; Schwabe, E.; Posada, B.O.; Silva, D.; Poupeau, G.; Toro, G.; Hermelin, M.; Schwabe, E.; Posada, B.O.; Silva, D.; Poupeau, G.; Universidad EAFIT. Departamento de Ciencias; Geología Ambiental y TectónicaAge of weathered andesitic volcanic ash layers, which cover most of the landscape in the Central Cordillera northern massif plateaus in Colombia, were determined through the use of fission track counting in volcanic zircon crystals. A stoneline, exposed in many outcrops, which corresponds to the lower limit of younger tephras, gave ages between 350 ka and 440 ka. Andosols lying above the stoneline could not be dated. Paleolake deposits in the study area and surroundings were dated at about 2 Ma; older ashes deposited on the Rio Negro erosion surface (SII) below the stoneline level gave ages between 3.4 and 5.4 Ma. The study area is located about 150 km from the volcanic center. The ash layers decrease in thickness only slightly with distance, as perceived from profiles sampled in flat areas. We conclude that the only noticeable erosional event in the region was the emplacement of the stone-line. This erosion event which was relatively short and mild in intensity, as part of the underlying soil derived from quartzdiorite was preserved. This soil formed above a saprolite with a thickness which may reach 150 m, a fact which supports the inference that the plateaus have been stable for millions of years, in contrast to the high erosion rates observed in the surrounding steep slopes and canyons. © 2006 Gebrüder Borntraeger.Ítem Human induced discharge diversion in a tropical delta and its environmental implications: The Patía River, Colombia(ELSEVIER SCIENCE BV, 2012-03-06) Restrepo, Juan D.; Kettner, Albert; Universidad EAFIT. Departamento de Geología; Ciencias del MarThe Patía River, the number one in terms of sediment yield ~1500tkm -2yr -1 draining the western South America, has the most extensive and well developed delta on the Pacific coast, measuring 1700km 2. During the Holocene, nature forced the Patía delta to the south; however, a major water diversion, starting in 1972, diverted the Patía flow to the Sanguianga River, the latter, a small stream draining internal lakes from the Pacific lowlands. This human induced discharge diversion shifted the active delta plain back to the north and changed the northern estuarine system into an active delta plain. Overall, major environmental consequences of this discharge diversion in terms of morphological changes along the delta coast and distributary channels, are evidenced by: (1) coastal retreat along the abandoned delta lobe; 63% of the southern shoreline is retreating at maximum rates of 7myr -1, with a corresponding coastal land loss of 106myr -1; (2) transgressive barrier islands with exposed peat soils in the surf zone; (3) abandonment of former active distributaries in the southern delta plain with associated closing of inlets and formation of ebb tidal deltas; (4) breaching events on barrier islands; and (5) distributary channel accretion in the northern delta plain by morphological processes such as sedimentation (also in crevasses), overbank flow, increasing width of levees, interdistributary channel fill, and colonization of pioneer mangrove. The Sanguianga Mangrove National Park (SMNP), the largest mangrove reserve in Colombia, measuring 800km 2, lies in this former estuary, where major hydrologic and sedimentation changes are occurring. Observed environmental changes in the SMNP, include (1) seaward advance of the sub-aqueous delta front at the Sanquianga inlet evidenced by an increase in tidal flat area from 5.4Mm 2 in 1986 to 14Mm 2 in 2001; (2) freshening conditions in the Sanguianga distributary channel, a hydrologic change that has shifted the upper estuarine region (salinity <1psu) downstream; (3) downstream advance of freshwater vegetation, which is invading channel banks in the lower and mixing estuarine zones; (4) die-off of approximately 5200ha of mangrove near the delta apex at Bocas de Satinga; and (5) recurrent periods of mangrove defoliation due to a warm plague. Further analysis indicate that during the past two decades, processes such as mangrove erosion in the delta shore, are the result of a short-term relative sea-level rise of 5.1mmyr -1 for the 1984-2006yr-period, after the devastating tsunami of 1979. In the Patía catchment, erosion rates have been more pronounced during the 1970-1980 and 1990-2000 decades, as a result of land degradation and deforestation. Preliminary results indicate that relative resent anthropogenic influences on the Patía River drainage basin have altered the deltaic environment and beyond significantly. © 2011 Elsevier B.V.Ítem Sediment load trends in the Magdalena River basin (1980–2010): Anthropogenic and climate-induced causes(ELSEVIER SCIENCE BV, 2018-02-01) Restrepo, J.D.; Escobar, H.A.; Universidad EAFIT. Departamento de Geología; Ciencias del MarThe Colombian Andes and its main river basin, the Magdalena, have witnessed dramatic changes in land cover and further forest loss during the last three decades. For the Magdalena River, human activities appear to have played a more prominent role compared to rainfall (climate change) to mobilize sediment. However, environmental authorities in Colombia argue that climate change is the main trigger of erosion and floods experienced during the last decade. Here we present the first regional exercise addressing the following: (1) what are the observed trends of sediment load in the northern Andes during the last three decades? and (2) are sediment load trends in agreement with tendencies in land use change and climate (e.g., precipitation)? We perform Mann-Kendall tests on sediment load series for 21 main tributary systems during the 1980–2010 period. These gauging stations represent 77% of the whole Magdalena basin area. The last decade has been a period of increased pulses in sediment transport as seen by the statistical significant trends in load. Overall, six subcatchments, representing 55% of the analyzed Magdalena basin area, have witnessed increasing trends in sediment load. Also, some major tributaries have experienced changes in their interannual mean sediment flux during the mid- 1990s and 2005. Further analysis of land cover change (e.g., deforestation) indicates that the basin has undergone considerable change. Forest cover decreased by 40% over the period of study, while the area under agriculture and pasture cover (agricultural lands 1 and 2) increased by 65%. The highest peak of forest loss on record in the Magdalena basin, 5106 km2 or 24% of the combined deforestation in Colombia, occurred during the 2005-2010 period. In contrast, Mann-Kendall tests on rainfall series for 61 stations reveal that precipitation shows no regional signs of increasing trends. Also, increasing trends in sediment load match quite well with the marked increase in forest clearance during the 1990–2000 and 2005–2010 periods. Such signs of increasing sediment fluxes should not be attributed to climate change and rainfall variability alone. As a whole, the Magdalena, one of the top 10 rivers in terms of sediment delivery to the ocean (184 Mt y- 1), and its tributaries have experienced increasing trends in sediment load during the 1980–2010 period; increases in close agreement with trends in land use change and deforestation. During the last decade, the Magdalena River drainage basin has witnessed an increase in erosion rates of 34%, from 550 t km- 2 y- 1 before 2000 to 710 t km- 2 y- 1 for the 2000–2010 period, and the average sediment load for the whole basin increased to 44 Mt y- 1 for the same period. Similar to the global picture of human contribution to sediment generation, the rate of anthropogenic soil erosion in the Magdalena basin probably exceeds the rate of climate-driven erosion by several orders of magnitude. © 2016 Elsevier B.V.