15. Sediment Transfers from the Andes of Colombia during the Anthropocene

This chapter reviews data, models, and analyses on Anthropocene-impacted sediment fluxes in the Andes of Colombia and provides examples on how direct human alteration has increased sediment flux during the last decades. Firstly, it describes the context of the northern Andes in terms of sediment production within the whole Andes Cordillera. Secondly, it presents a summary of major land cover changes witnessed in the region from 8000 years ago to the beginning of large-scale land transformation that occurred in Colombia during the last three decades and analyzes major human-induced drivers of change. Also, trends in sediment load during the 1980–2010 period are documented. Finally, it compares modern and prehuman conditions of sediment flux by using some applied models in global and Colombian rivers.

An inventory of per capita anthropogenic land cover change (ALCC) from 8 ka to _AD 2000 for the Andes of Colombia reveals that a nearly pristine environment existed until 3 ka. Two thousand years later, by _AD 1, ALCC only slightly increases. From _AD 1500 to _AD 1600, the ALCC scenarios show a decrease in anthropogenic land use in the Andes, as the indigenous populations of the Americas succumbed to disease and war brought by European explorers and colonists. The collapse of large precontact populations with advanced agriculture, which were especially concentrated in Mesoamerica and the Andes, led to high amounts of land abandonment. The low levels of ALCC shown at _AD 1500 are almost entirely abandoned 100 years after conquest. By _AD 1687, anthropogenic land use in the Andes accelerated with the spread of colonies and nations founded by Europeans. The Americas only start to result in substantial amounts of ALCC emissions during the last centuries.

Further studies on historical patterns and drivers of landscape change in Colombia since 1500 confirm that land conversion in the Andes started five centuries ago. The transformed area in the Andean region rose from 15 M ha in 1500 to 42 M ha in 2000. During the last two centuries, the annual rate of forest-transformed area increased two orders of magnitude, from 4330 ha y⁻¹ in 1800 to 171,190 ha y⁻¹ in 2000. By the year 2000, 80% of the natural vegetation in the Andes was cleared, with 20% remaining as scattered remnants. An assumed value of 30% was cleared in preconquest agricultural landscapes (before 1500), increasing to 80% in 2000. Demographic impacts of colonization and the introduction of cattle were major drivers of change.

Findings of land use and sediment load trends indicate that the extent of erosion within the Andes of Colombia has severely increased over the last 30 years. For example, the last decade has been a period of increased pulses in sediment transport and rates of deforestation as seen by the statistical significant trends in load and by a marked increase of 241% in forest clearance. As a whole, the Andean drainage basins have witnessed an increase in erosion rates of 33%, from 550 t km⁻² y⁻¹ before 2000 to 710 t km⁻² y⁻¹ for the 2000–2010 period. Levels of sediment transport are one order of magnitude higher in modern times than during prehuman conditions. The differences between prehuman and modern sediment load in South American rivers were more pronounced for the Magdalena River, with a difference ranging between −100 and −150 Mt. y⁻¹. Thus, during pristine conditions and according to the observed total load of the Magdalena, 184 Mt. y⁻¹, the Magdalena could have had an annual sediment load between 34 and 84 Mt. y⁻¹ during prehuman times. Further results indicate that 35% of the sediment load in the Colombian Andes is due to deforestation; 1690 Mt. of sediments were produced due to forest clearance over the last three decades. Much of the river catchments (79%) are under severe erosional conditions due in part to the clearance of more than 80% natural forest during the last 500 years.