Human impacts on headwater fluvial systems in the northern and central Andes

Abstract

South America delivers more freshwater runoff to the ocean per km 2 land area than any other continent, and much of that water enters the fluvial system from headwaters in the Andes Mountains. This paper reviews ways in which human occupation of high mountain landscapes in the Andes have affected the delivery of water and sediment to headwater river channels at local to regional scales for millennia, and provides special focus on the vulnerability of páramo soils to human impact. People have intentionally altered the fluvial system by damming rivers at a few strategic locations, and more widely by withdrawing surface water, primarily for irrigation. Unintended changes brought about by human activities are even more widespread and include forest clearance, agriculture, grazing, road construction, and urbanization, which increase rates of rainfall runoff and accelerate processes of water erosion. Some excavations deliver more sediment to river channels by destabilizing slopes and triggering processes of mass-movement. The northern and central Andes are more affected by human activity than most high mountain regions. The wetter northern Andes are also unusual for the very high water retention characteristics of páramo (high elevation grass and shrub) soils, which cover most of the land above 3000 m. Páramo soils are important regulators of headwater hydrology, but human activities that promote vegetation loss and drying cause them to lose water storage capacity. New data from a case study in southern Ecuador show very low bulk densities (median 0.26 g cm − 3 ), high organic matter contents (median 43%), and high water-holding capacities (12% to 86% volumetrically). These data document wetter soils under grass than under tree cover. Effects of human activity on the fluvial system are evident at local scales, but difficult to discern at broader scales in the regional context of geomorphic adjustment to tectonic and volcanic processes.