**3. Impact of human activities on erosion processes and the effect on the environment**

Sedimentological processes operate on the earth's crust over thousands of years. Climatic factors are the main drivers of the processes of erosion, sediment transport and deposition over different time-scales. When subjected to the action of natural forces, soil particles are transferred to other sites within the watershed. Sediment processes may occur in several forms: surface erosion, erosive formations such as channels and gullies, mass transfer including collapsed riverbanks and hillside landslides. When incorporated into the river system, sediment is carried by the flow to downstream areas, where sedimentation may occur. It is important to note that erosion processes occur in a continuous and dynamic system, which is in constant reworking and subject to geomorphologic changes (Walling, 2006). Therefore, erosion processes can be due to storm events or the wind action over dunes formations. Extreme events such as high magnitude floods may produce highly significant geomorphologic changes unrelated to human intervention. Natural events cause weathering processes on mineral rocks, which are subjected to erosion and transported to sedimentary formations, where they are subjected to other chemical processes.

Although erosion processes are directly linked to climate factors, human activity tends to accelerate their impact on the environment, provoking considerable negative effects (Dedkov & Moszherin, 1992). According to Panin (2004), the suspended sediment load released into oceans annually in continental regions varies between 15-20 GT.year-1. Historically, erosion processes have increased worldwide as a result of several different types of human activities, including agriculture, mining, urbanization and industry (Walling & Fang, 2003). Intensification of these erosion processes has led to long-term negative social and economic impacts. For example, siltation of reservoirs built for hydropower production can cause economic losses that affect society as a whole. Mahmood (1987) estimated that reservoir storage capacity in the world decreased by approximately 1% every year as a result of siltation, causing an annual loss of US\$ 6 billion. This impact is far-reaching considering that approximately 40% of the worldwide river system capacity is stored in large dams (Vörösmarty et al. 2003).

Deforestation of native vegetation for agriculture and wood extraction are the main causes of erosion over the world. Ives & Messerli (1989) presented a model illustrating how changes in the population structure of Nepal in the 1950s affected natural processes

*transfer* of resources such as water, nutrients and soil to the receiving patch (referred to as the *reserve*) downslope. Patches within a semi-arid landscape are typically formed by plants, under which soils tend to have higher organic matter, nutrients and microbial activity. The rainfall and subsequent transfer of materials to the patch triggers a *pulse* of biological activity, which in turn produces positive feedbacks including the formation of stronger or new soil aggregates that improve soil stability and water infiltration (Belnap et al., 2005). Human activities that disrupt this positive feedback loop between the abiotic and biological activities, for example native vegetation clearance or overgrazing, can lead to negative impacts on the system. For example, removal of vegetation will reduce organic matter input to the soil, which can lead to decreased microbial activity and poorer soil structure and

**3. Impact of human activities on erosion processes and the effect on the** 

formations, where they are subjected to other chemical processes.

large dams (Vörösmarty et al. 2003).

Sedimentological processes operate on the earth's crust over thousands of years. Climatic factors are the main drivers of the processes of erosion, sediment transport and deposition over different time-scales. When subjected to the action of natural forces, soil particles are transferred to other sites within the watershed. Sediment processes may occur in several forms: surface erosion, erosive formations such as channels and gullies, mass transfer including collapsed riverbanks and hillside landslides. When incorporated into the river system, sediment is carried by the flow to downstream areas, where sedimentation may occur. It is important to note that erosion processes occur in a continuous and dynamic system, which is in constant reworking and subject to geomorphologic changes (Walling, 2006). Therefore, erosion processes can be due to storm events or the wind action over dunes formations. Extreme events such as high magnitude floods may produce highly significant geomorphologic changes unrelated to human intervention. Natural events cause weathering processes on mineral rocks, which are subjected to erosion and transported to sedimentary

Although erosion processes are directly linked to climate factors, human activity tends to accelerate their impact on the environment, provoking considerable negative effects (Dedkov & Moszherin, 1992). According to Panin (2004), the suspended sediment load released into oceans annually in continental regions varies between 15-20 GT.year-1. Historically, erosion processes have increased worldwide as a result of several different types of human activities, including agriculture, mining, urbanization and industry (Walling & Fang, 2003). Intensification of these erosion processes has led to long-term negative social and economic impacts. For example, siltation of reservoirs built for hydropower production can cause economic losses that affect society as a whole. Mahmood (1987) estimated that reservoir storage capacity in the world decreased by approximately 1% every year as a result of siltation, causing an annual loss of US\$ 6 billion. This impact is far-reaching considering that approximately 40% of the worldwide river system capacity is stored in

Deforestation of native vegetation for agriculture and wood extraction are the main causes of erosion over the world. Ives & Messerli (1989) presented a model illustrating how changes in the population structure of Nepal in the 1950s affected natural processes

lower soil storage capacity.

**environment** 

on several scales. It is estimated that the global area dedicated to agriculture has increased five times over the last 200 years, prompted by population growth and higher food demand (UNEP, 1995). On the other hand, reservoir construction and the damming of water and sediment significantly reduce the amount of sediment reaching floodplains and estuaries. A recent survey using long-term records of large basins subjected to the impact of human activity found that, in some cases, increased sediment in river systems may not impact deltas and estuaries (Dai & Tan, 1996; Walling, 2000; Walling & Fang, 2003) due to sediment retention in reservoirs located upstream. Thus, erosion processes reflect the combined action of climate factors and disturbances in the basin as a result of unsustainable human activities.

Urbanization may also cause substantial changes in hydrologic behavior and erosive processes (Taylor, 2007). Land occupation of urban areas brings together the production of liquid and solid residues that, if not adequately collected, may be detrimental to water and sediment quality. Urbanization is associated with building construction and infrastructure service. Paved surfaces in urban environments result in lower amounts of water infiltration, which, in turn, can produce adverse social and economic impacts such as floods. In developing countries, it can be observed that urban development does not commonly occur in line with infrastructure and urban services investments. Often public services such as health and education are inadequate and planning and provisions to prevent or cope with extreme events (e.g. prevention measures and land occupation control) are lacking. In addition, urban occupation generates sediment contaminated by toxic substances (heavy metals, pesticides, oils, organic compounds), which can adhere to the fine fractions in the fluvial environment (Robertson et al., 2003; Lecoanet et al., 2003). Primary sources of sediment contamination in urban areas are domestic sewage and the construction of buildings and roadways. The presence of contaminants significantly affects aquatic organisms that feed on the sediment, which highlights the implications of land management on other parts of the system.
