Problems related to soil degradation and sedimentation
Soil degradation directly affects the economy of producers due to the fact that it dramatically reduces their productivity as a direct consequence of the loss of the fertile layer of topsoil. Further, sedimentation, or processes related to the movement and deposition of eroded soils within the hydrologic system, affects processes like increased turbidity, color, and other processes that are strictly mechanical in nature, such as erosion, and silting up of the river beds and other water bodies and coastal zones. The sedimentation processes make the river waters turbid, preventing sunlight from penetrating and limiting the production of primary producers (plants) for aquatic life to consume. This affects the reproduction and survival of some economic species, including fish. In the coastal zone, sedimentation affects the populations of reefs, ecosystems that involve and are crucially important to the reproduction of marine species with a high commercial value.
The soil of the basin is fertile and can be farmed using mechanized agricultural techniques. Generally speaking, the soil in the North of Costa Rica and the area bordering Lake Nicaragua and the eastern part of the Basin is poorly drained in the lower areas, with drainage improving at higher altitudes. The volcanic characteristics of the region have provided soils with a high content of ash, hence richer. However, the natural processes of erosion and soil loss, compounded by inappropriate (unsustainable) agricultural practices, inappropriate forestry techniques, and the lack of soil and water conservation practices have resulted in major losses of fertile topsoil in different parts of the basin. Similarly, erosion is accompanied by a decline in the replenishment of aquifers which, in addition to reducing the availability of underground water, alters the hydrological regime of water bodies, producing greater and increasingly frequent floods during the rainy season and low flow rates during the dry season. Changes in land use, without taking into account its potential, such as the extension of commercial agriculture and overexploitation of forests for wood exacerbate the erosion problems facing the SJRB. In much less degree, traditional land clearing practices in preparation for agricultural work, such as slash and burn practices, are also contributors to the increase of erosion processes in the SJRB.
The sedimentation process affects the basin’s water resources to a large extent, both in terms of the quality and in terms of its aquatic flora and fauna. However, it is not possible to quantify this problem because there is very little information available on the environment of the entire SJRB. Historically, the San Juan River has undergone a natural process of sedimentation, which has led to discharges through two sites: the North San Juan Bay or Lagoon in Nicaragua, which is high in sediment, and the mouth of the Colorado River in Costa Rica, which produces the highest flow rate discharge. Nevertheless, the upper and mid-level parts of the San Carlos River sub-basin are the most critical in terms of sedimentation, mainly because they have volcanic soil associated with a mountainous terrain, very rugged residual soil, hilly residual terrain, in addition to torrential rainfall and inappropriate soil use.
The particular production of sediment in suspension in this portion of the Basin has been determined at the Terrón Colorado station, where the figures reached 817 ton/km2/year; at the Peñas Blancas station, the figures were 700 ton/km2/year; and at the Puerto Viejo and Veracruz stations on the Sarapiquí River where the figures are close to 216 ton/km²/year. While these latter figures are significantly lower than those assessed for the San Carlos River basin, they are significant in terms of turbidity. According to comments from farmers in the Los Chiles canton, about 15,000 hectares of soil are farmed using mechanized agricultural techniques each year for crop development. Erosion is reported to be visible in these agricultural areas, and production is reported to be decreasing.
The upper part of the Frío River sub-basin has residual soils with hilly relief and a high risk of erosion, compounded by the implementation of unsustainable agricultural practices in the area, which excludes soil and water conservation practices. The specific production of sediment calculated at the Guatuso station was 298 ton/km²/year and 181 ton/km²/year at the Venado station. These amounts are less than the input from the San Carlos River sub-basin but have the same implications for turbidity.
The upper part of the Zapote River sub-basin is made up of very rugged residual soil and very shallow soil on mountainous terrain. The sediment transported in this sub-basin is deposited at the outlet of Lake Nicaragua, where the San Juan River is formed. Although the turbidity of the waters indicates a high level of solids in suspension, the specific production of sediment in this sub-basin has not been estimated because no information is available.
In the north of the San Juan River subsystem, overuse of the soil by the extension of commercial agriculture, overexploitation of forests for wood, weather, land, and topographic conditions combine to cause severe erosion by water in certain areas, which affects the sustainability of the resources in the zone and increases the transport of sediment toward the San Juan River basin. The zones considered to be critical areas in this sector of the sub-basin are the Negra, Sábalos and Santa Cruz rivers. There are no measurements of the sediment deposited in these rivers, hence the inability to quantify the specific sediment deposits.
Lake Nicaragua acts as a reservoir for sediment and a sink for most of the pollutants that reach its waters; because of the large size of the lake, much of the sediment that reaches it is deposited there and does not reach the San Juan River. Also, because of the lake’s great dilution capacity, the concentration of pollutants is considerably reduced. The problems of erosion and of sediment carried to the lake are significant, mainly due to deforestation and the practice of unsustainable agriculture, even though it does not reach the levels indicated for the former sub-basins. This is fundamentally due to the fact that the terrain is less rugged and rainfall levels are lower.
Nevertheless, as a result of this situation, many tons of soil are lost annually, thereby diminishing the productivity of agricultural land through the loss of fertile topsoil as a result of the rains.
The same situation occurs in livestock rearing, in which case the erosion caused by water is accentuated by the movement of herds through pastures, since most of these farms rear stock on a large scale and do not practice livestock rotation. This is exacerbated by the general practice of turning forestry land into pastures.
Erosion by water also seriously affects rural roads, resulting in bad roads for marketing agricultural products, thereby increasing the price of transporting products to markets. Quite often, it is not possible to get production to retail outlets, causing severe economic losses for producers. In the Lake Nicaragua subsystem, these problems of erosion by water occur on the slopes of the Mombacho volcano, and extend to the vicinity of Nandaime, in the sub-basins of the Ochomogo, Malacatoya, Tecolostote, Mayales, and Acoyapa rivers. However, the specific production of sediment in this subsystem has not been estimated, as the necessary information is not available. Additional, inappropriate road design and construction also contributes to the acceleration of the erosion processes.
The sediment from the upper and middle sectors of the basin rises mainly through the San Carlos and Sarapiquí rivers. Human activities exacerbate this natural process. Comparing aerial photographs taken in different years, one observes islets formed during the past five years as a result of the sedimentation process. It is well known that during the dry season, places in the San Juan River are not navigable by the small craft or rowboats used to transport people in the region, owing to the heavy sedimentation taking place. The soil degradation process becomes more acute as a result of population pressure. As population growth is high, wider areas are claimed for unsustainable agricultural activities. The construction of waterworks like dams or microdams, and mining, without incorporating the environmental variable, places additional stress on the fragile land and accelerates erosion processes.
At the transboundary level, sedimentation is a major problem given that the accelerated erosion process that is occurring in the basin affects the San Juan river and the coastal area of the basin, due to the fact that these areas receive the sediment. To halt, reduce, and control this erosion process, and consequently reduce the problems caused by sedimentation, it is necessary to develop soil and water conservation programs that would allow for the integration of these practices into the agricultural activities being developed in the basin. The creation of tax incentives for those producers who appropriately manage natural resources could be a motivation to halt and control the erosion and sedimentation process occurring in the SJRB.
Land use planning and the preparation of plans for basin management are also necessary. These plans should seek to develop socioeconomic activities based on land use capacity, and encourage the acquisition of more precise information on the dynamics of erosion, sedimentation, and pollution of water bodies, as guidelines for control. These actions can hardly be successful without the involvement of producers and civil society as a whole, through environmental education programs, the creation of economic alternatives, and the development of monitoring and information systems that provide a better awareness of the actions being advocated.
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