Impact of Erosion, Mass Wasting, and Sedimentation on Human Activities in the Río Grande Basin, Jujuy Province, Argentina

Nearly all dry mountainous regions are affected by severe erosion, floods, and debris flows during times of intense precipitation. The lithology, geologic structure, and climate in Jujuy Province, Argentina combine to place at serious risk the people who live along the Río Grande, the major river that drains the east side of the Cordillera Oriental and the west side of the Sierras Subandinas. Nearly all precipitation falls during summer (January–March) with little during the remainder of the year; most of the basin is semiarid to arid, although the southern end has a humid subtropical climate. Relief is great, as much as 4,000 m in 15 to 20 km, and the streams that drain the mountains have high gradients. Rapid runoff during the wet season changes the Río Grande, nearly dry most of the year, into a torrent that erodes bridge abutments and floods the valley floor. Precambrian- age low grade metaclastics shatter in the frost climate of the higher mountains and become source material for debris flows that damage or bury roads, railroads, and houses. Poorly consolidated deposits of late Tertiary rocks and Quaternary deposits along the sides of the valley erode readily and yield large volumes of sediment in addition to that coming from higher parts of the basin. These processes are common to all rivers that drain mountainous regions, but lack of understanding of the geomorphic regime of the rivers has led to unexpected hazardous conditions that are expensive to correct. Most communities are along transportation routes that follow the river. Many bridges and levees are inadequately designed for the high runoff rates and sediment load, thus are frequently damaged or destroyed. Dam construction causes aggradation upstream and scouring downstream. Housing in the city has expanded onto the floodplain; flood walls to protect the residents of these “barrios de emergencia” constrict the channel. Application of adequate geomorphic and geologic information provided prior to new construction may reduce the costs of future repair