Avulsion and capture of El Tejar Creek: an example of geological risk related to lateral migration and tree cutting down, Tucumán, Argentina.

MOYANO, M.S. Y S.M. GEORGIEFF

Miramar – Córdoba

Congreso; Third Joint Meeting of ICSU Dark Nature; 2005

Third Joint Meeting of ICSU Dark Nature

El Tejar Creek flows onto a gentle plain in West - East direction in Tucumán Province, Northwest Argentina. Its catchment´s area is related to Pueblo Viejo River, which is a permanent braided gravel river. Pueblo Viejo River avulsioned and partially captured twice El Tejar Creek channels causing floods in a city of 35,000 people and surrounding zone. The avulsion on this area is related to the intense trees cutting down at the mountain foot and an increase of more than 190 mm in the mean annual precipitation for the last 30 years, which have added more water and sediments into channels. Two detail maps were performed with GPS and digital theodolite at the joining point of El Tejar Creek and Pueblo Viejo River in the summer of 2002, before and after the main peakflow. They show a rapid lateral migration of Pueblo Viejo´s channels following the north - northeast regional slope, wich means in direction of El tejar Creek. Sedimentary deposits on the floodplain shows that the peakflow reaches more than 2.5 m high over its mean level and lateral migration up to 1 m. Pueblo Viejo River had a braided pattern before 2002 summer and after peakflow channel become meandering. This pattern change will produce an increase of erodability in the margin which faces El Tejar Creek, moreover this stream has a steeper slope than Pueblo Viejo River. Furthermore, after last peakflow the channel floor of both streams are at the same topographic level. Only a set of old unstable jetties separate these streams. It is a high priority to build new jetties and dredge the chute of Pueblo Viejo River to channel the main flow.