HR: 1330h
AN: H52A-1155    [PDF]
TI: Movement of Sediment Associated With Lowered Reservoir Levels in the Rio La Venta Arm of the Presa Netzahualcoyotl, Chiapas, Mexico
AU: Hilton, S
EM: shilton@fs.fed.us
AF: Pacific Southwest Research Station, 1700 Bayview, Arcata, CA 95521 United States
AU: * de la Fuente, J
EM: jdelafuente@fs.fed.us
AF: Klamath National Forest, 1312 Fairlane Road, Yreka, CA 96097 United States
AU: Lisle, T E
EM: tlisle@fs.fed.us
AF: Pacific Southwest Research Station, 1700 Bayview, Arcata, CA 95521 United States
AU: Velasquez, J
EM: portillo@conanp.gob.mx
AF: Comision Nacional de Areas Naturales Protegidas, Carretera Pan Americana km 1050 Hotel Coiteco, Ocozocuautla, Chi 29040 Mexico
AU: Allison, B
EM: ballison@fs.fed.us
AF: Klamath National Forest, 1312 Fairlane Road, Yreka, CA 96097 United States
AU: Olson, B
EM: bjolson@fs.fed.us
AF: Klamath National Forest, 1312 Fairlane Road, Yreka, CA 96097 United States
AU: Quinones, R
EM: rquinones@fs.fed.us
AF: Klamath National Forest, 1312 Fairlane Road, Yreka, CA 96097 United States
AB: A joint sedimentation study is currently underway at the Netzahualcoyotl reservoir in Chiapas, Mexico, involving the Comision Nacional de Areas Naturales Protegidas (CONANP) of the Secretaria de Medio Ambiente y Recursos Naturales and the USDA Forest Service. The reservoir is adjacent to the Reserva de la Biosfera, Selva El Ocote, administered by CONANP. Ongoing research is intended to provide watershed and reservoir managers with strategies to protect the resources of Rio La Venta canyon. The Rio La Venta arm of the reservoir is incised into karst terrain, with near-vertical limestone walls up to 300 meters high. The canyon is fed by two rivers, Rio La Selva and Rio Negro, and is surrounded by pristine tropical forest. The majority of the clastic sediment (predominantly sand and fine gravel) entering the reservoir originates in the headwaters of the two rivers which are underlain by weathered and dissected granitic terrain. Rapid sedimentation of the partially inundated canyon poses a threat to the aquatic ecosystem, as well as to recreational resources. Longitudinal and transverse profiles were surveyed in the inundated canyon in March of 2002 and repeated in April of 2003 when the reservoir level was 15 meters lower. The 2002 longitudinal profile shows an inflection from a slope of 0.0017 to one of 0.0075 at 7.2 km downstream of the mouth of Rio Negro. In 2003, the two slopes remained the same, but the bed lowered about 5 meters and the inflection point moved downstream about 2.3 km. We calculated that reservoir lowering in 2003 allowed the transport of 2.5 million cubic meters of sand further out into the reservoir. This volume is more than the average annual rate of filling up to the 2002 level since 1984 when sedimentation was not as advanced (De la Fuente et al., 2002), which was calculated disregarding loss of sediment to the main reservoir. Field observations at late dry season low flows in 2003 revealed active transport of sand and pebbles and formation of fresh bars. Transverse profiles showed a flat bed with multiple submerged channels, consistent with high sediment mobility. Sediment stored in exposed bars up to 4 meters in height consisted of light colored sand beds alternating with dark organic-rich silt and clay beds ranging from a few centimeters to 50 cm in thickness. These beds were deposited in fluvial and lacustrine environments respectively. We conclude that: a) Lowering of the reservoir allowed sediment to be transported out of the canyon; b) Long periods of inundation of the canyon allow accumulation of very fine sediment (clay/silt/organic) which may affect fish habitat; c) Sediment accumulation in the canyon appears to be a balance between accumulation at high reservoir levels and removal at lower levels. Continuing research will examine: past reservoir levels; future changes in channel geometry; sediment sources, particularly recently burned areas adjacent to the reservoir; and fish populations and habitat. These studies will improve our understanding of sedimentation patterns and processes and our ability to make management recommendations for Netzahualcoyotl reservoir.
DE: 1803 Anthropogenic effects
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1857 Reservoirs (surface)
SC: Hydrology [H]
MN: 2003 Fall Meeting