The environmental impact of flow regulation in a tropical delta : the case of the Manamo distributary in the Orinoco delta, Venezuela

2011-10-14T13:56:45Z (GMT) by Giuseppe Colonnello Bertoli
The regulation of the Manama river, one of the major Orinoco distributary channels, has instigated changes in the hydrological and sedimentological regimes of its channel and surrounding wetlands, covering one-third of the Delta area. The dam has altered the hydrodynamics of the Manama. While the average flow of its channel was reduced from 10% to 0.5% (200 m3/s), of the annual discharge of the Orinoco River. The adjacent Macareo distributary, which is not regulated, discharges 11% (4,000 m3/s) annually. During the seasonal high-water period, the water level in the Macareo rises approximately 7 m, while in the Manama is restricted to an periodic oscillation of 0.7 to 1.2 m due mainly to the tidal influence. This flow reduction not only affects the accretion and erosive processes of the banks, due to the decrease of transport of sediments, but also the composition, distribution and diversity of aquatic plants in the region, particularly along the river shores where the species diversity tend to be lower in the regulated river than in the unregulated. The change in the hydrologic dynamic has promoted broad floating meadows to develop in the upper reaches of the Manama river creating patches of rafting meadows. The cessation of the annual flooding has allowed an extensive intrusion of saline water from the sea, which only occur during the dry season. This has enabled halophytic species, particularly mangrove communities to expand their distribution upriver and has also forced the migration of part of the Delta inhabitants Creole and Indians, resulting in a modification of the distribution of the forest and marsh ecological communities due to the overexploitation of their subsistence resources. The particular characteristics of the region that have remained mostly undisturbed since the dam construction several decades ago, permit a comparison of 30 years period of the long-term physical and biological characteristics of a regulated (Manamo) and unaltered (Maca reo) distributary. This thesis investigates these changes and propose a model of the river regulation effects on the ecosystem properties and an equilibrium model of the disturbance and the gain of a new steady state of the ecosystems compartments.