CONSEQUENCES OF WATER DIVERSION IN ARID COASTAL SYSTEMS WITHIN THE RIO GRANDE, COLORADO, AND OTHER WORLD DELTAS

Nearly 500 million people live on and adjacent to modern deltas, although anthropogenic modifications have made these low-lying coastal systems increasingly vulnerable to environmental degradation. Little research has been conducted on arid-region deltas in North America. For example, the Rio Grande (Rio Bravo) delta along the south Texas-northeast Mexico border, has been significantly impacted by drought during the past 6 years, while its coast is periodically modified by hurricanes and storm surges. Moreover, seaward portions of the delta plain are affected continuously by land subsidence and consequent relative sea-level rise.

Although these natural processes played a major role in the Rio Grande delta development, human influences now prevail and are resulting in accelerated degradation of this system. Water and sediment influx to the delta has been extensively modified by a series of upstream dams and reservoirs to such a degree water and sediment are no longer discharged in the western Gulf of Mexico. Loss of river sediment influx to the delta front has brought about widespread erosion and degradation of coastal (including wetland) environments. Moreover, irrigation and reclamation projects have caused major changes in the flow of water and sediment within the delta. Construction of >2000 km of closely spaced ship channels, irrigation canals and ditches, most bordered by levees, precludes widespread flooding and sediment accretion on the delta plain. Major impacts caused by reduced flow of the delta water and sediment regime include 1) increased salinization of surface and ground water; 2) reduced water quality (i.e., increased chemical loading) leading to decreased soil productivity and degradation of natural habitats; and 3) accelerated coastal erosion.

The other large arid-climate delta in North America, the Colorado formed in the northern Gulf of California high-tide setting, has also been extensively modified by artificial water diversion and, consequently, is undergoing destructional rather than progradational development. Comparative study of the Rio Grande and Colorado with other world deltas in arid-climate settings, such as Egypt's and India's densely populated Nile and Indus deltas, Iraq's largely drained Basrah delta, and northern Australia's less modified systems is warranted.