Sedimentation in the Colorado River delta and Upper Gulf of California after nearly a century of discharge loss

摘要

Human intervention in the hydrologic basin of theColorado River has eliminated water discharge and sedimentsupply to the river's mouth and its delta. After~95 years ofapplying strong flow control policies, the previous sedimentarybudget in the delta has changed, the delta now being entirelyexposed to the hydrodynamic forces in its basin. In order to assessthe anthropogenic impact of water diversion on deltaicsedimentary processes and short-term delta geomorphic evolution,textural and mineralogical analyses were performed on 68sediment samples collected from the river, estuary, beach,continental shelf, delta plain and desert. The results show aconsistent NE to SW textural gradient on the shallow-marineplatform adjacent to the Colorado River delta (an area known asthe Upper Gulf of California),and a net cross-basinal sedimenttransport occurring in the same direction. Two opposing littoraltransport components exist along the Sonoran and BajaCalifornian coasts: (1) net sediment transport from SE to NWalong the Sonoran coast deliver sediments from the ocean into theestuarine basin through the Sonora channel of the deltaic system ofthe Colorado River; and (2) sediment transport from N to S alongthe Baja Californian coast removes sediments from the estuarinebasin of the Colorado River into the Northern Gulf of California(NGC). The residual sediment transport pattern observed suggestsa counterclockwise path of exchange of materials between theestuarine basion of the Colorado River and the NGC. Multivariatecluster and factor analyses of the heavy-mineral data reveal theexistence of a sedimentary system dominated by two end-members,representing two heavy-mineral sedimentary provinces: (1) theSonora province is characterized by a heavy-mineral suite of garnetand zircon (G-Z), whose sediment supply originates from the sandysediments of the Sonora Mesa deposits and Sonora's GranDesierto; and (2) the Baja California province is characterized by ahornblende-epidote-pyroxene (H-E-P) suite, whose sedimentsoriginate from the earlier supply of the Colorado River. Since theColorado River is no longer supplying sediments from its drainagebasin, the H-E-P-rich sediments now actively dispersed in thesystem have mainly originated through intense reworking of thedelta. Moreover, G-Z-rich sediments are starting to invade areaspreviously dominated by H-E-P sediments. On the basis of theprevailing hydraulic regimen, textural and mineralogicalcharacteristics of the sediments, and hydrographic and bathymetricinformation from the region, a hypothetical sediment dynamicsmodel for the delta region is proposed, in which the tidallyinfluenced, wave-dominated, sediment-reworked, slightly erodingSonoran coast changes into a tide-dominated, depositional, veryslowly prograding Baja Californian coast. This study shows thathuman intervention of the hydrologic basin of the Colorado Riveris not only responsible for inducint drastic hydrologic changes inits estuary (i.e., from brackish to hypersaline), but also forinducing drastic changes in the hydrographic circulation of thereceiving basin (i.e., from long-basinal to cross-basinal). Thesechanges are ultimately responsible for the relocation of massivevolumes of the delta's sediment inventory, and for the seriousecological impact of habitat loss of indigenous species, such as thenow endangered Totoaba (Totoaba macdonaldi) and Vaquita(Phocoena sinus). 1999 Elsevier Science B.V.All rights reserved.