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

Human intervention in the hydrologic basin of the Colorado River has elimin ated water discharge and sediment supply to the river's mouth and its delta . After similar to 95 years of applying strong flow control policies, the p revious sedimentary budget in the delta has changed, the delta now being en tirely exposed to the hydrodynamic forces in its basin. In order to assess the anthropogenic impact of water diversion on deltaic sedimentary processe s and short-term delta geomorphic evolution, textural and mineralogical ana lyses were performed on 68 sediment samples collected from the river, estua ry, beach, continental shelf, delta plain and desert. The results show a co nsistent NE to SW textural gradient on the shallow-marine platform adjacent to the Colorado River delta (an area known as the Upper Gulf of California ), and a net cross-basinal sediment transport occurring in the same directi on. Two opposing littoral transport components exist along the Sonoran and Baja Californian coasts: (1) net sediment transport from SE to NW along the Sonoran coast delivers sediments from the ocean into the estuarine basin t hrough the Sonora channel of the deltaic system of the Colorado River; and (2) sediment transport from N to S along the Baja Californian coast removes sediments from the estuarine basin of the Colorado River into the Northern Gulf of California (NGC). The residual sediment transport pattern observed suggests a counterclockwise path of exchange of materials between the estu arine basin of the Colorado River and the NGC. Multivariate cluster and fac tor analyses of the heavy-mineral data reveal the existence of a sedimentar y system dominated by two end-members, representing two heavy-mineral sedim entary provinces: (1) the Sonora province is characterized by a heavy-miner al suite of garnet and zircon (G-Z), whose sediment supply originates from the sandy sediments of the Sonora Mesa deposits and Sonora's Gran Desierto; and (2) the Baja California province is characterized by a hornblende-epid ote-pyroxene (H-E-P) suite, whose sediments originate from the earlier supp ly of the Colorado River. Since the Colorado River is no longer supplying s ediments from its drainage basin, the H-E-P-rich sediments now actively dis persed in the system have mainly originated through intense reworking of th e delta. Moreover, G-Z-rich sediments are starting to invade areas previous ly dominated by H-E-P sediments. On the basis of the prevailing hydraulic r egime, textural and mineralogical characteristics of the sediments, and hyd rographic and bathymetric information from the region, a hypothetical sedim ent dynamics model for the delta region is proposed, in which the tidally i nfluenced, wave-dominated, sediment-reworked, slightly eroding Sonoran coas t changes into a tide-dominated, depositional, very slowly prograding Baja Californian coast. This study shows that human intervention of the hydrolog ic basin of the Colorado River is not only responsible for inducing drastic hydrologic changes in its estuary (i.e., from brackish to hypersaline), bu t also for inducing drastic changes in the hydrographic circulation of the receiving basin (i.e., from long-basinal to cross-basinal). These changes a re ultimately responsible for the relocation of massive volumes of the delt a's sediment inventory, and for the serious ecological impact of habitat lo ss of indigenous species, such as the now endangered Totoaba (Totoaba macdo naldi) and Vaquita (Phocoena sinus). (C) 1999 Elsevier Science B.V. All rig hts reserved.