CHANNEL SHIFTING OF THE RIO-GRANDE, SOUTHERN RIO-GRANDE RIFT - IMPLICATIONS FOR ALLUVIAL STRATIGRAPHIC MODELS

Presented here is the history of channel course changes of the Rio Gra nde, based on Bureau of Reclamation and other maps, from prior to 1844 (exact date unknown) to 1912 over an 80 km stretch of the semi-arid M esilla and upper Hueco half grabens of the southern Rio Grande rift. T he historical Rio Grande was a pebbly sand bedload stream, occupying a relatively narrow (less than or equal to 8 km) floodplain incised abo ut 100 m into Pliocene and early Pleistocene alluvium. Prior to comple tion of Elephant Butte Dam in 1916, the river experienced late spring floods due to snowmelt near its headwaters. Throughout the 68+ year hi story recorded here the river channel had variable width (1300-100 m) and sinuosity (1.9-1.2), and displayed kilometres of lateral migration of both straight and sinuous stretches, neck cutoff of meander loops, and large- and small-scale avulsions. Three major avulsions occurred in different parts of the Mesilla basin: (1) between 1844 and 1852 in the north-central part of the basin, (2) in 1865 in the northern part of the basin, and (3) between 1903 and 1912 in the southern part of th e basin. The length scale of the avulsions (distance from node to reoc cupation of previous channel) ranged from 17 to 30 km, and the 1865 av ulsion is documented to have occurred in one flood season. The histori cal Rio Grande is unique among other rivers described in the literatur e because of the high frequency of channel migration and avulsion. It sheds doubt on the applicability of sedimentary models requiring time- dependant (levee growth) avulsion functions. (C) 1998 Elsevier Science B.V. All rights reserved.