SURFICIAL GEOLOGIC MAP OF THE MIDDLE RIO GRANDE VALLEY FLOODPLAIN, FROM ISLETA DIVERSION DAM TO ELEPHANT BUTTE RESERVIOR, NEW MEXICO

We prepared detailed 1:24,000-scale surficial geologic maps of the inner Middle Rio Grande Valley (MRGV) floodplain. The map area extends from the Isleta Diversion Dam south to the upstream high-water elevation of Elephant Butte Reservoir (about 140 km). We delineated historical and modern surficial geologic units and characterized associated vegetation patterns, using 1935 and 2001 aerial photography. The maps depict the floodplain landforms and deposits including: point bars, crevasse splays, meander scrolls, paleo-channels, alluvial fans, artificial fill (levees). This mapping provides detailed geologic data along the central part of the Rio Grande, and provides a valuable baseline of information on the late Holocene geologic conditions in the inner MRGV.

We also prepared GIS layers and databases for digital analysis of fluvial changes in the MRGV during the time period from 1935 to 2001. Comparing the distribution and extent of the surficial geologic deposits from the two time windows enables: (1) documentation of changes that have occurred to the fluvial system over a span of 65 years, and (2) characterization of fluvial processes that operate within the river system. From initial analysis of these maps, we conclude that between 1935 and 2001 the active channel of the Rio Grande became substantially narrower and straighter, as concluded previously by other investigators. In addition, the pre-1935 paleo-channels preserved locally throughout the inner valley are much more sinuous than both the 1935 and 2001 channels, and that the floodplain associated with these channels is dominated by crevasse splay deposits. The floodplain directly associated with the 1935 channel is characterized by point bars, in-stream bars, and local scroll bars on the insides of substantial channel bends. In contrast, the floodplain associated with the 2001 Rio Grande channel is dominated by local point bars and small in-stream bars. Thus, the surficial geologic maps help document the characteristics of the Rio Grande prior to substantial engineering modification, and help demonstrate how the Rio Grande and its tributaries responded in time and space to land-use changes, hydrologic changes, channel modification, and/or the presence of engineered structures.