A qualitative investigation of the spatio temporal development of the river morphology of the Rio Grande in White Rock Canyon, New Mexico (United States) as a result of the construction of Cochiti Dam

Oberhofer, Petra

The objective of the study is to show the response of the Rio Grande upstream of Cochiti Dam (in White Rock Canyon) due to its closure in 1973. In order to assess the adjustment of the river's morphology and to differentiate between natural (due to water and sediment discharge) and human induced (the closure of Cochiti Dam) change, surveyed the channel morphology between 1935 and 1991. The change of the fluvial morphology is documented using the longitudinal profile of the channel, its cross section and planform, and sediment data. The approach is based on the assumption that the fluvial system is constantly attempting to establish an equilibrium. Rivers are three-dimensional, and the elements of the channel geometry, i.e. planform, profile, and cross section, are interdependent. The channel geometry is also dependent on water and sediment discharge,which a redetermined by the parameters in the drainage area (climate, topography, geology and sediment, vegetation. and land use). Therefore, the parameters and dynamics of the drainage area were analyzed, as well as of the study area in White Rock Canyon. Furthermore, analysis was done on the elements directly influencing the channel morphology in White Rock Canyon: sediment and water discharge, and the water level in Cochiti Lake. The core of the study is the interpretation of aerial photos from 1935, 1951, 1963, 1975, 1982, 1986, and 1991, and the following transformation of the maps in a GIS (Geographical Information System). First, the planform of the Rio Grande was manually mapped on mylar using an APY (Analytical Photogrammeter by Yzerman). The orthophoto maps of the US Army Corps of Engineers, Albuquerque District 1981/1986 (scale 1:4800) were used as base maps (plates 6,7, and 8) in order to establish a georeferencing system (New Mexico State Plane Coordinate System, Central Zone, Transverse Mercator Projection, 2,000 ft-grid) for the maps drawn from the aerial photographs and showing the planform of the Rio Grande.