The topography of the basin reflects the differential resistance of rock units to weathering and erosion. The highest parts of the basin drain Precambrian Granite of the Nacimiento range and basic volcanics of the Mt. Taylor Complex. Intermediate elevation mesas are on Mesozoic sandstones (e.g. Point Lookout Fm.) and the lowest areas are on Mesozoic shales (e.g. Mancos Fm.).

The distribution of soils and vegetation is also influenced strongly by topography and geology. This Normalized Density Vegetation Index (NDVI) image from July 5, 1989, represents a pre-monsoon period in a dry year. The colors going from red-yellow-blue-green indicate increasing chlorophyll content. Areas of little or no chlorophyll are displayed as dark gray or black. We are using such images to compare wet and dry seasons in wet and dry years. The average rainfall in the basin varies annually between about 12 to 20 inches, and is delivered mostly by the summer monsoon. Comparisons of images between very wet and very dry intervals show many parts of the basin are very responsive to seasonal variations in precipitation. However, large areas, particularly in the Arroyo Chico drainage (to the north of Mt. Taylor), show little vegetation change with annual or seasonal precipitation variation. This information will be used to assess erosion potential. Natural vs. human controls on vegetation distribution are important to assessing impacts of grazing and other landuse practices on erosion and the overall Puerco sediment budget.

U.S. Department of the Interior
U.S. Geological Survey
This page is http://esp.cr.usgs.gov/rio_puerco/intro/overview.html
Maintained by Richard Pelltier
Last modified: 14:42:17 on 15-Mar-2006