The Rio Puerco project is an interdisciplinary, interdivisional study of the long-and short-term history of arroyo incision in the Rio Puerco basin of New Mexico. This basin produces more sediment than any other basin of its size in the United States. At its confluence with the Rio Grande, the Rio Puerco contributes less than 20% of the water and more than 80% of the sediment, thus having significant impact on downstream water quality and on the Rio Grande's riparian ecosystem.

The Rio Puerco has been studied extensively since the 1920's. Early investigations discovered that intense, basin-wide erosion began in the 1870's or 1880's. The first hypothesis for the cause (or initiator), of intense erosion was overgrazing following the introduction of cattle in the 1860's and 1870's. By the 1930's, scientists realized that high erosion rates began almost simultaneously in many basins of the Southwest and that Spanish settlement of the area in the 1500's had introduced large numbers of sheep, which provided an alternate and earlier source of overgrazing. The balance of scientific opinion shifted toward a hypothesis of regional climate change as the initiator of erosion. The exact nature of the climate change, however, was unknown and controversial; no definitive climate data are available to prove or disprove any proposed mechanism. In the 1970's scientists began to think that the Rio Puerco and other Southwestern basins undergo cycles of erosion, and that modest external influences (e.g., minor regional climate variations) could pseudo-synchronize these cycles.

Various aspects of Rio Puerco hydrology, sediment transport, and arroyo configuration were studied and documented periodically from the 1930's until the 1990's. During this period, the rate of sediment delivery to the Rio Grande diminished substantially and many of the arroyos in the lower basin began to fill and to colonize with vegetation. In the upper part of the basin, erosion and arroyo incision continue in the 1990's.

USGS is taking a new research approach, which has three major elements: (1) improve the chronology of sediment removal and deposition throughout the system, with emphasis on the major arroyos; (2) measure the rate of sediment movement in instrumented catchments within the basin; and (3) develop extensive geographic information and use it for analysis of erosion pattern and process. Task 3 is the responsibility of the Rocky Mountain Mapping Center. RMMC is also responsible for placing information from all tasks on the Web (see http://climchange.cr.usgs.gov/rio_puerco/ where geographic data from the project may be downloaded).

Three geographic analysis activities are under way:

• Development of precipitation fields for summer-monsoon thunderstorms. This work incorporates satellite weather observations; NEXRAD doppler radar observations; regional radiosonde observations of winds, temperatures, and humidity; and regional modeling. The work is being done in the Department of Atmospheric Sciences at Colorado State University.
• Modeling of water movement across hill slopes and into stream channels. This work is being done by R. Watts at RMMC and at Colorado State University's Cooperative Institute for Research in the Atmosphere.
• Modeling of the movement of water and sediment in the channel system. This work is being done in the Department of Civil Engineering at Colorado State University.

The project is expected to produce a clearer understanding of conditions and variations of conditions that are conducive to extensive erosion. This information will help land managers to understand when their rangeland and riparian resources are at risk, what changes they may expect from various climate and management scenarios, and what their management options may be.

Project Lead: Ray Watts