In northern New Mexico, the environmental plutonium bound to sedimentary particles is the most mobile in river systems, particularly the Rio Grande. This chapter describes the physical characteristics of the drainage basin into which Los Alamos National Laboratory has released plutonium. I review those characteristics of the basin that most strongly influence the movement of sediment and its associated plutonium: landforms, geology and soils, climate, vegetation, and precipitation. Precipitation and elevation provide the energy that is the primary driving force behind river processes in the Northern Rio Grande Basin. The geographic variation in stream flow and the temporal characteristics of its magnitude and frequency explain how water, sediment, and contaminants such as plutonium move through the system. An accurate accounting of stream flow is therefore essential to the development of a basinwide budget for water, sediment, and contaminants. Calculations for the mechanics of sediment transport (and the transport of associated contaminants) thus depend on measurements of stream flow from a variety of places within the system. In this chapter I examine the basic data for stream flow in the basin and then define and explain the temporal and geographical variation in the system’s river flows. The result is a regional stream-flow budget. The portion of the Northern Rio Grande emphasized in this book consists of the watershed upstream from the U.S. Geological Survey stream gage on the Rio Grande at San Marcial, at the headwaters of Elephant Butte Reservoir. The drainage network in this 71,700-sq-km area is the principal mechanism for the surface transport and storage of plutonium. The Rio Grande begins as a trickle of meltwater from a semipermenant snowbank at Stoney Pass in the San Juan Mountains in southwestern Colorado. Steep mountain tributaries are the primary sources of water, joining the main stem as it trends southeastward to the San Luis Valley and the Alamosa, Colorado, area. Additional mountain waters from the Rio Conejos, which drains the southern San Juan Mountains in southern Colorado, join the main stream as it flows southward into New Mexico. The northern Sangre de Cristo Mountains in Colorado generate surface runoff, but relatively little reaches the main river.
Observed Changes in Climate and Streamflow in the Upper Rio Grande Basin
