RioGrandeWaterSupplyandNeedsReport

• Basin Roundtable: Rio Grande
• Title: Water Supply & Needs Report for the Rio Grand Basin
• Date: 6/1/2006
• Author: CDM
• Basin Roundtables - Doc Type: Needs Assessment Documents

Section 3
Physical Environment of the Rio Grande Basin
IJ 1111

Range, and agriculture in the eastern plains and
elsewhere can impact the quality of the state's waters
and aquatic habitats. Habitat degradation, nutrient
loading, soil erosion, and increased stormwater runoff
are only a few examples of the concerns associated with
rapid urbanization, particularly in the mountain
recreational areas (Colorado Department of Public
Health and Environment [CDPHE] 2000).

Improving water quality and restoration and protection of
water bodies in Colorado is occurring through programs
such as the Total Maximum Daily Load (TMDL) process,
Gold Medal fisheries establishment, instream flow
programs, and federal and state listed threatened,
endangered, and species of special concern.

3.2 Rio Grande Basin Physical

Environment

3.2.1 Geography

The Colorado portion of the Rio Grande Basin, located in
south central Colorado, encompasses approximately
7,543 square miles, as shown in Figure 3-2. The largest
cities or towns in the basin are Alamosa (population
8,248) and Monte Vista (population 4,542) (DOLA 2003).

3.2.2 Climate

The San Luis Valley is a high mountain desert with cool
summers and cold winters. The majority of the
precipitation occurs as scattered summer afternoon
showers or winter snow (CGS 2003). Figure 3-3 shows
the mean annual precipitation over the Rio Grande
Basin. Average annual precipitation in the central part of
the Rio Grande Basin ranges from 6 to 9 inches.
Precipitation in the mountains is considerably greater.
For example, Wolf Creek Pass, located southwest of
South Fork, receives 49 to 56 inches of precipitation
annually (National Resources Conservation Services
[NRCS] 1999). Due to low humidity, abundant sunshine,
and warm temperatures, the average annual evaporation
rate often exceeds precipitation, ranging from 35 to
48 inches per year (National Oceanic and Atmospheric
Administration [NOAA] 1982).

CDIVI

3-2

3.2.3 Topography

The San Juan Mountains in the west, the Sangre de
Cristo Range in the northeast, the Culebra Range in the
southeast, and the Colorado-New Mexico state line in the
south define the Colorado portion of the Rio Grande
Basin. The San Luis Valley, a primary feature of the Rio
Grande Basin, extends from the foothills of the San Juan
Mountains eastward to the foothills of the Sangre de
Cristo range and has an average elevation of about
7,500 feet.

3.2.4 Land Use

Figure 3-4 (U.S. Geological Survey [USGS] 1992) shows
the land use in the Rio Grande Basin, while Table 3-1
summarizes the data. The majority of the San Luis Valley
is privately owned and the greater than 600,000 acres of
irrigated land is primarily used for agricultural operations
in the central portion of the basin. Areas in the valley that
are not irrigated are mostly classified as shrubland (24
percent) and grassland (31 percent). The San Juan and
the Sangre de Cristo mountain ranges are largely
forested.

Table 3.1 land Cover Data for the Rio Grande Basin

Basinwide

Statewide

Grassland
Forest
Shrubland
Planted/
Cultivated
Barren 158
Wetland 41
Open Water 35
Developed 14
TOTAL 7,543

Source: USGS 1992 NLCD

2.10/0
0.50/0
0.50/0
0.20/0

1.20/0
0.080/0
0.60/0
0.90/0

3.2.5 Surface Geology

Rocks of various geologic ages are exposed throughout
the Rio Grande Basin. Precambrian age crystalline rocks
such as granites, gneisses, and schists are found in the
Sangre de Cristo Mountains. A section of Paleozoic age
sedimentary rocks are present and are exposed along
the north and east side of the San Luis Valley. These
rocks have been divided into the Manitou Limestone,
Harding Sandstone, Fremont Limestone, Chaffee
Formation, Kerber Formation, and Minturn Formation
(Pearl 1980).

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