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The Middle Rio Grande: Its Ecology and Management

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The Middle Rio Grande: Its Ecology and Management The Middle Rio Grande: Its ecology and management Jeffery C. Whitney’ Abstract.-The Middle Rio Grande (MRG) riparian forest, or repre- sents the largest cottonwood gallery riparian forest in the southwestern United States. This reach of the Rio Grande extends from Cochiti Dam downstream 260 Km to San Marcial, New Mexico. It constitutes 8% of the river’s total length and 34% of if its length in New Mexico. The valley traverses three major biotic communities, as defined by Brown and Lowe (1980). The MRG reach can be subdivided into 4 reaches which coincide roughly with the 4 geologic basins or “grabens” along this portion of the Rio Grande Rift. This system has been affected by man’s activities throughout prehistoric and modern eras. The Rio Grande is regulated for water supply (primarily irriga- tion) and flood control. The effects of this interaction have contributed to the character of the riparian ecosystem in its current expressron. Over 40% of New Mexico’s population lives within the MRG reach. This paper will discuss the climate, geology, hydrology, subsequent river and factors which contribute to the past and current expressions of the riparian habitat associated with the Middle Rio Grande. INTRODUCTION changed Rio Grande ecosystems by altering flood cycles, channel geomorphology, processes, The Rio Grande is one of the longest rivers in and water quality and quantity. Such North America (1900 miles). The Rio originates in changes have influenced the biological diversity the southern Rocky Mountains of Colorado, flows and ecological functions of the MRG, altering the the whole length of New Mexico and forms the distribution, structure, and composition of riparian entire border between the state of Texas and the plant and animal communities. Republic of Mexico (fig. The Rio is the greatest The Rio Grande basin above El Paso, Texas, is source of permanent water in the desert southwest one of the oldest regions of agriculture in the other than the Colorado River. It is home to the United States. Agricultural activity extends back largest cottonwood forest in North America, centuries to prehistoric inhabitants of the Rio locally referred to as the “Bosque”. Grande valley (fig. 2.) and includes the seven- Human populations have increased dramatically teenth and eighteenth century Pueblo Indians and along the Rio Grande since European settlement. Spanish colonists, and European-Americans in the Human use of water for irrigation and consump- latter part of the nineteenth century (Wozniak tion, and human use of land for agriculture, urban 1987). More recent history of the region involves centers, livestock grazing and recreation have disputes and concerns over management, irriga- tion, and distribution and delivery of upstream Middle Rio Grande Coordinator, U.S. Fish and Wildlife Service, waters to downstream users in an attempt at fair Albuquerque, NM. sharing between concerned parties. Because of the 4 long history of agricultural activity, Rio Grande Colorado. Above Velarde the drainage basin area water is tied to public laws governing its convey- is about 27,325 sq. km., including the Closed Basin. ance, storage, and use. The close connection be- The Rio Chama, one of the most important tribu- tween legislation and flow of water through the taries in the study area, has its headwaters in the Rio Grande is largely responsible for the present Jemez, Conejos, and San Juan Mountains of New physical state of the river, floodplain, and associ- Mexico and Colorado. The MRG extends from ated riparian community. Changes in the flood- Cochiti Dam downstream 260 river km (160 mi) to plain ecology probably began shortly after human San Marcia1 (fig. The MRG constitutes 8% the settlement in the region, and change has continued River’s total length and 34% of its length in New relatively unabated with increasing population. Mexico. The middle valley’s direct drainage ac- (Bullard and Wells 1992). counts for 7% of the total Rio Grande drainage and about half of New Mexico’s direct drainage. LOCATION AND GEOLOGY The MRG is part of the larger Rio Grande system (fig. 1). The Rio Grande headwaters lie Hydrologic characteristics of the Rio Grande along the Continental Divide at elevations ranging Basin, such as infiltration, runoff, and sediment from 2,440 m to 3,660 m in the San Juan Mountains discharge, are dependent on the geology, geomor- of southern Colorado. The entire area of the Rio phic evolution of tributary basins and late Tertiary Grande drainage basin is about 470,000 sq. km of and Quaternary geologic and climatic history. which about sq. km. are in the United States Structural geology (such as faults and folds) of a and the remainder in Mexico (Hunt, 1974). The region governs spatial and geometric relations of river flows south from Colorado through the rock in that region. Geologic structures and length of New Mexico and then forms the interna- lithology influence the development of topographic tional boundary between Texas and the Republic features, river and tributary position, and landscape of Mexico. The drainage basin area above Elephant evolution. Tectonic activity can produce measur- Butte is about 76,275 sq. km., including 7,615 sq. able effects on channel and sediment transport km. in the Closed Basin of the San Luis Valley in characteristics 1983, 1985; Schumm 1986). Figure 1. Middle Rio Grande study area (from Bullard and Wells, 1992). 5 3 8 C a 154016001700 1800165018801896 1925 1960 1970 1980 1990 YEAR Figure 2. Historical account of acres of land under cultivation in the Middle Rio Grande Valley (acres/ (From Crawford et al, 1993). The location of the Rio Grande is controlled by the dominant geologic structure of the region, the Rio Grande Rift. The Rio Grande Rift is a linear topographic feature that separates the Great Plains Figure 3. Setting and institutional boundaries in the from the Colorado Plateau (Hawley 1978) moun- Middle Rio Grande (from Crawford et al, 1993). tain ranges, which can influence weather patterns, are a direct result of geologic processes. The rift, which in turn influence infiltration and runoff, active for at least 18 million years (Wilkins is landscape stability, soil development, and sedi- characterized by extension, seismicity, local tec- ment supply. Soil development is important tonic uplift, and volcanism et al. 1991). because natural, progressive changes in physical The location of early trade routes was influenced properties of soils occurring through time alter the by the spatial arrangement of mountain ranges nature of the land surface, including vegetation that were natural barriers to travelers. Indigenous communities, infiltration (decreases with increas- populations and early settlers in the region sought ing age), erosion, and runoff and discharge. areas of suitable climate, access, and availability of water. Thus, the presence of the Rio Grande Rift has influenced human settlement patterns in the REGIONS region. The extent and type of bedrock can influence The Rio Grande basin lies in five physiographic infiltration and runoff characteristics. These factors provinces: the Coastal Plain, the Great Plains, the can dramatically influence tributary basin evolu- Basin and Range, the Colorado Plateau, and the tion, discharge characteristics, main stem flow, and Southern Rocky Mountains (Hunt 1974). The MRG main stem evolution and integration (Leopold et and its tributaries are located within the latter al. 1964; Schumm 1977; Richards 1982; Kelson 1986; three provinces. From about Santa Fe southward, Wells et al. 1987, Bullard and Wells 1992). Bedrock the rift is in the Basin and Range Physiographic type influences vegetation types and densities, Province which separates the Colorado Plateau 6 Province to the west from the Great plains Prov- mid-October, lasting on average 160 days. In ince to the east. (Crawford et al. 1993). Socorro, the average period is 197 days, beginning The MRG valley is actually a series of basins. in Mid-May and lasting through late October. These (depressions) formed a series of The Rio Grande drainage basin is located in a linked, but slightly offset, depositional basins, each transitional climatic zone between the Gulf of of which contained its own ephemeral lake. Over Mexico and the Pacific rainfall provinces. Complex time, the surface water eroded canyons between meteorological conditions exist in this region, and the intervening bedrock sills that defined the these conditions are further complicated by the basins, integrating the area into the Rio Grande orographic influence of surrounding mountain river system and Wells 1992). The ranges and global circulation patterns. through-flowing ancestral Rio Grande drainage The MRG basin has an arid to semiarid climate developed into a single river about 5 million years typical of the southwestern United States. The ago (Lozinski et al. 1991). The basins in the Middle climate is characterized by abundant sunshine, low Rio Grande are: relative humidity, light precipitation, and wide diurnal temperature fluctuations. The average Santo Domingo Basin annual precipitation varies from 178 MM (7 in.) to White Rock Canyon to San Felipe 380 mm (15.25 in.) over two-thirds of the basin and may exceed 635 mm (25 in.) only in the high Albuquerque Basin mountain areas. Winters are generally dry, and San Felipe to snow rarely remains on the ground at low eleva- tions for more than 24 h. Snowfall in the high Basin mountains composes of the total annual precipitation; in the remainder of the basin snow- to San Acacia fall composes less than 25% of the annual precipi- Socorro Basin tation. Summer precipitation supplies almost half of the annual moisture. Most of the rain falls in San Acacia to San Marcia1 brief, though sometimes intense, convective thun- derstorms (fig. 4). These summer thunderstorms have a considerable moderating effect on daytime CLIMATE temperatures. Prevailing winds are from the southwest and typically are continuous during the The hydrology and morphology of the Rio spring months.
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