Lucas, S.G., Morgan, G.S. and Zeigler, K.E., eds., 2005, New Mexico’s Ice Ages, New Mexico Museum of Natural History and Science Bulletin No. 28. 115 RIVER RESPONSES TO ICE AGE (QUATERNARY) CLIMATES IN NEW MEXICO FRANK J. PAZZAGLIA Department of Earth and Environmental Science, Lehigh University, 31 Williams, Bethlehem, PA 18015, [email protected] Abstract–Rivers shape landscapes by responding to a range of external forces including climate and climate change. This paper examines the general role that Quaternary climates play in river channel form and process and how rivers in New Mexico have responded specifically to Pleistocene and Holocene climatic changes. The impact of Quaternary climates on New Mexican streams is cast in the context of how channel form and process has evolved through the Phanerozoic and in light of a large and growing data set for measurable changes in channel form and watershed hydrology over human time scales. Changes in channel form and process are commonly equivocal; there is no single unique response that follows from a given change in climate. Neverthe- less, geomorphologists have developed process-response models that limit the range of expected responses and help guide an interpretation of how Quaternary climates ultimately have shaped the New Mexican landscape. Long-term landscape evolution trends like the rate of valley incision and human-scale trends like the arroyo incision-aggradation cycle are discussed in the context of Quaternary climates. The general conclusion is that climate change through the Quaternary has had a major impact on pacing the rate of valley incision and defining the geomorphic thresholds that control arroyo incision, but that other factors, including tectonics and humans, also contribute. Climate and climate change remains, however, the most important factor in river form and process over Quaternary time scales. Leopold et al., 1966), and John Miller’s treatises on small mountainous INTRODUCTION channels (Miller, 1958; Miller and Wendorf, 1958). More recent studies, New Mexico has enjoyed a long history of process fluvial geo- including Wells et al. (1982), Love and Young (1983), Gellis (1998), and morphology research consistent with the close link between water as a Clapp et al. (2001), have expanded our understanding of river behavior, resource, and the cultural and economic development of the state. Some particularly along the lines of quantifying sediment yields over decadal of the earliest records of humans in the state, indeed in all of North time scales. These and other studies stress the importance of climate and America, are the Clovis and Folsom archeological sites, both of which are climate change in shaping channel form and function, but they also ac- set in the context of river valleys. Later, Anasazi and Pueblo cultures knowledge that anthropogenic factors such as agriculture and grazing managed rivers for their water resources in constructing impressive cities have played a role in historic river changes. Unfortunately, the precise in an otherwise arid and hostile environment. Spanish colonization of role of anthropogenic activities has proven difficult to quantify, leaving the state was guided by the course of the Rio Grande del Norte (Rio natural river changes throughout the Quaternary as the best analogue for Grande), and the subsequent rise of large cities and modern agriculture contemporary river behavior or any predictions of future behavior, espe- under American statehood again was shaped by rivers. Over the time cially in the context of global warming. scales that humans use and modify water resources and the landscape, This paper will first explore Quaternary rivers in the context of New Mexico’s rivers have changed. That change has had a direct eco- river form and process interpreted from New Mexico’s tectonic setting nomic impact, both positive and negative, and helps drive continued and the Phanerozoic rock record. It then will introduce some general research in the form and processes of New Mexico’s streams. Over long, concepts of river forms and processes and explain the prevailing models geologic time scales, New Mexico’s rivers have also changed, and the for how geomorphologists envision channels responding to climate change. magnitude of that change far exceeds what has been historically recorded Data on long-term and short-term changes for several major New Mexi- over the past 500 years. The location, size, shape, pattern, and discharge can rivers are presented and compared. These observations are used to of New Mexico’s rivers ultimately arise from the tectonic setting of the draw some general conclusions on the pace of New Mexican landscape state (King, 1965). Although an interesting topic unto itself, the rela- evolution, the effective sensitivity of New Mexican rivers to climate tionship between tectonics and river form and function will not be con- change, and predicted behavior of New Mexican rivers in the future. sidered in this paper. Rather, this paper will explore climate as the NEW MEXICO’S RIVERS proximate cause for changes in river pattern form and function over time periods ranging from recorded human history (100-102 years) to the The relatively arid lands of New Mexico are drained by large Quaternary (105-106 years), which includes the last 10,000 years of streams that owe their existence to headwaters in lofty mountains, and geologic time called the Holocene. Using the well known and dramatic their courses to the general tectonic setting. New Mexico lies at the climatic shifts that are the hallmark of the Quaternary, geomorphologists juxtaposition of five major physiographic provinces, the Southern Rocky have built an understanding of how climate influences river form and Mountains in the north, the Colorado Plateau in the west, the High function. This understanding is key to contemporary efforts to manage Plains in the east, the Basin and Range in the south, and the Rio Grande resources of a semi-arid land under ever-increasing pressures of a grow- rift, extending south to north, nearly bisecting the state into two equal ing population. parts (Fig. 1). The Rio Grande, which heads in the San Juan Moun- The behavior of New Mexican rivers during the Quaternary has tains of southwestern Colorado, is the axial drainage of the rift. It has been well documented and is often cited as being representative of arid a few important tributaries that head on the rift flanks, including the and semi arid lands world-wide. Particularly important process stud- Red River, Rio Chama, and Rio Puerco. Otherwise, most of the drain- ies include Kirk Bryan’s work in the Rio Grande rift (Bryan, 1925; age that heads on the rift flank flows away from the rift and across 1926; 1938; 1940), Luna Leopold’s research on arroyo development either the High Plains, like the Canadian and Pecos Rivers, or the Colo- and evolution (Leopold, 1951a,b; 1976; Leopold and Miller, 1956; rado Plateau, like the San Juan, Chaco, and Puerco (of the west) rivers. 116 FIGURE 1. Color shaded relief map of New Mexico showing the major rivers of New Mexico, including those discussed in this paper. JR=Jemez River, AH=Arroyo Hondo, RG = Rio Guadalupe, RR=Red River, RC=Rio Costilla. A=Alamogordo, ABQ=Albuquerque, C=Clovis, CB=Carlsbad, F=Farmington, G=Gallup, LC=Las Cruces, R=Roswell, SC=Silver City, SF=Santa Fe, SR=Santa Rosa, T=Taos. Most of the above mentioned rivers maintain perennial flows with highly WHAT IS THE QUATERNARY FROM THE PERSPECTIVE OF variable amounts of sediment discharge. The Rio Puerco (rift drainage A RIVER? tributary to the Rio Grande), in particular, is often noted for producing some of the highest measured suspended sediment yields of any stream Rivers and fluvial processes play the most important role in set- globally (Nordin, 1963). With the exception of the above mentioned ting the pace of landscape evolution. Geomorphological interpretations streams, and those on Figure 1, most of New Mexico’s rivers are ephem- of river form and process and the resulting influence on the landscape eral, some flowing seasonally and others flowing only during infrequent, evolution is colored by the fact that the Earth system currently resides in large magnitude precipitation events. a global ice house with permanent ice at the poles. This climate is Tectonics have imprinted the general drainage pattern of New characterized by large latitudinal shifts in climate. Quaternary climates, Mexico’s streams on the landscape, but it has been climate that has which appear to be an expression of the normal condition of the Earth shaped the size and form of their channels. Streams draining the High system from the human perspective, are in fact rare events when viewed Plains and Colorado Plateau have ancestries reaching back to the Laramide over the entire Phanerozoic timescale (Fig. 2). The fluvial forms and orogeny (40-80 Ma). The Laramide drainage was disrupted by the processes we observe today, and their response to climate, may be poorly development of the rift and the subsequent growth of the Rio Grande as represented in the rock record of fluvial deposits. its master, axial stream. Climate has modulated the discharge of the Rio Fluvial and fluvial-deltaic deposits are well represented in the Grande, particularly in the Quaternary, and has played a role in the sedimentary rock record of the Phanerozoic (Fig. 2). Similar deposits are hydrologic integration of tectonically distinct basins in the rift. Al- known from the Precambrian, but the near total absence of biostratigra- though its inception is coincident with the beginnings of rifting in the late phy limits paleoecological and specific depositional environment inter- Oligocene-early Miocene, the Rio Grande became a complete, free-flow- pretations. In contrast, Phanerozoic fluvial and fluvial-delatic deposits ing, integrated drainage, reaching the Gulf on Mexico only within the last are rich in biostratigraphic and paleoecologic data, and from these geolo- 500-700 k.y.