Quaternary) Climates in New Mexico

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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. (Pazzaglia and Hawley, 2004). gists have pieced together a general story of how fluvial form and process 117

FIGURE 2. Geologic time scale illustrating major changes in river form and process through the Phanerozoic. has changed over the past 543 m.y. There are at least four major innovations in the history of life that have had a profound impact on fluvial form and process: (1) the colonization of the land by land plants, (2) the development of true, ecologi- cally diverse forests with a tall canopy and understory, (3) the evolution and subsequent spread of grasses, and (4) the local impact of channel modifying animals like beavers or humans. Lower Paleozoic fluvial deposits are almost exclusively braided channel and braided alluvial plain facies. Channel forms tend to be wide and shallow and characterized by transverse bars. Floodplain facies are rare in these braidplains, but where preserved, tend to lack extensive pedogenic modification. By the Late Devonian, however, fluvial deposits begin to diversify with the narrow- ing and deepening of channels, especially in the fluvial-deltaic setting, and the increase in the number of preserved floodplain facies. The Old Red Sandstone/Catskill Delta of the Caledonides and Appalachians, respectively, are good examples of a Late Devonian fluvial deposit where the channel width to depth ratio is decreased with respect to lower Paleozoic channels, presumably by plant root channel bank stabilization. Low and middle latitude Carboniferous coal swamps increase the frequency of root stabilization of channel banks and floodplain expan- sion begun in the Late Devonian. Dense forest cover in the Carbonifer- ous enhanced pedogenesis, resulting in the greater production of fines that were ultimately vertically accreted on floodplains and continued to enhance channel bank stability. Carboniferous fluvial deposits are the first to exhibit widespread meandering as well as braided channel facies including clear epsilon cross bedding consistent with point bar deposition. Meandering channels began to dominate the sedimentological record by the middle Tertiary as grasses, which had evolved by the late Eocene, grew to global distributions. Grasses provide the most effective means of stabilizing channel banks, further reducing the channel width to depth. FIGURE 3. Photographs of (a) a meandering channel, Rio San Antonio, Valles Grasses also have a wide distribution in arid lands that typically receive Caldera, (b) the point bar of a meandering channel, Rio Puerco, Cuba, NM (c) seasonal, intense precipitation. Channel bank stabilization and the en- braided-anastomosing channel(s), oblique airphoto of the Sandia Mountains piedmont, and (d) a bedrock channel and knickpoint, Rio Frijoles. 118 hanced infiltration capabilities of grasses foster steady, rather than flashy, discharges, another characteristic of meandering, rather than braided channels. Lastly, recruitment of large woody debris into channels, something that is typically accomplished during large floods, has been enhanced by various animals including beaver, to name a Cenozoic example. Large woody debris is known to foster a step-pool and pool-riffle channel morphology. THE SHAPES OF NEW MEXICO’S STREAMS General Channel Morphology New Mexico’s rivers exhibit four major channel morphologies, each consistent with its geologic and hydrologic setting (Fig. 3). Most large, trunk streams such as the Rio Grande and Pecos Rivers have alluvial braided channels. Human restrictions like levees and dams have worked to narrow and deepen what were otherwise broad, shallow braided channels. Braided channels are characterized by unstable channel banks, a high proportion of bedload to suspended load, and flashy discharges, both on a seasonal and individual storm basis. Alluvial channels in New Mexico with a low proportion of bedload to suspended load, a high percentage of fines, stable channel banks, and relatively stable discharges tend to have more narrow, deep and meandering patterns. The Rio Puerco is a good example of a meandering channel entrenched into its own alluvium, where channel bank stability is provided by the high percentage of fines. The upper reaches of the Jemez River is a good example of a meandering channel in an upland, moister setting where channel bank stability is provided by meadow grasses. Mountain channels are both alluvial and bedrock in nature and exhibit pool-riffle and step-pool morphologies well cited in the recent geomorphological literature (Miller, 1958; Wohl and Merritts, 2001). The specific hydraulic geometry of these channels is well documented for streams of the Sangre de Cristo Mountains (Miller, 1958). Channels that are deeply incised into bedrock have their form FIGURE 4. (a) Plot of stream discharge as a function of drainage area for New and function shaped by the relative hardness of the substrate. These Mexican streams (semi arid climate) vs. streams in western Washington state (humid channels tend to be steep and have numerous knickpoints related to temperate climate). (b) Plot of stream channel width as a function of watershed area changes in rock hardness. The lower reaches of small streams that are (modified from Mitchell, 1999). Solid triangles indicate bankfull channel widths, tributary to the Rio Grande such as the Rio Frijoles, Arroyo Hondo, or gray circles indicate base flow channel widths. The b values refer to the slope of the Red River are good examples of bedrock channels. The Rio Grande regression lines, which is the power dependence in the equation Width = k(Drainage itself has several predominantly bedrock reaches such as the Taos gorge Area)b. and White Rock Canyon. Significant, incised bedrock reaches occur on other major New Mexico rivers including the San Juan, Chaco, Ca- homogeneous, presumably relatively unfractured substrates like granite nadian, Pecos, San Francisco, and Gila. tend to generate proportionally more discharge per unit area than compa- Watershed-Channel Hydraulics rable areas underlain by softer, more heterogeneous and presumably relatively more fractured sedimentary rocks (Kelson and Wells, 1989). Rivers respond to climate because climate has a first-order ef- Streams draining granitic watersheds tend to have more peaked discharges fect on the amount of water and sediment that gets delivered to the that generate more erosive power and bedrock incision with respect to channel. Furthermore, climate dictates whether the water and sedi- larger watersheds underlain by sedimentary rocks. For a large popula- ment are delivered steadily on an annual basis, or with a large variation tion of gaged watersheds in New Mexico, discharge tends to scale to on a seasonal basis. In order to understand the observed effects of watershed area raised to the 0.5-0.7 power. In contrast, for a similar Quaternary climates on New Mexico’s channels detailed below, the population of similar-sized drainage basins on the west side of the Cas- scaling relationships between drainage basin area and discharge, and cades in the Pacific northwest, discharge scales linearly with drainage channel shape and discharge, must be understood. For small alluvial area (Mitchell, 1999). These results are significant from the perspective channels in the mountains of New Mexico, some of these hydraulic that modern spatial variations in effective precipitation stand as a proxy geometry relationships are well known (Miller, 1958). Similar data to the effects of temporal changes in Quaternary climate. Channel form exist for the numerous, small, ephemeral channels in the more arid and process under relatively arid conditions is represented by the current parts of the landscape (Leopold and Miller, 1956; Leopold et al., 1966). warm, arid, seasonal Holocene climate, and the cooler, wetter, less sea- Changes in channel form, process, and hydrology for perennial sonal Pleistocene climates would drive channel form and process to alluvial channels are less well known, but important from the stand- approximate the hydrologic conditions of wetter regions not currently point that they record the channel response to climate change. Rivers represented in New Mexico. that fall into this category include, but are not restricted to the Rio Jemez, Arroyo Hondo, the Red River, and Rio Costilla. These streams CLIMATE, CLIMATE CHANGE, AND RIVER RESPONSE were investigated by Kelson and Wells (1989) and Mitchell (1999; General Process-Response Model Fig. 4). Their findings indicate that the scaling between discharge and drainage basin area is dependent upon both rock type and climate. Hard, A general model for fluvial response to climate change follows 119

FIGURE 5. The Bull process-response model (modified from Bull, 1991). from decades of climatic geomorphology research on fluvial systems where the channel and deposits in the lower part of a watershed can be stratigraphically linked to glacial deposits in the upper part of a watershed (Fig. 5; Schumm, 1965; Bull, 1991). The model that has emerged does not only apply to only glaciated watersheds, but is fairly well- suited to describe a majority of New Mexico’s rivers whose headwaters are typically in lofty mountains that experienced glaciation in the Quaternary. All rivers incise their valleys at a rate driven by the rate of base level fall. In tectonically active settings, the base level fall is rapid, typically in the range of 1 to 5 mm/yr. By comparison, the rate of base level fall in New Mexico averages an order of magnitude slower (0.1 to 0.25 mm/yr; Pazzaglia and Hawley, 2004); nevertheless, at these rates, valleys hundreds of meters deep can be carved during the past 1– FIGURE 6. A process-linkage model (modified from Meyer et al., 1995). 2 m.y. of the Quaternary. Over 105 to 106 year time spans, the vertical incision rate is nearly steady, but over 103 to 104 year time spans the is known to vary across millennial time spans between relative warm, incision rate is unsteady. This unsteadiness and its effect on fluvial dry years when winter snowpacks are small and summer convective terrace genesis is illustrated in Fig. 5. The aggradation periods in Fig. storm frequency is large, and relatively wet years, when winter snow- 5 are stratigraphically linked upstream to glacial heads of outwash and packs are large and summer convective storms frequency is depressed. temporally linked to times of increased sediment and water discharge Large, stand-clearing fires tend to occur during the dry summers follow- to the channel. In contrast, the incision periods in Fig. 5 do not have an ing below normal winter snowpacks. The dry summers, along with the up basin glacial stratigraphic equivalent and are temporally linked to fires result in frequent debris flows moving sediment off of hillslopes times of decreased sediment and water discharge to the channel. Riv- where it accumulates on the valley flanks, atop the floodplain. During ers aggrade, depositing alluvium in their valleys during glacial climates these times the large channels tends to be relative narrow, braided, and because an overall steeper channel is required to carry the increased incising into their floodplain, processes fostered by flashy discharges sediment load. In converse, rivers incise, deepening their valleys dur- and lack of fine sediments (Fig. 6b). In contrast, the relative lack of fires ing interglacial climates, because less steep channels are needed to during wet summers results in fewer debris flows, so sediment is effec- transport the reduced sediment load. In this way, Quaternary climates tively kept on hillslopes. During these times, the large channels tend to have a profound impact on channel form and gradient. The frequent be relatively wide, meandering, and incising laterally into the alluvial fans aggradation-degradation cycles observed in river valleys globally is a formerly deposited on the valley flanks. These processes are favored by hallmark of Quaternary climates. Similar aggradation-degradation stable discharges and the delivery of fines that vertically accrete a flood- cycles may be less common for the rest of the Phanerozic, except for plain (Fig. 6a). Floodplain construction lags behind the initial liberation times like the Pennsylvanian when stratigraphy suggests numerous and of sediment from hillslopes by several thousand years. So the cut-and- frequent base level changes consistent with glacio-eustasy. fill cycles of the large channels, although driven by climate, are out of phase with the actual climatic change that liberated the sediment from the Process Linkage hillslopes in the first place. Similar process linkages and time lags are The connections between the processes that deliver sediment envisioned for glacial-interglacial climatic cycles of the Pleistocene that 5 from hillslopes and those that move sediment through the fluvial sys- play out over 10 , rather than the millennial time scales illustrated in this tem are called process linkage. Even though the climatic influence on example. the fluvial system has already been established, changes in sediment QUATERNARY CLIMATES AND RIVER DEPOSITS flux to the channel may not be in phase with climate changes because of the time lags associated with sediment storage on hillslopes. An Climate during the Quaternary is characterized by 105 year scale example of process linkage that plays out over Holocene (104 yrs) time waxing and waning of mean average temperature by ~10oC and the growth scales in the northern Rocky Mountains illustrates the connections be- and shrinkage of continental and mountain glaciers (Fig. 7). The first tween climate, watershed hydrology, the liberation of sediment from major expanse of a major Northern Hemisphere continental ice sheet is hillslopes, and response of the fluvial system (Meyer et al., 1995; Fig. typically taken as occurring 2.45 Ma. This event is recorded by 6). Climate in the northeastern portion of Yellowstone National Park dropstones in North Atlantic ocean sediments of this age, as well as a 120

FIGURE 7. The record of Quaternary climate change from stable isotope geochemistry of ocean bottom sediments (modified from various sources including Braun, 1989). Note break in time scale between 1.2 and 1.4 Ma. UBT = Upper Bandelier Tuff; LBT = Lower Bandelier Tuff. major increase in the amount of the heavier isotope of oxygen (18O) in that the fluvial system was strongly impacted by middle and late Pleis- ocean waters corresponding to an increase in global ice volume. The tocene glacial-interglacial cycles because tephras, erupted at opportune antiquity of this first major glacial-interglacial climate change makes it times, are interbedded with terraces. Two tephras with wide distribu- difficult, if not impossible to recognize in the New Mexican landscape. tion, the Lava Creek B (Yellowstone) and Bishop (Mammoth, CA) are It is possible that general fluvial aggradation such as the Tuerto or Ancha interbedded with fluvial deposits corresponding to marine oxygen iso- formations that characterized the rift flank through the Pliocene, may tope stages 16 and 20, respectively. The Lava Creek B tephra is particu- have ceased at about this time as streams incised following aggradation larly widespread throughout the Rocky Mountain region and speaks to (Koning et al., 2002). the broad effect that climate had on driving aggradation of fluvial systems The latest Pliocene and virtually all of the early Pleistocene is during the transition from glacial stage 16 climates to interglacial stage 15 characterized by frequent climate changes occurring on a 41 k.y. peri- climate conditions. Similarly, aggradation of the Rio Grande at the close odicity (Fig. 7). Deposition, incision, or cut and fill cycles are also of isotope stage 6, resulting in the deposition of the “segundo alto” difficult to identify during this time interval. Some fluvial systems, terrace and Los Duranes Formation (Lambert, 1968; Connell and Love, like the Rio Grande, were likely aggrading through this time. Other 2001) is known from interbedded lavas of the Albuquerque volcanoes systems, like the Rio Jemez, were incising, as there are no deposits in (156±29 ka; Peate et al., 1996). the Jemez valley between the base of the Upper Bandelier Tuff (1.2 Locally, rivers are known to aggrade, even during the climate changes Ma) and a terrace alluvium that contains the Lava Creek B tephra (0.64 associated with interstadial, rather than glacial-interglacial transitions. Ma). Rivers draining the Jemez Mountains, for example, typically have an Large amplitude glacial-interglacial cycles lasting approximately alluvial terrace containing the El Cajete tephra (Cochiti Canyon; Aby, 100 k.y. distinguish the middle and late Pleistocene from the late 1997) or Banco Bonito clasts (Rio Jemez; Formento-Trigilio and Pazzaglia, Pliocene and early Pleistocene. There have been at least ten major 1998), both of which are 60 k.y. old and mark the transition from isotope glacial-interglacial climate changes in the past 1 Ma. Glacial climates stage 4 to isotope stage 3. are generalized as relatively cooler and wetter, with a general depres- HOLOCENE CLIMATES sion of the tree line, and current low-standing arid to semiarid regions being covered by a more savanna-like vegetative assemblage (Spaulding The Holocene is the last 10 k.y. of the Quaternary and represents et al., 1983; Spaulding and Graumlich, 1986; Betancourt et al., 1990). just the latest of several major interglacial climate episodes that have Interglacial climates are generally warmer and drier, but the big differ- existed periodically over the past 1 m.y. Although Holocene climatic ence may be the greater intensity of precipitation and seasonality with variability has likely been less extreme than a Pleistocene glacial-intergla- respect to the glacial climates (Holliday, 1989, 1997, 2000). cial cycle, millennial scale, hemispheric, or global changes in mean annual Most of the major rivers in New Mexico, paced by the Rio Grande, temperature and precipitation are present in high resolution stratigraphic entered a phase of generally incising during the middle and late Pleis- records (Bond et al., 1997). In New Mexico, the record of playas, tocene. The record of that incision and Quaternary climate change is interspersed with widespread desiccation for basins such as the Estancia preserved deposits of paleo-valley bottoms called terraces along the basin demonstrates that there have been relatively wetter and dryer flanks of the river valleys. A terrace is both a landform and an periods each lasting several thousand years (Allen, 1993; Allen and Ander- allostratigraphic deposit composed of three main components, an ero- son, 1993, 2000; Allen and Hawley, 1991; Holliday, 1997). Leopold sional basal unconformity called a strath, a depositional top called a (1951b) was among the first to suggest precipitation intensity and sea- tread, and the intervening terrace alluvium (Fig. 8). Strath terraces are sonality as driving Holocene-scale cut and fill cycles in New Mexican characterized by thin terrace alluvium and planar or nearly planar basal fluvial systems. Such variations in rainfall intensity may be linked to unconformities. The thickness of the alluvium for strath terraces is hemispheric circulation patterns like ENSO (El-Niño Southern Oscilla- consistent with the prevailing scour depth of the channel for which it is tion; Gutzler, 2000), but such links remain to be definitively established. associated. In contrast, fill terraces are underlain by thick alluvium Complex responses to Holocene climate changes are the rule, rather consistent with a period of valley aggradation. The basal unconformities than the exception, making it difficult to link a specific climate to a of fill terraces are commonly irregular, consistent with the burial of specific channel response. A complex response is defined as a family of paleo-valley bottoms. related channel processes that cascade from a single stimulus (Schumm, For the western United States including New Mexico, we know 1973). For example, a Holocene increase in precipitation intensity might 121

FIGURE 9. Reconstructed longitudinal profiles and terrace stratigraphy of the Jemez River. The terrace longitudinal profiles and associated stratigraphic horizons are FIGURE 8. Photograph of a terrace deposit of the Canadian River. plotted with respect to the stream longitudinal profile. Inset shows long term rates of river incision. cause a major ephemeral channel to incise. That incision is seen as a base level fall for the tributary channels that also incise, delivering sediment back to the trunk channel. The increase in sediment delivery to the trunk and there are no data to link the braided or meandering facies to a specific channel overwhelms its ability to transport it, causing aggradation and climate; however, the general coincidence of dated terraces with the cooler, reversal of the initial climate-induced incision. In this way, channels wetter glacial stages of the oxygen isotope curve (Fig. 7) suggest that the draining small watersheds might incise due to an increase in precipitation observed variations in channel form and function likely follow from intensity, whereas larger trunk channels initially incise, but later aggrade. climatically-induced changes in watershed hydrology and delivery of Complex responses make the links between climate change and channel sediment from hillslopes as outlined above in the process response and response at millennial time scales equivocal. process linkage models. The age of Qt1 is known to be ~610 ka (glacial isotope stage 16) from reworked Lava Creek B tephra preserved in the TRENDS IN RIVER RESPONSE TO CLIMATE middle sandy facies (Rogers, 1996). Qt2 has been dated to approximately 500 ka (marine oxygen isotope stage 12) by travertine cemented Long Term; Terraces, Terrace Stratigraphy, and Incision gravels near Soda Dam (Goff and Shevenell, 1987) and Qt3 has a re- Several river valleys in New Mexico have been particularly good ported age of 315 – 180 ka (marine oxygen isotope stages 10 to 8) from recorders of Quaternary climate change through the preservation of an ESR (Electron Spin Resonance) date on travertine-cemented quartz flights of river terraces (see Pazzaglia and Hawley, 2004 for a compre- grains (Rogers et al., 1996). There are no independent ages for Qt4 but it hensive review). The Jemez River in north-central New Mexico that is known to be older than 60 ka, the numerically-determined age of the drains the southern flank of the Jemez Mountains (Fig. 1) is notewor- next youngest terrace. Both correlation to the oxygen isotope curve, as thy because of its wide distribution and vertical separation of numeri- well as calibrated amino acid racemization ages (Rogers and Smart, 1996) cally dated terraces (Rogers, 1996; Rogers and Smart, 1996; Formento- from ubiquitous mollusks in the fine-grained facies suggest an approxi- Trigilio, 1997; Formento-Trigilio and Pazzaglia, 1998). mate 150 ka age (marine oxygen isotope stage 6) for this terrace. There are seven major alluvial deposits in the middle portion of Terrace Qt5 differs in texture and stratigraphy from Qt1-Qt4 in the Jemez valley, in the vicinity of its confluence with the Rio Guadalupe. that it is coarser grained, and lacks the coarse-fine-coarse facies stacking The highest and oldest of these alluvial deposits (Qg) overlies a paleo- pattern. Qt5 varies in thickness from 1 to 5 meters, overlies a high relief valley bottom of high local relief cut into upper Paleozoic sedimentary strath, and contains mostly a volcanic provenance including rounded rocks. This alluvium is composed of well stratified, well rounded, boulders of Banco Bonito rhyolite, some of which are greater than a interbedded, volcanic provenance gravel and sand, with lesser amounts meter in diameter. The Banco Bonito rhyolite was deposited during the of Paleozoic sedimentary rocks. Deposit thickness varies from 1 to 15 El Cajete eruption approximately 60 ka so deposition of Qt5 occured m and is exposed locally beneath the upper (1.2 Ma) and lower (1.6 during the transition from a cool interstadial isotope stage 4 to warmer Ma) Bandelier Tuff. Poorly consolidated ash at the base of the Bandelier interstadial isotope stage 3. ignimbrites contains climbing ripples indicating its deposition in the The modern Jemez River below the Rio Guadalupe confluence is ancestral Rio Jemez channel, which at this time was an alluvial stream predominantly an alluvial river, flowing atop 25 meters of mixed prov- draining a predominantly volcanic upland. enance sediment that unconformably overlies Paleozoic, Mesozoic, and Four major fill terraces (Qt1 to Qt4), locally paired, are inset Cenozoic bedrock. This late Pleistocene and Holocene alluvium re- below Qg. Terraces Qt1 through Qt3 have planar straths, but Qt4 has a sembles terraces Qt1 through Qt4 in texture and stratigraphy. high-relief strath and clearly represents alluvial burial of Terrace ages and strath elevations can be plotted to determine the paleotopography. All four fill terraces share a common stratigraphy, long term rate of incision of the Jemez valley (Fig. 9). At the confluence sedimentology, and provenance. The deposits are approximately 10-25 reach, the long term rate of incision has been approximately 0.17 mm/yr m thick and composed of mixed volcanic, plutonic, and sedimentary (170 m/m.y.). Incision rates increase downstream and decrease upstream rock including limestone and chert (mostly Pedernal Chert), and gra- from the confluence reach as indicated by the diverging and converging nitic clasts. Texture and bedding in the deposits follow a distinct pat- terrace-long profile pattern, respectively (Fig. 9). Emplacement of the tern of a basal, coarse-grained channel facies gravel, conformably over- Bandelier Tuff 1.2 Ma, base level fall paced by the Rio Grande, and lain by a cross-bedded overbank and crevasse-splay facies gravelly sand possible continued uplift in the Jemez Mountains, are driving incision and silt, unconformably overlain by a second channel facies of coarse- and producing the accommodation space for terrace deposition and pres- grained gravel. This stratigraphy reflects a change in the overall channel ervation. Quaternary glacial-interglacial climates are providing changes geometry and pattern from braided at the beginning and end of valley in watershed hydrology and hillslope sediment flux that makes the inci- alluviation to meandering in the intervening phase. Numeric age control sion unsteady, and punctuated by valley alluviation events preserved as is not precise enough to know how long the valley-filling phase lasted, Qg, terraces, and the modern valley alluvium. 122 received the greatest popular attention as being the cause of the incision episode, but there are little data to actually support this contention. In 1846, some 20 years before any American settlement of New Mexico and in places removed from traditional Spanish livestock grazing, Lt. Simpson of the Army of the West marched his team across the Rio Puerco near Cabezon. The team had to cut down the banks to transport the cannon because the channel there was already incised 10 m with steep walls (Bryan, 1925). Furthermore, numerous geologic and archeological studies utilizing the exposed alluvium of the incised valley bottoms demonstrate that there have been several cutting and filling cycles during the Holocene (Bryan, 1940; Leopold et al., 1966; Love, 1980; Love and Young, 1983; Balling and Wells, 1990), the previous one occurring approximately 3000 years ago. There is no a priori reason, there- fore, to conclude that land use practices alone can account for the incision episode for New Mexico’s arroyos between ~1880-1950. An alternative explanation for driving incision appeals to a redis- tribution of annual precipitation from the relatively cooler climates of the Little Ice Age to the warmer climates of the 20th century. During the Little Ice Age (~1650-1850 AD) New Mexico received most of its annual moisture as winter, frontal precipitation that tends to be of relatively low intensity. In the warming since the Little Ice Age, New Mexico receives virtually the same amount of annual precipitation, but it comes proportionally more now in the summer as intense, convective storms (Leopold, 1951b, 1956). The increase in precipitation intensity and more peaked hydrographs are typically associated with more channel scour and subsequent incision. In summary, there probably is not one single cause to the incision-aggradation cycles of New Mexico’s arroyos. Ephemeral channels are known to have metastable gradients in part because of how sediment is transported (Leopold and Miller, 1956; Patton and Schumm, 1975, 1981) so there exist a variety of potential triggers, natural and anthropogenic that can cause a lowering of the channel gradient (incision) or steepening (aggradation) (Bryan, 1940) over relatively short FIGURE 10. (a) Photograph of the valley wall of the Rio Puerco west of Los Lunas. time scales. Numerous cut and fill cycles are evident. (b) Historic decrease in Rio Puerco sediment CONCLUSIONS yield resulting from current alluviation in the Rio Puerco channel following deep incision at the beginning of the last century (modified from Gellis, 1998). Climate change remains one of the most, if not the most important, influences on river form and process in the Quaternary. The streams Short-Term: The Cut And Fill Cycle Of Arroyos we observe in New Mexico, and elsewhere, are shaped to accommo- date the large amplitude and frequent changes in climate manifest as One way that Quaternary climate changes may have had a direct glacial-interglacial cycles at Pleistocene time scales and millennial- impact on humans and human activities is through major incision and scale wet-dry cycles at Holocene time scales. These kinds of climatic aggradation of channels over decadal time spans (Fig. 10). Such rapid changes are not well represented in the Phanerozoic. Together with channel processes can seriously disrupt agricultural and grazing land evolutionary trends like the emergence of grasses, modern rivers may uses with economical consequences. Most of the American southwest, not have direct analogues in the geologic record. including New Mexico, is characterized by deeply incised, steep-walled, The lowering of valleys by river incision can be a steady process flat-bottomed ephemeral channels called arroyos. Historical accounts when viewed over 105-106 yr time scales, but is decidedly unsteady of the precise width and depth of these channels in New Mexico are when viewed over 104-105 yr time scales. Quaternary climate change numerous as they alternatively presented opportunities, or barriers to is the single most important control on that unsteadiness and it is mani- agriculture and transportation. fest in New Mexico as river terraces over Pleistocene time scales, and A period of arroyo incision and associated tributary gullying of arroyo cut and fill cycles over Holocene (103 yr) time scales. Long term the New Mexico is generally agreed to have commenced in the latter rates of valley lowering in New Mexico range from about 50-200 m/ part of the 19th century and continued through the first half of the 20th m.y., and many valleys are flanked by terraces that correspond to one or century (Bryan, 1925; Hack, 1942; Antevs, 1952; Leopold, 1956). The more glacial-interglacial induced valley aggradation – valley excava- cause of this period of rapid incision has been ascribed to climatic tion events. In the Holocene, a period of arroyo incision and associated changes associated with the end of the Little Ice Age (Bryan, 1940; tributary gullying is generally agreed to have commenced in the latter Leopold, 1951b; 1976; Balling and Wells, 1990), overgrazing and re- part of the 19th century and continued through the first half of the 20th lated unsustainable land use practices (Thornethwaite et al., 1942), or century. The cause of this period of rapid incision has been ascribed to natural metastability of the gradient of ephemeral channels in a gen- climatic changes associated with the end of the Little Ice Age, but other eral semi-arid climate (Bryan, 1925; Leopold et al., 1966; Leopold, factors, including overgrazing and related unsustainable land use prac- 1976). In the latter part of the 20th century, aggradation of the formerly tices, or the natural metastability of the gradient of ephemeral channels incised channels appears to be the general response observed across a in a general semi-arid climate, must also be considered. wide range of channel sizes (Leopold et al., 1966; Gellis, 1998), al- River responses to climate change are typically equivocal; there though there remain local instances of intense gullying. is no single unique response that follows from a given change in cli- Overgrazing and associated land use practices have probably mate. Nevertheless, through a wide range of studies, geomorpholo- 123 gists have developed process-response models that limit the range of governing evolution allows for the study of paleontology. However, expected responses and help guide an interpretation of how Quater- these process-response models are robust enough to provide a starting nary climates ultimately shape landscapes. The availability and trans- point for interpreting river responses to Quaternary climate. port of sediment along the channel gradient are central to these models that predict aggradation during climates that foster high sediment yield ACKNOWLEDGMENTS and large, stable discharges, and incision during climates that shut off The author thanks NMMNH for an invitation to present this pa- the sediment supply and foster flashy discharges. Process-response per. John Hawley provided helpful suggestions on an early version of models are not a rigid set of rules, nor do they always allow for unique this paper. Formal reviews by Tom Gardner and Sean Connell greatly interpretations of geomorphic data in contrast, for example, as the rules improved the manuscript.

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