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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.

INTRODUCTION

New Mexico has enjoyed a long history of process fluvial geo-morphology research consistent with the close link between water as aresource, and the cultural and economic development of the state. Someof the earliest records of humans in the state, indeed in all of NorthAmerica, are the Clovis and Folsom archeological sites, both of which areset in the context of river valleys. Later, Anasazi and Pueblo culturesmanaged rivers for their water resources in constructing impressive citiesin an otherwise arid and hostile environment. Spanish colonization ofthe state was guided by the course of the Rio Grande del Norte (RioGrande), and the subsequent rise of large cities and modern agricultureunder American statehood again was shaped by rivers. Over the timescales that humans use and modify water resources and the landscape,New Mexico’s rivers have changed. That change has had a direct eco-nomic impact, both positive and negative, and helps drive continuedresearch in the form and processes of New Mexico’s streams. Over long,geologic time scales, New Mexico’s rivers have also changed, and themagnitude of that change far exceeds what has been historically recordedover the past 500 years. The location, size, shape, pattern, and dischargeof New Mexico’s rivers ultimately arise from the tectonic setting of thestate (King, 1965). Although an interesting topic unto itself, the rela-tionship between tectonics and river form and function will not be con-sidered in this paper. Rather, this paper will explore climate as theproximate cause for changes in river pattern form and function over timeperiods ranging from recorded human history (100-102 years) to theQuaternary (105-106 years), which includes the last 10,000 years ofgeologic time called the Holocene. Using the well known and dramaticclimatic shifts that are the hallmark of the Quaternary, geomorphologistshave built an understanding of how climate influences river form andfunction. This understanding is key to contemporary efforts to manageresources of a semi-arid land under ever-increasing pressures of a grow-ing population.

The behavior of New Mexican rivers during the Quaternary hasbeen well documented and is often cited as being representative of aridand semi arid lands world-wide. Particularly important process stud-ies include Kirk Bryan’s work in the Rio Grande rift (Bryan, 1925;1926; 1938; 1940), Luna Leopold’s research on arroyo developmentand evolution (Leopold, 1951a,b; 1976; Leopold and Miller, 1956;

Leopold et al., 1966), and John Miller’s treatises on small mountainouschannels (Miller, 1958; Miller and Wendorf, 1958). More recent studies,including Wells et al. (1982), Love and Young (1983), Gellis (1998), andClapp et al. (2001), have expanded our understanding of river behavior,particularly along the lines of quantifying sediment yields over decadaltime scales. These and other studies stress the importance of climate andclimate change in shaping channel form and function, but they also ac-knowledge that anthropogenic factors such as agriculture and grazinghave played a role in historic river changes. Unfortunately, the preciserole of anthropogenic activities has proven difficult to quantify, leavingnatural river changes throughout the Quaternary as the best analogue forcontemporary river behavior or any predictions of future behavior, espe-cially in the context of global warming.

This paper will first explore Quaternary rivers in the context ofriver form and process interpreted from New Mexico’s tectonic settingand the Phanerozoic rock record. It then will introduce some generalconcepts of river forms and processes and explain the prevailing modelsfor how geomorphologists envision channels responding to climate change.Data on long-term and short-term changes for several major New Mexi-can rivers are presented and compared. These observations are used todraw some general conclusions on the pace of New Mexican landscapeevolution, the effective sensitivity of New Mexican rivers to climatechange, and predicted behavior of New Mexican rivers in the future.

NEW MEXICO’S RIVERS

The relatively arid lands of New Mexico are drained by largestreams that owe their existence to headwaters in lofty mountains, andtheir courses to the general tectonic setting. New Mexico lies at thejuxtaposition of five major physiographic provinces, the Southern RockyMountains in the north, the Colorado Plateau in the west, the HighPlains in the east, the Basin and Range in the south, and the Rio Granderift, extending south to north, nearly bisecting the state into two equalparts (Fig. 1). The Rio Grande, which heads in the San Juan Moun-tains of southwestern Colorado, is the axial drainage of the rift. It hasa few important tributaries that head on the rift flanks, including theRed River, Rio Chama, and Rio Puerco. Otherwise, most of the drain-age that heads on the rift flank flows away from the rift and acrosseither the High Plains, like the Canadian and Pecos Rivers, or the Colo-rado Plateau, like the San Juan, Chaco, and Puerco (of the west) rivers.

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, fjp3@lehigh.edu

Abstract–Rivers shape landscapes by responding to a range of external forces including climate and climatechange. This paper examines the general role that Quaternary climates play in river channel form and processand how rivers in New Mexico have responded specifically to Pleistocene and Holocene climatic changes. Theimpact of Quaternary climates on New Mexican streams is cast in the context of how channel form and processhas evolved through the Phanerozoic and in light of a large and growing data set for measurable changes inchannel form and watershed hydrology over human time scales. Changes in channel form and process arecommonly 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 andhelp 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 arroyoincision-aggradation cycle are discussed in the context of Quaternary climates. The general conclusion is thatclimate change through the Quaternary has had a major impact on pacing the rate of valley incision and definingthe 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 andprocess over Quaternary time scales.

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Most of the above mentioned rivers maintain perennial flows with highlyvariable amounts of sediment discharge. The Rio Puerco (rift drainagetributary to the Rio Grande), in particular, is often noted for producingsome of the highest measured suspended sediment yields of any streamglobally (Nordin, 1963). With the exception of the above mentionedstreams, and those on Figure 1, most of New Mexico’s rivers are ephem-eral, some flowing seasonally and others flowing only during infrequent,large magnitude precipitation events.

Tectonics have imprinted the general drainage pattern of NewMexico’s streams on the landscape, but it has been climate that hasshaped the size and form of their channels. Streams draining the HighPlains and Colorado Plateau have ancestries reaching back to the Laramideorogeny (40-80 Ma). The Laramide drainage was disrupted by thedevelopment of the rift and the subsequent growth of the Rio Grande asits master, axial stream. Climate has modulated the discharge of the RioGrande, particularly in the Quaternary, and has played a role in thehydrologic integration of tectonically distinct basins in the rift. Al-though its inception is coincident with the beginnings of rifting in the lateOligocene-early Miocene, the Rio Grande became a complete, free-flow-ing, integrated drainage, reaching the Gulf on Mexico only within the last500-700 k.y. (Pazzaglia and Hawley, 2004).

WHAT IS THE QUATERNARY FROM THE PERSPECTIVE OFA RIVER?

Rivers and fluvial processes play the most important role in set-ting the pace of landscape evolution. Geomorphological interpretationsof river form and process and the resulting influence on the landscapeevolution is colored by the fact that the Earth system currently resides ina global ice house with permanent ice at the poles. This climate ischaracterized by large latitudinal shifts in climate. Quaternary climates,which appear to be an expression of the normal condition of the Earthsystem from the human perspective, are in fact rare events when viewedover the entire Phanerozoic timescale (Fig. 2). The fluvial forms andprocesses we observe today, and their response to climate, may be poorlyrepresented in the rock record of fluvial deposits.

Fluvial and fluvial-deltaic deposits are well represented in thesedimentary rock record of the Phanerozoic (Fig. 2). Similar deposits areknown from the Precambrian, but the near total absence of biostratigra-phy limits paleoecological and specific depositional environment inter-pretations. In contrast, Phanerozoic fluvial and fluvial-delatic depositsare rich in biostratigraphic and paleoecologic data, and from these geolo-gists have pieced together a general story of how fluvial form and process

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=ArroyoHondo, RG = Rio Guadalupe, RR=Red River, RC=Rio Costilla. A=Alamogordo, ABQ=Albuquerque, C=Clovis, CB=Carlsbad, F=Farmington, G=Gallup, LC=LasCruces, R=Roswell, SC=Silver City, SF=Santa Fe, SR=Santa Rosa, T=Taos.

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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 coloni-zation of the land by land plants, (2) the development of true, ecologi-cally diverse forests with a tall canopy and understory, (3) the evolutionand subsequent spread of grasses, and (4) the local impact of channelmodifying animals like beavers or humans. Lower Paleozoic fluvialdeposits are almost exclusively braided channel and braided alluvial plainfacies. Channel forms tend to be wide and shallow and characterized bytransverse bars. Floodplain facies are rare in these braidplains, but wherepreserved, tend to lack extensive pedogenic modification. By the LateDevonian, 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 OldRed Sandstone/Catskill Delta of the Caledonides and Appalachians, re-spectively, are good examples of a Late Devonian fluvial deposit wherethe channel width to depth ratio is decreased with respect to lowerPaleozoic channels, presumably by plant root channel bank stabiliza-tion. Low and middle latitude Carboniferous coal swamps increase thefrequency 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 finesthat were ultimately vertically accreted on floodplains and continued toenhance channel bank stability. Carboniferous fluvial deposits are thefirst to exhibit widespread meandering as well as braided channel faciesincluding clear epsilon cross bedding consistent with point bar deposi-tion. Meandering channels began to dominate the sedimentological recordby the middle Tertiary as grasses, which had evolved by the late Eocene,grew to global distributions. Grasses provide the most effective meansof stabilizing channel banks, further reducing the channel width to depth.Grasses also have a wide distribution in arid lands that typically receiveseasonal, intense precipitation. Channel bank stabilization and the en-

FIGURE 2. Geologic time scale illustrating major changes in river form and processthrough the Phanerozoic.

FIGURE 3. Photographs of (a) a meandering channel, Rio San Antonio, VallesCaldera, (b) the point bar of a meandering channel, Rio Puerco, Cuba, NM (c)braided-anastomosing channel(s), oblique airphoto of the Sandia Mountainspiedmont, and (d) a bedrock channel and knickpoint, Rio Frijoles.

118hanced infiltration capabilities of grasses foster steady, rather than flashy,discharges, another characteristic of meandering, rather than braided chan-nels. Lastly, recruitment of large woody debris into channels, somethingthat is typically accomplished during large floods, has been enhanced byvarious animals including beaver, to name a Cenozoic example. Largewoody debris is known to foster a step-pool and pool-riffle channelmorphology.

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). Mostlarge, trunk streams such as the Rio Grande and Pecos Rivers havealluvial braided channels. Human restrictions like levees and damshave worked to narrow and deepen what were otherwise broad, shal-low braided channels. Braided channels are characterized by unstablechannel banks, a high proportion of bedload to suspended load, andflashy discharges, both on a seasonal and individual storm basis.

Alluvial channels in New Mexico with a low proportion ofbedload to suspended load, a high percentage of fines, stable channelbanks, and relatively stable discharges tend to have more narrow, deepand meandering patterns. The Rio Puerco is a good example of a me-andering channel entrenched into its own alluvium, where channel bankstability is provided by the high percentage of fines. The upper reachesof the Jemez River is a good example of a meandering channel in anupland, moister setting where channel bank stability is provided bymeadow grasses.

Mountain channels are both alluvial and bedrock in nature andexhibit pool-riffle and step-pool morphologies well cited in the recentgeomorphological literature (Miller, 1958; Wohl and Merritts, 2001).The specific hydraulic geometry of these channels is well documentedfor streams of the Sangre de Cristo Mountains (Miller, 1958).

Channels that are deeply incised into bedrock have their formand function shaped by the relative hardness of the substrate. Thesechannels tend to be steep and have numerous knickpoints related tochanges in rock hardness. The lower reaches of small streams that aretributary to the Rio Grande such as the Rio Frijoles, Arroyo Hondo, orRed River are good examples of bedrock channels. The Rio Grandeitself has several predominantly bedrock reaches such as the Taos gorgeand White Rock Canyon. Significant, incised bedrock reaches occuron other major New Mexico rivers including the San Juan, Chaco, Ca-nadian, Pecos, San Francisco, and Gila.

Watershed-Channel Hydraulics

Rivers respond to climate because climate has a first-order ef-fect on the amount of water and sediment that gets delivered to thechannel. Furthermore, climate dictates whether the water and sedi-ment are delivered steadily on an annual basis, or with a large variationon a seasonal basis. In order to understand the observed effects ofQuaternary climates on New Mexico’s channels detailed below, thescaling relationships between drainage basin area and discharge, andchannel shape and discharge, must be understood. For small alluvialchannels in the mountains of New Mexico, some of these hydraulicgeometry relationships are well known (Miller, 1958). Similar dataexist for the numerous, small, ephemeral channels in the more aridparts of the landscape (Leopold and Miller, 1956; Leopold et al., 1966).

Changes in channel form, process, and hydrology for perennialalluvial channels are less well known, but important from the stand-point that they record the channel response to climate change. Riversthat fall into this category include, but are not restricted to the RioJemez, Arroyo Hondo, the Red River, and Rio Costilla. These streamswere investigated by Kelson and Wells (1989) and Mitchell (1999;Fig. 4). Their findings indicate that the scaling between discharge anddrainage basin area is dependent upon both rock type and climate. Hard,

homogeneous, presumably relatively unfractured substrates like granitetend to generate proportionally more discharge per unit area than compa-rable areas underlain by softer, more heterogeneous and presumablyrelatively more fractured sedimentary rocks (Kelson and Wells, 1989).Streams draining granitic watersheds tend to have more peaked dischargesthat generate more erosive power and bedrock incision with respect tolarger watersheds underlain by sedimentary rocks. For a large popula-tion of gaged watersheds in New Mexico, discharge tends to scale towatershed area raised to the 0.5-0.7 power. In contrast, for a similarpopulation of similar-sized drainage basins on the west side of the Cas-cades in the Pacific northwest, discharge scales linearly with drainagearea (Mitchell, 1999). These results are significant from the perspectivethat modern spatial variations in effective precipitation stand as a proxyto the effects of temporal changes in Quaternary climate. Channel formand process under relatively arid conditions is represented by the currentwarm, arid, seasonal Holocene climate, and the cooler, wetter, less sea-sonal Pleistocene climates would drive channel form and process toapproximate the hydrologic conditions of wetter regions not currentlyrepresented in New Mexico.

CLIMATE, CLIMATE CHANGE, AND RIVER RESPONSE

General Process-Response Model

A general model for fluvial response to climate change follows

FIGURE 4. (a) Plot of stream discharge as a function of drainage area for NewMexican streams (semi arid climate) vs. streams in western Washington state (humidtemperate climate). (b) Plot of stream channel width as a function of watershed area(modified from Mitchell, 1999). Solid triangles indicate bankfull channel widths,gray circles indicate base flow channel widths. The b values refer to the slope of theregression lines, which is the power dependence in the equation Width = k(DrainageArea)b.

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from decades of climatic geomorphology research on fluvial systemswhere the channel and deposits in the lower part of a watershed can bestratigraphically linked to glacial deposits in the upper part of a water-shed (Fig. 5; Schumm, 1965; Bull, 1991). The model that has emergeddoes not only apply to only glaciated watersheds, but is fairly well-suited to describe a majority of New Mexico’s rivers whose headwa-ters are typically in lofty mountains that experienced glaciation in theQuaternary. All rivers incise their valleys at a rate driven by the rate ofbase level fall. In tectonically active settings, the base level fall israpid, typically in the range of 1 to 5 mm/yr. By comparison, the rate ofbase level fall in New Mexico averages an order of magnitude slower(0.1 to 0.25 mm/yr; Pazzaglia and Hawley, 2004); nevertheless, at theserates, valleys hundreds of meters deep can be carved during the past 1–2 m.y. of the Quaternary. Over 105 to 106 year time spans, the verticalincision rate is nearly steady, but over 103 to 104 year time spans theincision rate is unsteady. This unsteadiness and its effect on fluvialterrace genesis is illustrated in Fig. 5. The aggradation periods in Fig.5 are stratigraphically linked upstream to glacial heads of outwash andtemporally linked to times of increased sediment and water dischargeto the channel. In contrast, the incision periods in Fig. 5 do not have anup basin glacial stratigraphic equivalent and are temporally linked totimes of decreased sediment and water discharge to the channel. Riv-ers aggrade, depositing alluvium in their valleys during glacial climatesbecause an overall steeper channel is required to carry the increasedsediment load. In converse, rivers incise, deepening their valleys dur-ing interglacial climates, because less steep channels are needed totransport the reduced sediment load. In this way, Quaternary climateshave a profound impact on channel form and gradient. The frequentaggradation-degradation cycles observed in river valleys globally is ahallmark of Quaternary climates. Similar aggradation-degradationcycles may be less common for the rest of the Phanerozic, except fortimes like the Pennsylvanian when stratigraphy suggests numerous andfrequent base level changes consistent with glacio-eustasy.

Process Linkage

The connections between the processes that deliver sedimentfrom hillslopes and those that move sediment through the fluvial sys-tem are called process linkage. Even though the climatic influence onthe fluvial system has already been established, changes in sedimentflux to the channel may not be in phase with climate changes becauseof the time lags associated with sediment storage on hillslopes. Anexample of process linkage that plays out over Holocene (104 yrs) timescales in the northern Rocky Mountains illustrates the connections be-tween climate, watershed hydrology, the liberation of sediment fromhillslopes, and response of the fluvial system (Meyer et al., 1995; Fig.6). Climate in the northeastern portion of Yellowstone National Park

is known to vary across millennial time spans between relative warm,dry years when winter snowpacks are small and summer convectivestorm frequency is large, and relatively wet years, when winter snow-packs are large and summer convective storms frequency is depressed.Large, stand-clearing fires tend to occur during the dry summers follow-ing below normal winter snowpacks. The dry summers, along with thefires result in frequent debris flows moving sediment off of hillslopeswhere it accumulates on the valley flanks, atop the floodplain. Duringthese times the large channels tends to be relative narrow, braided, andincising into their floodplain, processes fostered by flashy dischargesand lack of fine sediments (Fig. 6b). In contrast, the relative lack of firesduring wet summers results in fewer debris flows, so sediment is effec-tively kept on hillslopes. During these times, the large channels tend tobe relatively wide, meandering, and incising laterally into the alluvial fansformerly deposited on the valley flanks. These processes are favored bystable discharges and the delivery of fines that vertically accrete a flood-plain (Fig. 6a). Floodplain construction lags behind the initial liberationof sediment from hillslopes by several thousand years. So the cut-and-fill cycles of the large channels, although driven by climate, are out ofphase with the actual climatic change that liberated the sediment from thehillslopes in the first place. Similar process linkages and time lags areenvisioned for glacial-interglacial climatic cycles of the Pleistocene thatplay out over 105, rather than the millennial time scales illustrated in thisexample.

QUATERNARY CLIMATES AND RIVER DEPOSITS

Climate during the Quaternary is characterized by 105 year scalewaxing and waning of mean average temperature by ~10oC and the growthand shrinkage of continental and mountain glaciers (Fig. 7). The firstmajor expanse of a major Northern Hemisphere continental ice sheet istypically taken as occurring 2.45 Ma. This event is recorded bydropstones in North Atlantic ocean sediments of this age, as well as a

FIGURE 5. The Bull process-response model (modified from Bull, 1991).

FIGURE 6. A process-linkage model (modified from Meyer et al., 1995).

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major increase in the amount of the heavier isotope of oxygen (18O) inocean waters corresponding to an increase in global ice volume. Theantiquity of this first major glacial-interglacial climate change makes itdifficult, if not impossible to recognize in the New Mexican landscape.It is possible that general fluvial aggradation such as the Tuerto or Anchaformations that characterized the rift flank through the Pliocene, mayhave ceased at about this time as streams incised following aggradation(Koning et al., 2002).

The latest Pliocene and virtually all of the early Pleistocene ischaracterized by frequent climate changes occurring on a 41 k.y. peri-odicity (Fig. 7). Deposition, incision, or cut and fill cycles are alsodifficult to identify during this time interval. Some fluvial systems,like the Rio Grande, were likely aggrading through this time. Othersystems, like the Rio Jemez, were incising, as there are no deposits inthe Jemez valley between the base of the Upper Bandelier Tuff (1.2Ma) and a terrace alluvium that contains the Lava Creek B tephra (0.64Ma).

Large amplitude glacial-interglacial cycles lasting approximately100 k.y. distinguish the middle and late Pleistocene from the latePliocene and early Pleistocene. There have been at least ten majorglacial-interglacial climate changes in the past 1 Ma. Glacial climatesare generalized as relatively cooler and wetter, with a general depres-sion of the tree line, and current low-standing arid to semiarid regionsbeing covered by a more savanna-like vegetative assemblage (Spauldinget al., 1983; Spaulding and Graumlich, 1986; Betancourt et al., 1990).Interglacial climates are generally warmer and drier, but the big differ-ence may be the greater intensity of precipitation and seasonality withrespect to the glacial climates (Holliday, 1989, 1997, 2000).

Most of the major rivers in New Mexico, paced by the Rio Grande,entered a phase of generally incising during the middle and late Pleis-tocene. The record of that incision and Quaternary climate change ispreserved deposits of paleo-valley bottoms called terraces along theflanks of the river valleys. A terrace is both a landform and anallostratigraphic deposit composed of three main components, an ero-sional basal unconformity called a strath, a depositional top called atread, and the intervening terrace alluvium (Fig. 8). Strath terraces arecharacterized by thin terrace alluvium and planar or nearly planar basalunconformities. The thickness of the alluvium for strath terraces isconsistent with the prevailing scour depth of the channel for which it isassociated. In contrast, fill terraces are underlain by thick alluviumconsistent with a period of valley aggradation. The basal unconformitiesof fill terraces are commonly irregular, consistent with the burial ofpaleo-valley bottoms.

For the western United States including New Mexico, we know

that the fluvial system was strongly impacted by middle and late Pleis-tocene glacial-interglacial cycles because tephras, erupted at opportunetimes, are interbedded with terraces. Two tephras with wide distribu-tion, the Lava Creek B (Yellowstone) and Bishop (Mammoth, CA) areinterbedded with fluvial deposits corresponding to marine oxygen iso-tope stages 16 and 20, respectively. The Lava Creek B tephra is particu-larly widespread throughout the Rocky Mountain region and speaks tothe broad effect that climate had on driving aggradation of fluvial systemsduring the transition from glacial stage 16 climates to interglacial stage 15climate conditions. Similarly, aggradation of the Rio Grande at the closeof isotope stage 6, resulting in the deposition of the “segundo alto”terrace and Los Duranes Formation (Lambert, 1968; Connell and Love,2001) is known from interbedded lavas of the Albuquerque volcanoes(156±29 ka; Peate et al., 1996).

Locally, rivers are known to aggrade, even during the climate changesassociated with interstadial, rather than glacial-interglacial transitions.Rivers draining the Jemez Mountains, for example, typically have analluvial terrace containing the El Cajete tephra (Cochiti Canyon; Aby,1997) or Banco Bonito clasts (Rio Jemez; Formento-Trigilio and Pazzaglia,1998), both of which are 60 k.y. old and mark the transition from isotopestage 4 to isotope stage 3.

HOLOCENE CLIMATES

The Holocene is the last 10 k.y. of the Quaternary and representsjust the latest of several major interglacial climate episodes that haveexisted periodically over the past 1 m.y. Although Holocene climaticvariability has likely been less extreme than a Pleistocene glacial-intergla-cial cycle, millennial scale, hemispheric, or global changes in mean annualtemperature and precipitation are present in high resolution stratigraphicrecords (Bond et al., 1997). In New Mexico, the record of playas,interspersed with widespread desiccation for basins such as the Estanciabasin demonstrates that there have been relatively wetter and dryerperiods each lasting several thousand years (Allen, 1993; Allen and Ander-son, 1993, 2000; Allen and Hawley, 1991; Holliday, 1997). Leopold(1951b) was among the first to suggest precipitation intensity and sea-sonality as driving Holocene-scale cut and fill cycles in New Mexicanfluvial systems. Such variations in rainfall intensity may be linked tohemispheric circulation patterns like ENSO (El-Niño Southern Oscilla-tion; Gutzler, 2000), but such links remain to be definitively established.

Complex responses to Holocene climate changes are the rule, ratherthan the exception, making it difficult to link a specific climate to aspecific channel response. A complex response is defined as a family ofrelated channel processes that cascade from a single stimulus (Schumm,1973). For example, a Holocene increase in precipitation intensity might

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.

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cause a major ephemeral channel to incise. That incision is seen as a baselevel fall for the tributary channels that also incise, delivering sedimentback to the trunk channel. The increase in sediment delivery to the trunkchannel overwhelms its ability to transport it, causing aggradation andreversal of the initial climate-induced incision. In this way, channelsdraining small watersheds might incise due to an increase in precipitationintensity, whereas larger trunk channels initially incise, but later aggrade.Complex responses make the links between climate change and channelresponse at millennial time scales equivocal.

TRENDS IN RIVER RESPONSE TO CLIMATE

Long Term; Terraces, Terrace Stratigraphy, and Incision

Several river valleys in New Mexico have been particularly goodrecorders of Quaternary climate change through the preservation offlights of river terraces (see Pazzaglia and Hawley, 2004 for a compre-hensive review). The Jemez River in north-central New Mexico thatdrains the southern flank of the Jemez Mountains (Fig. 1) is notewor-thy because of its wide distribution and vertical separation of numeri-cally dated terraces (Rogers, 1996; Rogers and Smart, 1996; Formento-Trigilio, 1997; Formento-Trigilio and Pazzaglia, 1998).

There are seven major alluvial deposits in the middle portion ofthe Jemez valley, in the vicinity of its confluence with the Rio Guadalupe.The highest and oldest of these alluvial deposits (Qg) overlies a paleo-valley bottom of high local relief cut into upper Paleozoic sedimentaryrocks. This alluvium is composed of well stratified, well rounded,interbedded, volcanic provenance gravel and sand, with lesser amountsof Paleozoic sedimentary rocks. Deposit thickness varies from 1 to 15m and is exposed locally beneath the upper (1.2 Ma) and lower (1.6Ma) Bandelier Tuff. Poorly consolidated ash at the base of the Bandelierignimbrites contains climbing ripples indicating its deposition in theancestral Rio Jemez channel, which at this time was an alluvial streamdraining a predominantly volcanic upland.

Four major fill terraces (Qt1 to Qt4), locally paired, are insetbelow Qg. Terraces Qt1 through Qt3 have planar straths, but Qt4 has ahigh-relief strath and clearly represents alluvial burial ofpaleotopography. All four fill terraces share a common stratigraphy,sedimentology, and provenance. The deposits are approximately 10-25m thick and composed of mixed volcanic, plutonic, and sedimentaryrock including limestone and chert (mostly Pedernal Chert), and gra-nitic clasts. Texture and bedding in the deposits follow a distinct pat-tern of a basal, coarse-grained channel facies gravel, conformably over-lain by a cross-bedded overbank and crevasse-splay facies gravelly sandand silt, unconformably overlain by a second channel facies of coarse-grained gravel. This stratigraphy reflects a change in the overall channelgeometry and pattern from braided at the beginning and end of valleyalluviation to meandering in the intervening phase. Numeric age controlis not precise enough to know how long the valley-filling phase lasted,

and there are no data to link the braided or meandering facies to a specificclimate; however, the general coincidence of dated terraces with the cooler,wetter glacial stages of the oxygen isotope curve (Fig. 7) suggest that theobserved variations in channel form and function likely follow fromclimatically-induced changes in watershed hydrology and delivery ofsediment from hillslopes as outlined above in the process response andprocess linkage models. The age of Qt1 is known to be ~610 ka (glacialisotope stage 16) from reworked Lava Creek B tephra preserved in themiddle sandy facies (Rogers, 1996). Qt2 has been dated to approxi-mately 500 ka (marine oxygen isotope stage 12) by travertine cementedgravels near Soda Dam (Goff and Shevenell, 1987) and Qt3 has a re-ported age of 315 – 180 ka (marine oxygen isotope stages 10 to 8) froman ESR (Electron Spin Resonance) date on travertine-cemented quartzgrains (Rogers et al., 1996). There are no independent ages for Qt4 but itis known to be older than 60 ka, the numerically-determined age of thenext youngest terrace. Both correlation to the oxygen isotope curve, aswell as calibrated amino acid racemization ages (Rogers and Smart, 1996)from ubiquitous mollusks in the fine-grained facies suggest an approxi-mate 150 ka age (marine oxygen isotope stage 6) for this terrace.

Terrace Qt5 differs in texture and stratigraphy from Qt1-Qt4 inthat it is coarser grained, and lacks the coarse-fine-coarse facies stackingpattern. Qt5 varies in thickness from 1 to 5 meters, overlies a high reliefstrath, and contains mostly a volcanic provenance including roundedboulders of Banco Bonito rhyolite, some of which are greater than ameter in diameter. The Banco Bonito rhyolite was deposited during theEl Cajete eruption approximately 60 ka so deposition of Qt5 occuredduring the transition from a cool interstadial isotope stage 4 to warmerinterstadial isotope stage 3.

The modern Jemez River below the Rio Guadalupe confluence ispredominantly an alluvial river, flowing atop 25 meters of mixed prov-enance sediment that unconformably overlies Paleozoic, Mesozoic, andCenozoic bedrock. This late Pleistocene and Holocene alluvium re-sembles terraces Qt1 through Qt4 in texture and stratigraphy.

Terrace ages and strath elevations can be plotted to determine thelong term rate of incision of the Jemez valley (Fig. 9). At the confluencereach, the long term rate of incision has been approximately 0.17 mm/yr(170 m/m.y.). Incision rates increase downstream and decrease upstreamfrom the confluence reach as indicated by the diverging and convergingterrace-long profile pattern, respectively (Fig. 9). Emplacement of theBandelier Tuff 1.2 Ma, base level fall paced by the Rio Grande, andpossible continued uplift in the Jemez Mountains, are driving incisionand producing the accommodation space for terrace deposition and pres-ervation. Quaternary glacial-interglacial climates are providing changesin watershed hydrology and hillslope sediment flux that makes the inci-sion unsteady, and punctuated by valley alluviation events preserved asQg, terraces, and the modern valley alluvium.

FIGURE 8. Photograph of a terrace deposit of the Canadian River.

FIGURE 9. Reconstructed longitudinal profiles and terrace stratigraphy of the JemezRiver. The terrace longitudinal profiles and associated stratigraphic horizons areplotted with respect to the stream longitudinal profile. Inset shows long term rates ofriver incision.

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Short-Term: The Cut And Fill Cycle Of Arroyos

One way that Quaternary climate changes may have had a directimpact on humans and human activities is through major incision andaggradation of channels over decadal time spans (Fig. 10). Such rapidchannel processes can seriously disrupt agricultural and grazing landuses with economical consequences. Most of the American southwest,including New Mexico, is characterized by deeply incised, steep-walled,flat-bottomed ephemeral channels called arroyos. Historical accountsof the precise width and depth of these channels in New Mexico arenumerous as they alternatively presented opportunities, or barriers toagriculture and transportation.

A period of arroyo incision and associated tributary gullying ofthe New Mexico is generally agreed to have commenced in the latterpart of the 19th century and continued through the first half of the 20th

century (Bryan, 1925; Hack, 1942; Antevs, 1952; Leopold, 1956). Thecause of this period of rapid incision has been ascribed to climaticchanges associated with the end of the Little Ice Age (Bryan, 1940;Leopold, 1951b; 1976; Balling and Wells, 1990), overgrazing and re-lated unsustainable land use practices (Thornethwaite et al., 1942), ornatural metastability of the gradient of ephemeral channels in a gen-eral semi-arid climate (Bryan, 1925; Leopold et al., 1966; Leopold,1976). In the latter part of the 20th century, aggradation of the formerlyincised channels appears to be the general response observed across awide range of channel sizes (Leopold et al., 1966; Gellis, 1998), al-though there remain local instances of intense gullying.

Overgrazing and associated land use practices have probably

FIGURE 10. (a) Photograph of the valley wall of the Rio Puerco west of Los Lunas.Numerous cut and fill cycles are evident. (b) Historic decrease in Rio Puerco sedimentyield resulting from current alluviation in the Rio Puerco channel following deepincision at the beginning of the last century (modified from Gellis, 1998).

received the greatest popular attention as being the cause of the incisionepisode, but there are little data to actually support this contention. In1846, some 20 years before any American settlement of New Mexicoand in places removed from traditional Spanish livestock grazing, Lt.Simpson of the Army of the West marched his team across the RioPuerco near Cabezon. The team had to cut down the banks to transportthe cannon because the channel there was already incised 10 m withsteep walls (Bryan, 1925). Furthermore, numerous geologic and archeo-logical studies utilizing the exposed alluvium of the incised valley bot-toms demonstrate that there have been several cutting and filling cyclesduring the Holocene (Bryan, 1940; Leopold et al., 1966; Love, 1980;Love and Young, 1983; Balling and Wells, 1990), the previous one occur-ring approximately 3000 years ago. There is no a priori reason, there-fore, to conclude that land use practices alone can account for the incisionepisode 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 ofthe Little Ice Age to the warmer climates of the 20th century. During theLittle Ice Age (~1650-1850 AD) New Mexico received most of its annualmoisture as winter, frontal precipitation that tends to be of relativelylow intensity. In the warming since the Little Ice Age, New Mexicoreceives virtually the same amount of annual precipitation, but it comesproportionally more now in the summer as intense, convective storms(Leopold, 1951b, 1956). The increase in precipitation intensity andmore peaked hydrographs are typically associated with more channelscour and subsequent incision.

In summary, there probably is not one single cause to the inci-sion-aggradation cycles of New Mexico’s arroyos. Ephemeral chan-nels are known to have metastable gradients in part because of howsediment is transported (Leopold and Miller, 1956; Patton and Schumm,1975, 1981) so there exist a variety of potential triggers, natural andanthropogenic that can cause a lowering of the channel gradient (inci-sion) or steepening (aggradation) (Bryan, 1940) over relatively shorttime scales.

CONCLUSIONS

Climate change remains one of the most, if not the most impor-tant, influences on river form and process in the Quaternary. The streamswe observe in New Mexico, and elsewhere, are shaped to accommo-date the large amplitude and frequent changes in climate manifest asglacial-interglacial cycles at Pleistocene time scales and millennial-scale wet-dry cycles at Holocene time scales. These kinds of climaticchanges are not well represented in the Phanerozoic. Together withevolutionary trends like the emergence of grasses, modern rivers maynot have direct analogues in the geologic record.

The lowering of valleys by river incision can be a steady processwhen viewed over 105-106 yr time scales, but is decidedly unsteadywhen viewed over 104-105 yr time scales. Quaternary climate changeis the single most important control on that unsteadiness and it is mani-fest in New Mexico as river terraces over Pleistocene time scales, andarroyo cut and fill cycles over Holocene (103 yr) time scales. Long termrates of valley lowering in New Mexico range from about 50-200 m/m.y., and many valleys are flanked by terraces that correspond to one ormore glacial-interglacial induced valley aggradation – valley excava-tion events. In the Holocene, a period of arroyo incision and associatedtributary gullying is generally agreed to have commenced in the latterpart of the 19th century and continued through the first half of the 20th

century. The cause of this period of rapid incision has been ascribed toclimatic changes associated with the end of the Little Ice Age, but otherfactors, including overgrazing and related unsustainable land use prac-tices, or the natural metastability of the gradient of ephemeral channelsin a general semi-arid climate, must also be considered.

River responses to climate change are typically equivocal; thereis no single unique response that follows from a given change in cli-mate. Nevertheless, through a wide range of studies, geomorpholo-

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ACKNOWLEDGMENTS

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