Floodplain construction of the Rio Grande at El Paso, Texas, USA: response to Holocene climate change

Quaternary Science Reviews 65 (2013) 102e119

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Floodplain construction of the Rio Grande at El Paso, Texas, USA: response to Holocene climate change Stephen A. Hall a, *, John A. Peterson b,1 a b

Red Rock Geological Enterprises, 3 Cagua Road, Santa Fe, NM 87508, USA Graduate Studies, Sponsored Programs and Research, University of Guam, Mangilao, GU 96923, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 May 2012 Received in revised form 20 November 2012 Accepted 21 November 2012 Available online 16 February 2013

The Rio Grande is one of the larger rivers in North America, and the development of its floodplain is related to Holocene climate and climate change. The late Pleistocene through early Holocene channel is characterized by a meander or braided system with lateral cutting and backfilling, resulting in the valleywide deposition of massive to cross-bedded, fine-to-medium textured sand. The late Pleistoceneeearly Holocene floodplain is also the sand source for the adjacent Bolson sand sheet. The sand sheet stopped accumulating new sand 5000 yrs ago, an event directly related to the shutting off of the sand supply caused by the deposition of overbank muds that covered and sealed the floodplain surface. During the middle Holocene, the river may have dried intermittently with the floodplain becoming deflated and local sand dunes forming on the floodplain. After 5000 yrs the climate was less arid and the river shifted to a regime of increased flooding and overbank deposition of silt and clay. By 2500 yrs, a late Holocene period of wet climate resulted in further overbank deposition and the formation of a cumulic Mollisol across the floodplain, the Socorro paleosol. The period of wet climate corresponds to the Audubon Neoglacial and active rock glaciers in the southern Rocky Mountains, speleothem growth in nearby caves, and other evidence for wet-cool conditions in the region. After 1000 yrs, the climate became drier, and the deposition and accumulation of overbank muds by the flooding Rio Grande came to a halt. Even though the river has flooded often in historic times, and presumably during late prehistoric times as well, there is little evidence for deposition of overbank sediments on the floodplain since A.D. 1000. Accordingly, the present-day surface of the Lower Valley is ten centuries old. Three channels occur on the US side of the Lower Valley floodplain, and during the past 2500 yrs stream flow has shifted from one to the other by the avulsion process of channel reoccupation, although most flow has been in the Rio Grande channel, the largest of the three. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Alluvial chronology Fluvial geomorphology Holocene Paleoclimatology Rio Grande Texas

1. Introduction The alluvial geology of many small streams and arroyos in the southern Great Plains and American Southwest have been investigated, probably due in part to easy access to sedimentary sequences exposed in cut banks. The Holocene history of deposition and erosion incorporated within the wide floodplains of the larger rivers, where natural exposures are uncommon, is generally less well known. The Rio Grande falls among the latter. The Rio Grande floodplain is not uniform throughout its 3050 km length from southern Colorado to the Gulf of Mexico. Long

* Corresponding author. Tel./fax: þ1 505 466 7755. E-mail address: [email protected] (S.A. Hall). 1 GSSPR, University of Guam, P.O. Box 5354, UOG Station, Mangilao, GU 969235354, USA. Tel.: þ1 671 735 2169; fax: þ1 671 734 7403. 0277-3791/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2012.11.013

stretches of the river have a wide floodplain several kilometers across with a meandering to braided channel. These reaches are the San Luis Valley of southern Colorado, the Middle Rio Grande Valley and Mesilla Valley of New Mexico, and the Lower Valley of El Paso and the Lower Rio Grande Valley of South Texas. These long reaches with a wide floodplain are separated from each other by narrow bedrock canyons and bedrock nick-points where the floodplain and channel are considerably constricted. One of the issues of river geomorphology concerns the Holocene history of the different segments or ‘valleys’ of the Rio Grande, whether each segment has an independent response to climate change or if the history of floodplain deposition and erosion is the same throughout the river. The channel of the Rio Grande has undergone substantial changes in the past century. Almost universally the channel has become more narrow and straighter, caused largely by historic increases in water withdrawal from the river and an accompanying decrease in discharge (Everitt, 1993; Jones and Harper, 1998; Pearce

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and Kelson, 2003, 2004; Dean and Schmidt, 2011; Dean et al., 2011). In an area of meandering in the San Luis Valley, the avulsion frequency has also diminished (Jones and Harper, 1998). The analysis of aerial photographs taken 1935 and 2001 from along the Middle Rio Grande Valley suggests that, during the twentieth century, the river has been changing from a meandering system to a braided channel with the active 2001 channel more narrow, straighter, and less sinuous than the pre-1935 channel (Pearce and Kelson, 2003, 2004). Flooding has been the mark of the river. The historical record indicates that between 1849 and 1942 the river flooded on an average of 1.9 years with a discharge greater than 10,000 cfs, and that during the Colonial and Mexican periods 1598e1846 severe major floods occurred nine times, often with floodwater covering the valley floor from Albuquerque to El Paso (Scurlock, 1998). One result of a major flood is that it can initiate a shift in active channel, such as documented in the Mesilla Valley (Mack and Leeder, 1998) and the Lower Valley of El Paso (this study). Much less is known about the earlier history of the Rio Grande floodplain. A few geomorphicestratigraphic studies have been initiated at archaeological sites on the floodplain, although in many reaches of the river the wide floodplain is covered by thick deposits laid down during the past few centuries, thus burying prehistoric sites that are not located on high ground. One of the concerns of archaeological studies is whether the channel has meandered across the valley, thereby eroding and removing prehistoric sites. In an evaluation of the floodplain in the Albuquerque basin, Sargeant (1987) noted that prehistoric sites are preserved in floodplain sediments, concluding that the river had not meandered. Our work in the Lower Valley of El Paso supports that conclusion, that the river has not moved out of a narrow channel zone and that the late Holocene sequence of floodplain sediments is intact. Even though channel conditions from 1935 aerial photographs suggest that the river has been shifting from a meander to a braided system in the Middle Rio Grande Valley (Pearce and Kelson, 2003), the pre-1935 meander channel cut and back-fill activity may not have affected the entire width of the floodplain, or the meander system may have been restricted to only some segments of the river along some stretches. Gustavson and Collins (1998) observed that, in the Lower Rio Grande Valley of South Texas, the Rio Grande channel has not meandered across the valley and that archaeological sites, once formed and buried in alluvium, are not eroded away by channel migration or widening. In the Albuquerque basin, the inner valley floodplain is mapped as the youngest inset fluvial deposit of the Rio Grande and formally named the Los Padillas Formation by Connell et al. (2007). They describe the formation as 15e29 m thick, pale-brown, fine- to coarse-grained sand and rounded gravel. Drill holes show that the base of the unit is gravel and unconformably overlies basin fill of the Santa Fe Group. The base of the Los Padillas Formation may be latest Pleistocene, although yet undated. Radiocarbon ages from two different buried archeological sites are 2800 and 5200 cal yrs, indicating a middle and late Holocene age of the upper part of the formation. Connell et al. (2007) suggest that the Los Padillas Formation can be traced from the north edge of the Albuquerque basin south to El Paso. The description of the Los Padillas Formation matches the general floodplain stratigraphy we found in the Lower Valley. However, the Holocene geology of the Rio Grande floodplain deposits is not everywhere uniform. In the Mesilla Valley of southern New Mexico, for example, Mack et al. (2011) describe and map the presence of one early Holocene terrace and two late Holocene terraces, the chronology of each terrace deposit established by radiocarbon dating. They further conclude that the timing of the incision that produced each terrace is related to a change in regional climate to increased aridity and that the Holocene terrace

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formation is unrelated to tectonic activity. Gustavson and Collins (1998) also conclude that the deposits that make up the three terraces at 2e5 m, 8e12 m, and 18e20 m above the channel in South Texas are Holocene in age. A buried soil in the intermediate 8e12 m terrace alluvium is radiocarbon dated late Holocene (this study, discussed later). The sequences of Holocene terraces described and mapped by Mack et al. (2011) and noted by Gustavson and Collins (1998) indicate a more complex picture than suggested by the floodplain stratigraphy in the Albuquerque basin (Connell et al., 2007) and the Lower Valley (this study). Nevertheless, as local sequences along the river are established, a comprehensive history of river deposition and erosion will emerge. Our study of the stratigraphy and geochronology of the floodplain deposits in the Lower Valley is a preliminary step in that direction. 2. Regional setting The Rio Grande valley in El Paso County, Texas, occupies the Hueco Bolson segment of the Rio Grande rift in the Basin and Range province of the western United States. The ancestral Rio Grande may have become established as a through-flowing river in the early Pleistocene about 2.25 Ma (Gustavson, 1991). After 0.78 Ma, the river shifted to a degradational phase with several episodes of downcutting and filling related broadly to glacialeinterglacial cycles (Mack et al., 2006). The Rio Grande today has a length of 3050 km, the fifth longest river in North America. Its watershed is 472,000 km2, excluding adjacent closed basins. The river begins at the Continental Divide in the San Juan Mountains of Colorado, flowing south through New Mexico and turning southeast at El Paso where it forms the international boundary between the United States and Mexico, eventually entering the Gulf of Mexico. The mean annual discharge is low because a large portion of its watershed is in semiarid to arid terrain. The long river was never navigable except by small boats. Since 1913, a number of reservoirs have been constructed on the river and its tributaries, and currently as much as 80% of its annual flow is diverted for agriculture and municipal uses. At El Paso today, it is not uncommon for the river to be completely dry. El Paso occurs at the northern edge of the Chihuahuan Desert and has an arid climate. The average annual rainfall is 224 mm (8.8 inches) and the average annual temperature is 17.3  C (63.2  F) (NOAA, 2010). 3. Methods 3.1. Stratigraphy The stratigraphy of the Rio Grande floodplain alluvium is not exposed in natural outcrops. Although many irrigation canals and drainage ditches cut through floodplain deposits, their walls are lined either with concrete or thick mud drapes and the alluvial stratigraphy is not visible. Accordingly, 50 trenches were excavated into Rio Grande floodplain sediments in an area of about 150 km2 in the Lower Valley of Trans-Pecos Texas. Trench localities are shown on the map (Fig. 1) and referred to by ‘T’ in the text. Additional stratigraphic information from the Rio Grande floodplain was obtained from trenches at Los Ranchos de Albuquerque, New Mexico, and near Laredo, Texas. Most of the Lower Valley trenches are less than three meters deep. It was found that once a trench penetrated the local shallow alluvial water table, especially in the Lower Valley, the walls of the trench immediately collapsed. Consequently, the investigation of floodplain stratigraphy was limited for the most part to the upper three meters of the deposits. As a result of a lower water table, it was possible to excavate deeper trenches at Los Ranchos de Albuquerque and Laredo. The geomorphology of the Rio Grande floodplain surface has been

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Fig. 1. Map of Lower Valley of El Paso with channels and trench localities; drawn from 1936 aerial photographs; Inset map: A, Los Ranchos de Albuquerque, Bernalillo County, New Mexico; L, Laredo, Webb County, Texas.

evaluated using the 1852 and 1855 international boundary survey maps (Emory, 1857), US Geological Survey topographic maps from 1988, black and white stereo-paired aerial photographs from 1936 and 1942e43, and color infrared aerial photographs from 1979 from NASA. 3.2. Radiocarbon dating All twenty-three of the radiocarbon ages from the Rio Grande alluvium in the Lower Valley are on bulk samples of fine-textured alluvium collected in 5- to 10-cm intervals of stratigraphy; nonarchaeologic buried wood or charcoal that could be used to date deposits were not encountered during the fieldwork. The radiocarbon ages were determined on the organic residues from the clayey sediment. The soluble humic acid fraction and sediment carbonate were removed by pretreatment. The laboratory procedures for the conventional radiocarbon ages from the Lower Valley are described in White and Valastro (1984). The five radiocarbon ages from Los Ranchos de Albuquerque and Laredo are AMS dates on organic matter from bulk sediment by Beta Analytic, Inc. and include both solid and soluble components; in all cases carbonate was removed during pretreatment (Table 1). Radiocarbon ages are presented as 14C years BP (yrs BP) or as calendar years BP before AD 1950 (cal yrs) based on the IntCal09 calibration data set

(Stuiver and Reimer, 1993; Reimer et al., 2009). Ages from other sources, such as OSL dating, uranium-series dating, and lichenometry are in calendar years (yrs). In an attempt to assess the Lower Valley alluvium for recycled ancient organic matter, pollen samples were analyzed from three alluvial sections. The late Holocene pollen assemblages are all typical of local northern Chihuahuan Desert plant communities. None of the pollen samples yielded pre-Quaternary spores, pollen, or dinoflagellates (Holloway, 1994; personal communication) that would indicate the presence of recycled old carbon thereby producing radiocarbon ages that could be too old for the age of the alluvium. 4. Lower Valley floodplain geomorphology The Rio Grande valley undergoes a major constriction at El Paso/ Juarez. Upstream from El Paso in the Mesilla Valley, the floodplain is 6 km wide. Within a short distance, however, the bedrock foothills of the Franklin and Juarez mountains constrict the valley to a narrow floodplain 0.24 km in width. Downstream from El Paso the valley abruptly widens to 8 km, forming the Lower Valley of El Paso. The Lower Valley is the only land suitable for irrigation farming in the El Paso area. Although prehistoric irrigation canals are unknown, an early historical record indicates that the Spanish

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Table 1 Radiocarbon dates from Rio Grande floodplain alluvium, Texas and New Mexico. Field loc., depth

Lab No.a

Lower Valley of El Paso, Texas Trench 1, 50e60 cm Tx-7376 Trench 3, 125e130 cm Tx-7444 Trench 6, 45e50 cm Tx-7377 Trench 8, 60e70 cm Tx-7514 Trench 9, 160e165 cm Tx-7512 Trench 12, 133e136 cm Tx-7513 Trench 14, 38e41 cm Tx-7516 Trench 14, 133e138 cm Tx-7515 Trench 16, 200e210 cm Tx-7446 Trench 18, 10e30 cm Tx-7441 Trench 18, 40e60 cm Tx-7443 Trench 18, 70e90 cm Tx-7442 Trench 19, 180e190 cm Tx-7445 Trench 21, 20e30 cm Tx-7883 Trench 22, 82e92 cm Tx-7886 Trench 22, 120e135 cm Tx-7885 Trench 24, 55e65 cm Tx-7884 Trench 25, 135e145 cm Tx-7882 Trench 33, 100e110 cm Tx-8130 Trench 33, 160e170 cm Tx-8132 Trench 35, 46e56 cm Tx-8131 Trench 50, 40e50 cm Tx-8544 Trench 50, 110e120 cm Tx-8543 Los Ranchos de Albuquerque, New Mexico El Prado Road NW 45e50 cm (artificial fill) Beta-222391 65e70 cm Beta-225920 78e83 cm Beta-222390 138e143 cm Beta-222389 Laredo, Webb County, Texas 200, 210e220 cm Beta-262458

Measured radiocarbon age (BP)

d13C &

Corrected radiocarbon age (BP)

2-Sigma calibrated age (cal years BP)b

1170 2260 1600 3290 1620 2350 1980 3820 3180 1890 2220 2300 7350 1020 1900 2260 2320 3380 2450 1980 1300 1350 1690

 60  60  60  60  50  70  60  90  60  50  60  60  120  50  50  60  60  60  60  60  50  60  50

19.1 23.4 21.4 22.4 24.5 24.2 23.7 22.2 23.3 20.2 21.7 21.1 24.0 20.0 21.5 22.1 24.6 21.7 22.3 21.4 20.2 22.5 21.4

1270 2280 1650 3330 1630 2360 2000 3860 3200 1970 2280 2360 7370 1100 1950 2310 2330 3430 2490 2040 1380 1390 1750

                      

60 60 60 70 50 70 60 90 60 50 60 60 120 50 50 60 60 60 60 60 50 60 50

1064e1294 2125e2367 1409e1696 3396e3720 1402e1627 2320e2706 1857e2115 4068e4451 3325e3570 1818e2051 2125e2367 2304e2702 7970e8389 928e1095 1776e2002 2150e2488 2153e2500 3556e3847 2429e2735 1871e2150 1233e1381 1226e1404 1549e1744

2460 1980 1790 2110

   

40 40 40 40

23.9 23.7 22.3 23.8

2480 2000 1830 2130

   

40 40 40 40

2433e2719 1867e2060 1693e1871 1995e2303

1570  40

19.2

1670  40

1513e1696

Note: Tx ages are on humus (solid organic particles) from clayey alluvium; Beta ages are from bulk clayey sediment. a Tx, Radiocarbon Laboratory, University of Texas at Austin, Texas, USA; conventional ages Beta, Beta Analytic, Inc., Miami, Florida, USA; AMS ages. b INTCAL 09; Reimer et al., 2009; Stuiver and Reimer, 1993; >90% relative area.

introduced irrigation farming in the valley in 1659e1661 (White, 1950; Ackerly, 1994). During the Pueblo Revolt of 1680 in northern and central New Mexico, the Spanish along with the Piro, Tompiro, and Tiwa fled to El Paso, establishing the pueblos of Ysleta del Sur and Socorro del Sur, the first large communities in the area. Irrigation agriculture in the Lower Valley expanded at that time (Peterson et al., 1994). Historical records also indicate that in 1726 the valley incorporated a number of irrigation ditches that diverted water from the Rio Grande (White, 1950; Ackerly, 1994). The surface of the floodplain has been dramatically altered in the twentieth century by expanded irrigation farming and flood control. Because of the irregular surface of the floodplain and the requirements of gravity-fed irrigation, topographically high areas were leveled and low areas were filled in with sediment from other places, such as occurred at Ysleta (Leach, 1996) and Los Ranchos de Albuquerque (this study, discussed later). “Most of the soils used in farming along the Rio Grande have been altered in one way or another.Old channels of the river have been filled and leveled; sandy material has been added to clayey soils to make the surface layer less clayey; and clayey material has been mixed into the surface layer of sandy soils” (Jaco, 1971, p. 58). Indeed, according to the information in the El Paso County soil survey, 87.5% of the irrigated cropland on the Rio Grande floodplain has been leveled by a combination of scraping high places and filling low areas (Jaco, 1971, p. 9e18). The straightening of the Rio Grande channel and construction of levees further altered the floodplain, resulting in a 70e150-m wide zone of disturbance and “made land” on the US side of the floodplain along the USeMexico border (Jaco, 1971). In the present study, the stratigraphy exposed in 50 trenches shows

that the upper 25e40 cm of the floodplain sediments are everywhere disturbed, a consequence of more than three centuries of irrigation farming in the valley and the twentieth century leveling of farm fields. Maps and aerial photos show the presence of four channels on the Rio Grande floodplain downstream of El Paso in the Lower Valley: two presently-abandoned channels on the US side, the Rio Grande channel, and an abandoned channel on the Mexico side; the old channel in Mexico was not investigated (Fig. 1). The channels on the US side are all shown clearly and named in the 1852 map produced by José Salazar Ilarregui, representing the Government of Mexico for the international boundary commission. Major José Salazar Ilarregui (1823e1892) was a well-respected engineer and administrator, serving the Republic of Mexico throughout his life; his papers are housed in the Special Collections of the University of Texas at Arlington. The 1852 Salazar map is detailed, showing roads, canals, farm fields, settlements, and the old abandoned river channels on both sides of the international boundary. The Rio Grande is called Rio Bravo del Norte on the 1852 map. Two abandoned channels on the United States side of the Salazar map are named Rio Viejo del Bracito and Rio Viejo de San Elizario (the spelling on the 1852 map is Rio Viejo de San Elzeario) (Emory, 1857). We use the names from the Salazar map with modern spellings. A second map, produced in 1855 by Maurice von Hippel for the United States international boundary commission, was not as detailed as the Salazar map. Von Hippel was a civilian surveyor; he worked on the southern boundary of New Mexico and led surveys of seven maps of the Rio Grande between El Paso and the Big Bend (Rebert, 2011). Only one of the old channels is shown on the 1855

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von Hippel map, the Rio Viejo del Bracito, and it is called “Old River or Rio Viejo” (Emory, 1857). The Rio Viejo de San Elizario of the Salazar map is not shown on the von Hippel map. The locations and course of the channels are verified by 1936 black and white stereo aerial photographs as well as by aerial photos taken in 1942e1943 and in 1979, although by 1979 large segments of the abandoned channels are obscured by artificial fill and urbanization. Aerial photographs from 2011 show that the old abandoned channels in the Lower Valley are now almost entirely filled and obliterated by urbanization and farming. 4.1. Rio Grande channel The modern Rio Grande channel was rectified in the 1930s to stabilize the international border between Mexico and the United States; the treaty authorizing the straightening and making the channel permanent was signed in 1933 (United States of America, 1934). The newly rectified channel downstream from El Paso is shown on a series of black and white aerial photographs taken in December 1936. The channel prior to rectification stands out clearly; the just-abandoned dry channel floor appears as clean sand without the presence of water or riparian plants or weeds. The abandoned channel floor varies in width from 37 to 103 m. Vegetated scars from meander bends and loops on both sides of the channel have a maximum width of 1400 m. Comparison of old maps and aerial photographs show that the channel has made only small changes in its geometry and position in the Lower Valley in the past 150 years. The changes have all occurred within a comparatively narrow zone along the channel itself, no more than 1400 m in width and considerably less than that in most stretches of the river. An interesting notation on the 1855 von Hippel map, signed by Commissioner Emery and Major Salazar: “The two Maps [1852 and 1855] agree, except in the bed of the River [Rio Bravo del Norte or Rio Grande], which circumstance is the consequence of the two Surveys being made at different periods, six months apart, during which time the River changed its bed as it is constantly doing but within narrow limits” (Emory, 1857; Mueller, 1975). The alluvium exposed in our trenches near the pre-rectified channel is characterized by a succession of massive to laminated fine sand and silt and clay beds that represent near-channel overbank deposits. The channel may not have meandered at all during the late

Holocene except within a narrow zone. At T-14, located 570 m east of the pre-rectified channel, a reddish brown silty clay at 135 cm depth is radiocarbon-dated 3860  90 yr BP, the earliest age from the floodplain near the channel (Fig. 2). Thus, the channel zone appears to have been stable without a significant change in lateral position during the past 4300 cal yrs. 4.2. Rio Viejo del Bracito channel The Rio Viejo del Bracito channel, or ‘Rio Viejo’ on the 1855 von Hippel map, originates in present-day El Paso. The old channel diverges from the Rio Grande channel in the vicinity of Ascarate Park between 106 240 and 106 250 west longitude. It runs along the northern (eastern) edge of the Lower Valley where in some places it has eroded the edge of the valley escarpment (Fig. 1). The channel is sinuous, and the modern floor of the channel where preserved is about 3 m below the level of the floodplain and has a width of 65e 70 m, based on measurements from the 1936 aerial photographs. Taking into account the sediments exposed in T-12 on the channel floor that are derived from deposition in the channel itself, the maximum scouring depth of the old channel is about 4.8 m below the floodplain surface. The channel formed during the late Holocene. The radiocarbon age of a thin bed of red clay about 135 cm below the floor of the channel at T-12 is 2360  70 yrs BP (Fig. 3). Overbank deposits near the channel are similar to other deposits of overbank fines throughout the floodplain and incorporate the Socorro paleosol. The Bracito channel was active as early as 2500 years ago at the same time that the cumulic Socorro paleosol was beginning to form on the floodplain. This is the channel that carried the flow of the Rio Grande valley in the early nineteenth century until the flood of 1829 when the principal flow shifted from the Rio Viejo del Bracito channel and reoccupied the present-day Rio Grande channel. 4.3. Rio Viejo de San Elizario channel The Rio Viejo de San Elizario channel takes its name from the community of San Elizario past which it flows. On the 1852 Salazar map, the channel is spelled Rio Viejo de San Elzeario and the community is spelled San Elzeario, named for the thirteenth century French patron Saint Elzear. The Rio Viejo de San Elizario channel

Fig. 2. Radiocarbon dated stratigraphic sections, Lower Valley of El Paso; radiocarbon ages in

14

C years BP (Table 1).

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Fig. 3. Radiocarbon dated stratigraphic sections associated with the Rio Viejo del Bracito and Rio Viejo de San Elizario, Lower Valley of El Paso; radiocarbon ages are in 14C years BP (Table 1).

occupies a central position in the Lower Valley. Based on aerial photographs and field inspection, the channel appears to split off of the Rio Viejo del Bracito at a point about 2 km northeast of Socorro. The present-day channel has a sinuous pathway, similar to that of the Bracito channel (Fig. 4). It extends about 3 m below the level of the floodplain and varies in width from 46 to 68 m. The maximum

depth of the channel is about 4 m below the floodplain surface, based on the amount of sediment fill in T-24. A trench (T-24) excavated into the floor of the channel reveals a meter of reddish brown clay and massive brown clay. The base of the brown clay is radiocarbon dated 2330  60 yrs BP, indicating that the channel was active by about 2400 cal yrs, similar to the age

Fig. 4. Dashed white line is the old channel of the Rio Viejo de San Elizario; from NASA color infrared aerial photograph taken in 1979; this area is now largely urbanized; direction of flow in the channel is from left to right in this view.

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of the Bracito channel and the formation of the Socorro paleosol (Fig. 3). The stratigraphy of the floodplain deposits adjacent the channel does not indicate that the channel has meandered, a conclusion supported by aerial photographs and the 1852 Salazar map. Long-time residents of the community of San Elizario have told us that in the 1960s and before that time, the channel of the Rio Viejo de San Elizario held standing water and was home for ducks and cattails. Subsequently, the lowering of the water table has eliminated standing water in the old channel. In 1991, we observed that the channel is almost entirely dry, although in places salt cedar and small clumps of cattails persist and the surface sand of the channel floor is encrusted by puffy gypsum, precipitated upon evaporation of sulfate-rich groundwater close to the surface. 4.4. Historic channel shifting During the early part of the nineteenth century and when Mexico became independent from Spain in 1821, the principle flow of water in the Lower Valley was through the channel of the Rio Viejo del Bracito with the communities of Ysleta, Socorro, and San Elizario situated on the south side of the river. These communities are also shown on the south side of the Rio de el Norte on the 1727 map by Don Francisco Álvarez Barreiro (Barreiro, 1727) and on the south side of the Rio del Norte on the 1744 map by Fray Juan Miguel Menchero (Menchero, 1744; Peterson, 2002). If historically accurate, the Rio de el Norte and Rio del Norte on the coarse-scale Barreiro and Menchero maps from the eighteenth century are the same channel as the Rio Viejo del Bracito on the 1852 Salazar map and the Rio Viejo on the 1855 von Hippel map. In the spring of 1829, the Lower Valley flooded, and, when the water receded, the principle flow was in the channel of the Rio Grande (or Rio Bravo del Norte as it was called by the international boundary commission in the mid-nineteenth century). For several years water flowed in

both channels, leaving the communities of Ysleta, Socorro, and San Elizario on an island, locally called La Isla (Fig. 5). By 1848, however, flow had ceased in the Rio Viejo del Bracito and was confined to the Rio Grande channel. In that year, the Treaty of Guadalupe Hidalgo defined the international boundary as “the Rio Grande.following the deepest channel” which was the Rio Grande of today (Timmons, 1980, 1990). Since 1848, the ‘Rio Viejo’ channels have not carried flowing water. Thus, the 1829 shift in flow or avulsion from the Rio Viejo del Bracito channel to the Rio Grande channel occurred during one flood event, although the complete shift in flow may have occurred over a period less than 19 years. The length of the avulsion from the city of El Paso to near the community of Tornillo is 49.3 km, following the modern rectified channel. Other nearby stretches of the Rio Grande have undergone channel shifts in historic times. The best known case is the Mesilla Valley north of El Paso where the Rio Grande channel had been surveyed six times before 1844 and to 1912. The channel in 1844 and 1852 flowed between Mesilla and La Cruces, with Mesilla on the west bank of the river. During a major flood in 1865, the channel changed course and began flowing west of Mesilla, leaving Mesilla on east bank of the river where it remains today (Mack and Leeder, 1998). The Mesilla Valley and Lower Valley records of channel avulsion are mismatched with regard to specific flood events. The various shifts in channel position seem to have been triggered by different floods, each flood pushing (or not pushing) the Rio Grande channels beyond an undetermined threshold, resulting in an avulsion event in some cases and not in others. 5. Lower Valley floodplain stratigraphy Floodplain sediments and stratigraphy were investigated with 50 trenches located in different geomorphic settings on the

Fig. 5. Map of the three channels on the US side of the international boundary showing the location of the avulsion and channel reoccupation in 1829. Maps from 1727 to 1744 show the missions of Ysleta and Socorro on the south side of the river, indicating that river flow at that time was in the Rio Viejo del Bracito.

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floodplain over an area of about 150 km2, including topographic highs and lows, channel floors, adjacent channel margins, inside and outside meander bends, and in areas of eolian sand. The floodplain sediments exposed in the trenches are separated into two units based on a disconformity in the sequence. 5.1. Alluvial unit 2 At the base of most trenches occurs a light yellowish brown to very pale brown (10YR 6-7/3-4) fine-to-medium quartz sand. The unconsolidated sand is well sorted and can be massive, laminar, or with cross-beds less than 10 cm high; small pebble gravel to 12 mm diameter is occasionally present, and some sand exposures have iron staining. Silt and clay beds are rare. Sand-sized particles of muscovite are a conspicuous component of the sand and reflect the presence of granitic rocks in the upper drainage basin. Buried soil horizons are absent in the unit 2 sand. We interpret the unit 2 sand as representing channel deposits, formed when the Rio Grande was either a meandering or braided stream. The deepest trench exposure of the sand is 3.5 m; the depth of the base of the sand and the nature of the sediment below 3.5 m are less well known. Only one radiocarbon age was obtained from unit 2. A clay bed at 175e185 cm depth in T-19 is dated 7370  120 yrs BP or about 8200 cal yrs, the earliest age obtained from the floodplain in this study. Water wells on the Lower Valley floodplain generally encounter gravel at 20e25 m depth and the gravel is commonly overlain by 3e6 m of coarse sand; overlying the coarse sand up to the modern floodplain is fine sand and clay (various well reports, Texas Water Development Board). The fine sand and clay is late Holocene overbank alluvium (unit 1). Pending further investigation, we conjecture that the coarse sand and gravel represent latest Pleistocene and early Holocene channel deposits. 5.2. Alluvial unit 1 Unit 1 represents the upper 90e250 cm of fine textured alluvium on the Lower Valley floodplain. The lower unit 1 alluvium consists of light reddish brown to light brown to reddish yellow (5YR 6/3-4, 7.5YR 6/3-6) silty clay. The clay is massive and can have small sparry crystals of gypsum less than 2 mm in diameter. The clay has slickensides in places, and in some cases the upper 25 cm of the red clay bed can have sandfilled cracks. Occasional shells of Helisoma, a common aquatic snail, are present. In one case (T-25), a thin bed of the reddish clay represents a clay plug in a small buried channel that has no surface expression. In rare cases the clay can have small soft masses of calcium carbonate granules 5e10 mm in diameter that may be related to groundwater conditions. The lower deposits of unit 1 are probably associated with the Rio Grande and not the Rio Viejo channels. The earliest radiocarbon age from the lower unit 1 clays is 3860  90 yrs BP, or about 4300 cal yrs, from 135 cm depth at T-14 (Fig. 2). The upper unit 1 alluvium is predominantly very pale brown to pale brown to light yellowish brown to brown (10YR 4-7/3-4) silty clay, clayey silt, and very fine sandy silt. The silt and clay beds are commonly laminar to massive and contain small masses of fine gypsum crystals (discussed below). These fine textured deposits have occasional shells of Helisoma and Gyraulus, both common aquatic snails found in wet floodplain habitats. The fine-textured deposits are interpreted as overbank sediments, deposited across the floodplain during flood events. During the late stage of sediment accumulation on the Lower Valley floodplain, the clayey silt grades upwards to a hard, dark brown clay, interpreted as a cumulic A horizon soil or the Socorro paleosol.

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5.3. Socorro paleosol (new name) The upper-most surficial deposit on the broad present-day floodplain is characterized by a 40e130 cm thick, massive, hard, dark brown to brown (10YR3-4/3) silty clay. The clay content varies from 30 to 80% and with the amount of organic carbon ranging from 0.2 to 1.0%; the percentages of clay and organic carbon generally increase upwards. Carbonate ranges from 5 to 10%, generally increasing with depth although not visible in exposures. The clay has a blocky structure and, in a few rare cases, weak slickensides. Visible aggregates of gypsum from 0.3 to 2.0 mm in diameter occur throughout the clay. In one profile (T-18), the amount of gypsum ranged from 0.2 to 1.2% by weight (Fig. 6). The massive brown clay is interpreted as a cumulic A horizon soil with characteristics of a Mollisol; it is no longer forming (Fig. 7). In some cases the base of the soil has a gradual or diffuse contact with underlying sediment. In other cases, the base of the soil is a sharp depositional boundary. It is named the Socorro paleosol in this paper for its occurrence and first recognition (T-1) at the old community of Socorro, El Paso County, Texas. The Socorro paleosol occurs at the top of the sequence of floodplain deposits at eight trench localities where it has been radiocarbon dated (T-1, 6, 9, 18, 21, 22, 35, 50) as well as at the top of several other trench exposures across the Lower Valley where radiocarbon ages were not obtained. Cumulic soils, such as the Socorro paleosol, are products of slow aggradation. The stratigraphy of four localities of the Socorro paleosol (T-9, 22, 35, 50) show that the formation of the soil was temporarily halted by the local deposition of 20e50 cm of silt and very fine sand (Figs. 3 and 8). The silt and very fine sand beds are probably a consequence of a large flood or series of floods in the Lower Valley that interrupted the slow accumulation of the cumulic soil on the floodplain and temporarily burying it. After deposition of the silt and very fine sand, the formation of the cumulic Socorro paleosol resumed. The silt and sand beds are bracketed by radiocarbon ages, indicating that the flood event(s) occurred sometime during the period between c 1660 and 1300 cal yrs. 5.3.1. Radiocarbon age and correlation of the Socorro paleosol A series of eleven radiocarbon ages from the Socorro paleosol range from 2360  60 yrs BP (T-22) to 1100  50 yrs BP (T-21), indicating that the paleosol formed during the period 2500 to 1000 cal yrs (Table 1). The Socorro paleosol has a closer parallel to the late Holocene alluvial stratigraphy of the Great Plains than with alluvial sequences in the Southwest. It correlates directly with the Copan paleosol in the southern Great Plains. Both paleosols formed during the same late Holocene period of wet climate. The Socorro paleosol shares the same sedimentology, geomorphic position in an alluvial valley, environment of formation, and radiocarbon age that have been documented for the Copan paleosol. The Copan paleosol was named from an alluvial sequence in northeastern Oklahoma by Hall (1977) and subsequently has been identified and radiocarbon dated throughout the southern Great Plains (Hall, 1990; Hall et al., 2012a). 5.4. Gypsum in clay Visible aggregates of fine gypsum crystals occur abundantly in the clayey alluvium and cumulic Socorro paleosol, especially in the upper levels, appearing as small white spots and streaks in the brown clay. Gypsum occurs as very small aggregates (0.3e0.6 mm diameter) in soil pores. The gypsum-filled pores are aggregates of both round (1e2 mm diameter) and elongated (up to 7 mm) clumps (Figs. 7 and 9). Gypsum crystals occur intermixed with clay in aggregates of 0.6e0.8 mm diameter. Gypsum occurs only in clayey sediment and is absent in sand.

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Fig. 6. Particle size and chemistry of the Socorro paleosol at trench 18; the increased gypsum upwards may indicate upward movement of moisture with precipitation of gypsum in upper levels of the soil column.

A soil-gypsum profile from the Socorro paleosol indicates an upward movement and precipitation of gypsum high in the sediment column (Fig. 6). The water table, the source of the gypsum, was 3.5 m below the surface of the floodplain at the location of the soil profile (T-18). Permian gypsum and gypsiferous sandstone occur in the San Andreas and Hueco mountains of southern New

Mexico and adjacent Texas, providing a source of gypsum found in younger sediments, groundwater, and surface water in the Rio Grande valley. An early study of groundwater from shallow wells 2e6 m deep in the Rio Grande valley documented an average sulfate content of 545 ppm (Sayre and Livingston, 1945, p. 51, 156). More recent analyses of water from wells to 30 m depth between Socorro and San Elizario show 1000 to 4000 total dissolved solids and up to 4240 ppm sulfates (Alvarez and Buckner, 1980). The groundwater in the Rio Grande valley has a sufficiently high amount of sulfate to account for the gypsum in the Socorro paleosol and associated clay. Three centuries of irrigation farming in the Lower Valley using water derived from the Rio Grande may also contribute to the high gypsum content found in the clayey alluvium and Socorro paleosol. 6. Eolian sand on the floodplain

Fig. 7. Exposure of Socorro paleosol at trench 22; the band of silty very fine sand was deposited on the floodplain, interrupting the accumulation of the soil; the white specks are small aggregates of gypsum crystals.

A few areas of eolian sand and low dunes on the floodplain can be identified on the 1936 aerial photographs. The El Paso county soil survey names and maps the ‘Brazito loamy fine sand’ which coincides with areas of surficial deposits identified in our study as eolian sand. The Brazito soil is mapped on 784 acres or 0.1% of El Paso County (Jaco, 1971, p. 9). We investigated a small area of sand dunes northwest of San Elizario (Fig. 1). The landowner on whose property the dunes occur told us that most of the sand dunes were leveled in the late 1940s for farmland. The eolian deposits are massive very pale brown (10YR7/4) very fine-to-fine quartz sand and, in one small area where the dunes had not been leveled, extend about 5 m above the floodplain. At the margin of one patch of eolian sand (T-19), a radiocarbon age of 7370  120 yrs BP (c. 8200 cal yrs) was obtained from a massive brown fluvial clay below a 1.6-m thick deposit of eolian sand (Fig. 2). All of the preserved eolian deposits form topographic highs above the floodplain, and all of the dunes apparently have archaeological sites. An archaeological site on dune sand near our

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Fig. 8. Radiocarbon dated sections with the Socorro paleosol; radiocarbon ages are in

study trenches (T-19, 20) has El Paso brown ware ceramics indicating an age of A.D. 1000 to 1400 (Peterson, 2002). In a comment on the presence of archaeological sites on the Rio Grande floodplain in the Las Cruces, New Mexico, area just north of El Paso, Lehmer (1948, p. 1) observed that “The only [sites] on the valley floor were located in sand hills which had not been leveled into fields.” Other sand dunes on the Rio Grande floodplain in the Mesilla Valley are reported to have prehistoric sites (O’Laughlin, 1985). Extensive deposits of eolian sand, including twentieth century coppice dunes, mantle older upland surfaces along the floodplain escarpment and beyond and are part of the Bolson sand sheet in the Hueco Bolsone Tularosa Valley (Hall et al., 2010).

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14

C years BP (Table 1).

The age of the eolian deposits on the Lower Valley floodplain is poorly constrained. However, based on the evidence of the 8200year age of alluvium beneath eolian sand at one locality (T-19) and the earliest age for overbank deposition of alluvium on the floodplain (T-14) at 4300 cal yrs, the eolian deposits are coarsely bracketed by 8200 and 4300 cal years. Accordingly, we suggest that the accumulation of the eolian deposits on the Lower Valley floodplain occurred during the middle Holocene. The sand that forms the dunes on the floodplain probably originated from early Holocene channel deposits (unit 2) that would have been exposed across the valley floor at that time before being buried by younger overbank silt and clay deposits. 6.1. Sand source for the Bolson sand sheet

Fig. 9. SEM photograph of an aggregate of gypsum crystals from the Socorro paleosol.

The Bolson sand sheet, mapped on the floor of the Hueco Bolson of Texas and Tularosa Valley of New Mexico, is immediately adjacent and down-wind (northeast) of the Lower Valley (Hunt, 1978; Johnson, 1997; Collins and Raney, 2000, 2002) (Fig. 10). The sand that makes up the Bolson sand sheet was thought to have been derived from the underlying Plio-Pleistocene Camp Rice Formation (Seager et al., 1987). However, the Camp Rice fluvial sand and gravel deposits are capped by a stage IV calcrete paleosol, eliminating the Camp Rice as a possible source for eolian sand. We suggest instead that the sand source for the Bolson sand sheet is the Rio Grande floodplain in the Lower Valley. Deflation alignments on satellite imagery across eolian sand west of El Paso indicate a resultant wind direction of 23 N of E (Langford, 2000), a direction favoring sand transport from the valley floor to the sand sheet. The unit 2 channel deposits in the Lower Valley are massive to cross-bedded fine to medium quartz sand. The 20-m thick channel sand is early Holocene and perhaps latest

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Fig. 10. Map of the Rio Grande and the floodplain of the Lower Valley of El Paso, sand source to the Bolson sand sheet; sand accumulation ended 5000 yrs ago when the Lower Valley floodplain was first covered by overbank muds, thus sealing off the supply of new sand to the sand sheet.

Pleistocene. Based on OSL dating, the age of the Bolson sand sheet is 45,000 yrs (unit Q2) and 22,000e5000 yrs (unit Q3). Eolian sand deposition on the sand sheet ended ca 5000 yrs ago (Hall et al., 2010, 2012b). By 4300 cal years, overbank deposition of finetextured alluvial silt and clay across the Lower Valley covered and buried the early Holocene channel sand, sealing it off and removing it as a source for eolian sand deposits. The end of eolian sand accumulation on the Bolson sand sheet by 5000 yrs is consistent with the deposition of overbank fines beginning before 4300 cal yrs on the Lower Valley floodplain and the elimination of the floodplain as a sand source at that time. If this reconstruction is correct, it is noteworthy that the termination of eolian sand accumulation on the Bolson sand sheet coincides with a shift to wetter conditions that shut down the sand source. Furthermore, if the Rio Grande is the source for the sand in the Bolson sand sheet, the river would have had a sandy bed load and perhaps a braided channel during the late Pleistocene.

7. Other Rio Grande floodplain records The results of two pilot studies of floodplain alluvium, one upstream and one downstream from El Paso are presented. Although their floodplain depositional sequences differ from that of the Lower Valley of El Paso, they provide support for the floodplain constructionepaleoclimatic relationship observed in the Lower Valley. 7.1. Los Ranchos de Albuquerque, Bernalillo County, New Mexico The Rio Grande floodplain is 5 km wide at Los Ranchos de Albuquerque. The presence of prehistoric pueblos and other archaeological sites on the floodplain led Sargeant (1987) to conclude that the river had not meandered across its floodplain during late Holocene time, a conclusion supported by our work at Los Ranchos. A shallow trench located 0.98 km east of the present-day

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Rio Grande channel at Los Ranchos indicates that the floodplain has been stable at this locality without lateral erosion for at least the past 2100 cal yrs. The floodplain deposits are 1.1 m of overbank clayey silt to very fine sand overlying channel gravelly medium-to-coarse sand. The overbank fines are thin-bedded with shells of several species of aquatic snails and a pill clam, indicating a wet meadow habitat with temporary shallow standing water. AMS ages for the sequence indicate that the wet floodplain sediments were deposited between 2100 and 1700 cal yrs (Fig. 11). The moist floodplain habitat and associated deposits are consistent with the wetter late Holocene conditions that produced the Socorro paleosol. The floodplain sequence at Los Ranchos is locally covered by 54 cm of artificial fill. The fill sediment is massive clayey sand, forming an irregular basal contact with the underlying bedded overbank deposits. The AMS age of the fill clayey sand is about 800 cal yrs older than the younger overbank sediment. We were not able to discover where the fill sediment was obtained. Nevertheless, the presence of the fill material overlying intact deposits is an example of recent disturbance of the Rio Grande floodplain, similar to situations in the Lower Valley.

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1932; Gustavson and Collins, 1998). North of Laredo, we selected for study a terrace surface about 7 m above water level of the Rio Grande that corresponds to the intermediate 8e12 m terrace of Gustavson and Collins (1998). Two deep trenches on the terrace encountered a 50e70 cm thick grayish brown (10YR 5/2) massive clayey silt with blocky structure at about 2 m depth below the terrace surface (Fig. 12). The hard brown clayey silt contrasts with the soft pale brown to light brownish gray (10YR 6/2-3) laminar silt that make up the rest of the sediment columns. The brown clayey silt is interpreted as a cumulic A horizon soil and is identical to the Socorro paleosol. An AMS radiocarbon age on bulk sediment from the soil is 1670  40 yrs BP, falling within the period of formation of the Socorro paleosol. We correlate the buried paleosol in the middle terrace deposits at Laredo with the Socorro paleosol in the Lower Valley at El Paso.

7.2. Laredo, Webb County, Texas Three alluvial terraces have been documented along the lower Rio Grande valley in the vicinity of Laredo (Trowbridge,

Fig. 11. Stratigraphy section at El Prada, Los Ranchos de Albuquerque; u, unconformity; s, scouring surface; AMS radiocarbon ages are in 14C years BP (Table 1).

Fig. 12. Stratigraphic column from the intermediate terrace deposits of the Rio Grande north of Laredo, Webb County, Texas; the buried soil correlates with the Socorro paleosol; AMS radiocarbon age in 14C years BP (Table 1).

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8. Late Holocene paleoclimate Countless studies, reports, and papers have been released dealing with the late Quaternary paleoecology of the American Southwest. Yet, definitive reconstructions of environmental conditions during the late Holocene are uncommon. In contrast, the late Holocene paleoenvironmental record from the southern Great Plains is moderately well established. Below, we discuss some of the more significant regional studies that pertain to our study of the Rio Grande floodplain deposits near El Paso. 8.1. Historic Rio Grande discharge and weather records Most of the discharge of the Rio Grande is derived from snowmelt in the southern Rocky Mountains of Colorado and New Mexico. Highest discharge along the Rio Grande occurs during the months of May, June, and April, in that order (peak discharge at El Paso is in June) (Fig. 13). A slight increase in flow occurs in October, related to monsoon rains, but on average it is trivial compared with that related to spring snowmelt. These discharges are recorded at gaging stations along the Rio Grande as observed at preimpoundment years 1895 through 1913 (U.S. Geological Survey, 1960). From these data we suggest that the discharge levels of the Rio Grande during the Holocene are regulated by climatic conditions. Periods with low temperatures and high precipitation produce greater amounts of snowfall in the upper Rio Grande watershed, resulting in greater discharge from spring snowmelt.

Likewise, periods with high temperatures and low precipitation produce less snowfall in the watershed and, accordingly, lower discharge in the Rio Grande. Thus, we interpret the fluctuating behavior of the Rio Grande as related to Holocene climate change. 8.2. Neoglacial records from southern Rocky Mountains Episodes of late Holocene glaciation or Neoglaciation have been identified in the southern Rocky Mountains of Colorado. Of special interest to us, the Audubon glacial interval with associated geomorphic activity is documented between about 2300 and 1000 cal yrs (Benedict, 1985) (Fig. 14). Ninety percent mortality of lichen colonies also occurred during the Audubon, evidence for increased late-season snow cover (Benedict, 1993). Greater snow cover would lead to greater spring runoff events in the Rio Grande drainage basin, higher alluvial water tables, and increased incidence of flooding and deposition of overbank muds, along with the formation of a cumulic Mollisol such as the Socorro paleosol. Neoglacial activity in the southern Rocky Mountains of New Mexico is not yet identified. Glacial moraines once thought to be Neoglacial (Armour et al., 2002) are now known to be pre-Younger Dryas in age based on cosmogenic 10Be dating of boulders on the moraines (Davis et al., 2009). 8.3. Rock glaciers from southern Rocky Mountains Numerous rock glaciers, both active and inactive, have been documented throughout the southern Rocky Mountains of

Fig. 13. Snow, rainfall, and Rio Grande discharge prior to the construction of reservoirs in the drainage basin after 1913; the greatest discharge is related to runoff from spring snowmelt in the headwaters of the drainage basin; the slightly elevated discharge in October is from local rainfall during the late summer monsoon.

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Fig. 14. Correlation chart of the Lower Valley floodplain stratigraphy with regional paleoenvironmental records, all indicating the formation of the Socorro paleosol during wet-cool conditions; also, middle Holocene aridity shows up in some records; A, Lower Valley of El Paso, Texas (this study); B, Neoglaciation in the southern Rocky Mountains of central Colorado; C, Paleoclimate based on speleothem growth-band thickness, Carlsbad Caverns area, southeastern New Mexico; uranium series ages are revised in Rasmussen et al. (2006) indicating the end of band formation in one cave ca 1000 yrs; D, precipitation values calculated from a series of stable carbon isotope values from the Abo Arroyo alluvial sequence, central New Mexico; E, late Holocene alluvial sequence with the Copan paleosol from the southern Great Plains.

Colorado (Giardino and Vitek, 1988; Janke, 2007). Using growth curves of lichens, rock glacier activity in central Colorado occurred during three episodes at about 3080 yrs, 2070 yrs, and 1150 yrs (Refsnider and Brugger, 2007). Growth and movement of rock glaciers are regarded as an index to cold conditions. The latter two episodes correlate with the Audubon Neoglacial (discussed above), and all three episodes are within the late Holocene period of wetter climate in the region. 8.4. Speleothems from Guadalupe Mountains In general, many cave formations in the Southwest are a product of wetter climates during the Pleistocene, and Holocene formations are often missing due to comparatively overall drier conditions (Polyak et al., 2004; Brook et al., 2006). In a few cases, Holocene speleothems have been discovered and analyzed. Columnar stalagmites from the Carlsbad Caverns area of southeastern New Mexico have been cored, dated by uranium series, and the growth bands measured. Thick bands are equated with wet conditions, thin bands with present-day or dry conditions, and a hiatus or absence of growth bands is regarded as drier than present (Polyak and Asmerom, 2001). The period with the thickest sequence of growth bands occurs between 3000 and 1700 yrs, indicating wet conditions. The paleoecology of mite species encased in the speleothems supports a wet-climate interpretation (Polyak et al., 2001). In one case, stalagmite growth ceased about 1000 yrs, indicating a shift to dry conditions at the cave (Rasmussen et al., 2006). The speleothem record from the Carlsbad Caverns area is consistent with other late Holocene indicators of wet climate and a change to dry conditions after 1000 yrs (Fig. 14).

8.5. Stable carbon isotope record from western Great Plains A sequence of AMS-dated alluvium with accompanying d13C values provides a view of paleoclimate for the past 12,800 yrs at the western edge of the Great Plains in central New Mexico (Hall and Penner, 2013). The late Holocene is characterized by a period of greater moisture and cooler temperatures between 3300 and 1400 cal yrs (Fig. 14). At 1400 cal yrs, the climate shifted abruptly to dry-warmer conditions, although the change occurred about 400 yrs before the onset of warmer-drier conditions elsewhere. Regardless, the stable carbon isotope record of cool-wet climate compares favorably with other regional cool-wet paleoenvironmental indicators during the late Holocene. 8.6. Alluvial environments of the southern Great Plains The alluvial geology of the southern Great Plains has been moderately well studied during the past 40 years, most of the investigations a component of archaeological recovery projects. Although many streams exhibit some variation in their history of deposition and erosion, there are some patterns that appear across drainage basins from one stream to another. One notable similarity is the common presence of a late Holocene cumulic Mollisol near the top of many alluvial sequences, called the Copan paleosol (Fig. 14). Independent lines of paleoenvironmental evidence indicate that the paleosol formed during a period of slightly wetter and cooler climate in the southern Great Plains about 2300 to 1000 cal yrs ago, followed immediately by a period of drier conditions (Hall, 1990; Hall et al., 2012a). The period of wet-cool climate correlates with the formation of the Copan and Socorro paleosols.

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Table 2 Holocene stratigraphy, geomorphology, and climate, Lower Valley of El Paso, Texas. Time

Climate

Channel

Floodplain

Process

0e1000 years

Semiarid, similar to today; shift from wet to dry climate at 1000 yr Rainfall greater than present; cooler; persistent snowfields at high elevation in southern Rockies; Audubon Neoglacial, active rock glaciers, active speleothem growth Gradually less arid, increasing rainfall from hot/dry middle Holocene

Low discharge; Rio Grande channel stable within narrow zone; reduced channel avulsion to ‘Rio Viejos’ Greater discharge than present, originating from snowmelt in upper Rio Grande drainage basin; present-day channels well established; active channel avulsion Increasing discharge in stable channel on sandy floodplain; present-day channels begin to form across Lower Valley Lowest discharge; channel stable; Rio Grande may have shut down seasonally Greater discharge than present; sand and small gravel bed load; meandering or braided channel system across valley

Stable floodplain; low incidence of flooding; lowered water table

Little or no fluvial or eolian deposition on floodplain

High incidence of flooding, overbank deposition across valley; high alluvial water table

Deposition of overbank silt and clay; formation of Socorro paleosol, a cumulic Mollisol, at present-day floodplain surface; (upper unit 1)

Transition to new phase of floodplain construction by overbank flooding and vertical aggradation

Beginning overbank deposition of muds; (lower unit 1)

Floodplain stable; low alluvial water table; deflation of sandy valley floor

Little or no fluvial deposition; sand dunes form on stable floodplain

Floodplain construction by lateral cutting and backfilling by meandering or braided channel

Deposition of channel sand with cross-beds; (unit 2)

1000e2500 years

2500e5000 years

5000e6000 years

Peak of middle Holocene aridity; hot and dry climate

6000e10,000þ years

Greater rainfall than today but diminishing; transition from wet late Pleistocene to dry middle Holocene

9. Discussion The Rio Grande is a good example of an allogenic river. Its headwaters are in the southern Rocky Mountains where it receives much of its water from spring and early summer snowmelt. The river flows southward out of the mountains and through the semiarid and arid terrain of southern New Mexico and Texas. The downstream channel, floodplain, and their deposits are shaped by seasonal flooding events which in turn are largely the products of the changing amounts of snowmelt in the montane headwaters.

floods resulted in avulsion and others did not. Regardless, it is apparent in this case that the broader record of late Holocene floodplain aggradation and multiple channel formation is related to past climate, especially as connected to increased snowfall and snowmelt in the Rio Grande headwaters and flooding downstream. Within that template, conditions were set for avulsion by channel

9.1. Channel formation and avulsion The Rio Grande channels were established after the middle Holocene, the period when the river may have been intermittently dry and eolian sand accumulated on the floodplain. Between about 5000 and 4000 cal yrs ago, a shift to less arid climate resulted in increased river flooding and the initial deposition of overbank silty clay on the Lower Valley floodplain. Overbank deposits adjacent the Rio Grande channel are dated 4300 cal yrs. The formation of the Rio Viejo channels occurred somewhat later. Basal muds from the Rio Viejo del Bracito and Rio Viejo de San Elizario channels are dated 2500 and 2400 cal yrs, respectively. The Rio Viejo channels formed at the beginning of the period of high discharge from Audubon Neoglacial snowmelt in the headwaters. We speculate that increased discharge from large floods would move into low areas of the floodplain, resulting in the formation of the Rio Viejo channels. Once the series of three or more channels was established on the floodplain, the principal flow periodically shifted from one channel to another, especially during major floods. The avulsion in 1829 was from the active channel at that time (Rio Viejo del Bracito) to the pre-existing inactive channel (Rio Grande) on the Lower Valley floodplain. Avulsion by channel reoccupation is not uncommon and has been documented during the late Holocene in the Mississippi River (Aslan et al., 2005). Although the historic avulsions of the Rio Grande have occurred during major flood events, not all floods have resulted in channel change. The large floods of 1852, 1872, 1884, and 1904, for example, did not produce channel changes or avulsion, even though irrigation facilities on the floodplain were destroyed and small communities were washed out (Wozniak, 1997; Scurlock, 1998; Peterson, 2002). From our study, we cannot explain why some

Fig. 15. Summary stratigraphic diagram of Rio Grande floodplain deposits in the Lower Valley of El Paso, Texas; the number of calibrated radiocarbon ages is in 500-year intervals.

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Fig. 16. Cross-section sketch of the Rio Grande floodplain in the Lower Valley of El Paso, Texas; no scale.

formation, abandonment, and reoccupation during the late Holocene.

10. Summary and conclusions A summary of the stratigraphy and geomorphology of the Rio Grande floodplain in the Lower Valley of El Paso is outlined in Table 2 (Fig. 15). Specific conclusions from our investigations in the Lower Valley and our pilot studies at Los Ranchos de Albuquerque, New Mexico, and Laredo, Texas, are summarized below.  Three channels are identified on the Lower Valley floodplain: Rio Grande, Rio Viejo del Bracito, and Rio Viejo de San Elizario (a fourth channel in Mexico was not studied) (Fig. 16). The channels are shown and named on the 1852 Salazar map for the international boundary commission. The channels are visible on 1936 and more recent aerial photographs, although in 1991 many segments of the Rio Viejo channels were filled; by 2011, the old channels were almost entirely filled and obliterated.  The Rio Grande is the widest and deepest channel in the Lower Valley. It is the only channel exhibiting lateral migration and meander scars. Its present narrow channel zone and position on the floodplain were established by 4300 cal yrs. The earlier history of the channel is yet unknown.  The Rio Viejo del Bracito is today an inactive channel in the valley. However, prior to 1829 and perhaps before 1727, it carried the principal flow in the Lower Valley. During this period, the communities of Ysleta, Socorro, and San Elizario were on the south side of the river. In 1829 during a major flood, the river flow shifted to the Rio Grande channel, resulting in a large area of the valley, including Ysleta, Socorro, and San Elizario, becoming an island, La Isla, until flow in the Bracito channel ceased entirely by 1848. When the Rio Viejo del Bracito channel was finally abandoned by 1848, river flow was in the Rio Grande channel where it has remained with the three communities on its north side. The channel avulsion of 1829 was by the process of channel reoccupation.  The Rio Viejo de San Elizario channel has probably been dry during the past several centuries, except for standing water as a result of high water table. Both of the Rio Viejo channels were established and had flowing water by about 2500 cal yrs. At present, a history of channel occupation, abandonment, and reoccupation throughout the late Holocene has not been established.  The sediments in the upper 1e2.5 m of the Rio Grande floodplain are bedded and massive clay, silt, and very fine sand (unit 1) deposited by overbank aggradation beginning about 5000 cal yrs. The climate became drier 1000 cal yrs and overbank deposition halted, resulting in the stability of the floodplain. The age of the present-day floodplain surface is about A.D. 1000.

 The Socorro paleosol is a cumulic Mollisol and occurs in the upper 70e160 cm of the overbank deposits, generally at the surface of the present-day floodplain. It formed on the aggrading floodplain surface 2500 to 1000 cal yrs ago during a period of wet climate and high frequency of floods. The Socorro paleosol correlates with the Copan paleosol in the southern Great Plains.  The Rio Grande may have intermittently shut down during the hot-dry climate of the middle Holocene 6000 to 5000 yrs. Small patches of sand dunes formed on the floodplain at that time. The dunes form topographic highs on the floodplain, although most of the dunes were leveled for farm fields in the mid-twentieth century.  The late Pleistocene and early Holocene history of the inner valley is poorly known. The Rio Grande alluvium extends to 20e25 m depth where gravel and coarse sand are encountered in drill holes. Above the gravel and coarse sand is yellowish brown fine-to-medium textured sand that is massive or has cross-beds and occasional small pebble gravel (unit 2). The yellowish brown sand occurs at the base of all trench exposures. One radiocarbon age from it is 8300 cal yrs. We conclude that the ubiquitous sand represents channel deposits, formed by lateral cut and fill of a meandering channel or by a wide braided channel in the Lower Valley.  The late Pleistocene and early Holocene meandering-braided channel of the Rio Grande was the sand source for the Bolson sand sheet in the adjacent Hueco BolsoneTularosa Valley. The late phase of the Bolson sand sheet formed about 22,000e 5000 yrs, based on OSL dating. Beginning 5000 yrs, overbank deposition of muds on the Rio Grande floodplain cut off the sand supply to the sand sheet, resulting in the end of new eolian sand accumulation on the Bolson sand sheet.  The chronology and origin of floodplain deposits in the Lower Valley defines the preservation of prehistoric archaeological sites. It is unlikely that Paleoindian sites (13,000 to 7500 yrs) will be found in inner valley sandy alluvium due to lateral cutting of the channel during that period. Early and Middle Archaic sites (7500 to 3000 yrs) could be located at the base of late Holocene overbank deposits at 1e2.5 m depth. Late Archaic sites (3000 to 2000 yrs) could be incorporated in shallow overbank deposits. Mesilla or Pithouse sites (2000e950 yrs) may be within upper overbank deposits, including the Socorro paleosol. Doña Ana, El Paso (Pueblo), and Historic sites (950 yrs to present) occur at the modern floodplain surface and are not buried in the alluvium.  The intermediate terrace above the Rio Grande near Laredo, Texas, has a thick sequence of alluvium with a cumulic Mollisol buried 2 m below the terrace surface and dated 1600 cal yrs. It correlates directly with the Socorro paleosol in the Lower Valley floodplain. During the same period, an alluvial sequence at Los Ranchos de Albuquerque, New Mexico, is characterized by overbank fine-textured sediment with freshwater mollusks indicating late Holocene moist floodplain conditions.

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 At this stage of investigation, it has become apparent that the formation of the Rio Grande floodplain has been strongly influenced by climate, especially the wet environmental conditions of the late Holocene 2500e1000 yrs ago that resulted in the deposition of overbank muds and the formation of the Socorro paleosol. The presence of late Holocene deposits elsewhere indicating wet conditions, such as at Los Ranchos de Albuquerque and Laredo, suggest that the Rio Grande has responded broadly to a sub-continental regime of climate and climate change. And, as these studies continue and more is learned about the stratigraphy, chronology, and paleoecology of floodplain deposits up and down the river, it will not surprise us to see further broad patterns of fluvial response to climate along the Rio Grande.

Acknowledgments We thank Sam Valastro, Jr., University of Texas at Austin Radiocarbon Laboratory, for his patience and care with the radiocarbon samples from the Lower Valley. We thank James Abbott at the University of Texas at Austin for the sediment analyses of Lower Valley alluvium; John Jacobs at the Texas A & M Soil Laboratory for his careful analysis of texture and gypsum from the T-18 soil profile; Mary Jo Schabel at the Milwaukee Soil Laboratory for highresolution analyses of the sediment from Los Ranchos de Albuquerque and Laredo; Susan Hovorka, Bureau of Economic Geology, for providing the SEM photographs of the gypsum. We thank Nick Parker and Parametrix Inc., Albuquerque, for facilitating the study at Los Ranchos. We thank Joe Sanchez for his help in the field at Laredo and Brandon Young and Blanton & Associates, Austin, for their support. We thank Mike Blum for his helpful comments during the Lower Valley investigation. We thank the board and staff of the El Paso County Lower Valley Water District Authority for their support and promotion of this project. We thank the Texas Water Development Board for their interest and support of this work. We also thank David O. Brown, James Neely, Archaeological Research, Inc., El Paso, and Sandra Hicks of Hicks and Company, Austin, Texas, for their institutional support, and Timothy Graves for updating our information on the area. We especially acknowledge the many residents and landowners of the Lower Valley for sharing their personal observations on landscape changes on the floodplain and for access to their land during the course of our fieldwork from 1991 to 1997. We thank two anonymous reviews for helpful comments and suggestions. We want to express a special acknowledgment to our friend James B. Benedict (1938e2011) who was interested in the connection of the southern Plains and Rio Grande alluvial history to the Neoglacial record of Colorado Rockies.

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