Late Pleistocene outburst flooding of ice-dammed glacial Lake Missoula, and possible discharge from the Cordilleran Ice Sheet (CIS), catastrophically altered the northwestern United States landscape, yet little is known about potential infiltration of flood waters into the subsurface. This study provides compelling evidence for the presence of late Pleistocene CIS-related recharge waters in the Columbia River Basalt Aquifers (CRBAs) in central Washington. CRBA groundwaters with corrected 14C ages from 15.7 and 33.3 k yrs BP (during periods of flood events) have anomalously low δ18O values (−18.9 to −17.6‰), compared to late Pleistocene soil waters (−16.1 to −13.4‰) and modern precipitation in the region (average −15.9‰), consistent with CIS-related meltwater recharge. These results have implications for our understanding of megaflood phenomena on earth and Mars.
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 The landscape of the northwestern United States was dramatically altered during the late Pleistocene through multiple periods of catastrophic flooding of ice-dammed glacial Lake Missoula, as well as other possible modes of discharge from the Cordilleran Ice Sheet (CIS) (Figure 1a). These late Pleistocene floodwaters scoured the basalt bedrock surface in the Columbia Basin, carved spectacular erosional features, and deposited large volumes of sediments creating the “Channeled Scablands” [Bretz, 1925, 1969; Baker and Bunker, 1985; Baker et al., 1991a; Shaw et al., 1999; Baker, 2009b]. Large freshwater pulses on the order of 106 m3/s rushed towards the Pacific Ocean over days to weeks, giving rise to offshore megaturbidite flows to distances greater than 1100 km along the continental shelf [Zuffa et al., 2000; Benito and O'Connor, 2003; Normark and Reid, 2003].
 To date, there has been little investigation into the potential for floodwaters to recharge the underlying basalt aquifers in the region (Figure 1b). We hypothesize that a significant volume of glacial Lake Missoula floodwater infiltrated into the subsurface and has been stored in the Columbia River Basalt Aquifers (CRBAs) since the late Pleistocene. Herein, we provide stable water isotope evidence and radiocarbon ages of groundwaters that suggests late Pleistocene cataclysmic flooding events recharged underlying fractured basalt aquifers, despite the relatively short duration of flooding. Today, these floodwaters are an important agricultural and domestic water resource for central Washington [Frans and Helsel, 2005]. In addition, knowledge of their presence in the subsurface could prove useful for extending the analog of glacial Lake Missoula flooding to inferred ancient megaflooding on Mars [Baker, 1982, 2001, 2009a; Baker et al., 1991b].
2. Modern and Past Climate Change
 Modern climate in the Columbia Basin region is semi-arid with ∼200 mm of mean annual precipitation (MAP), with the majority of precipitation occurring in the winter months between October and April. The mean annual temperature (MAT) is ∼10.1°C with a minimum and maximum of 3.4°C and 16.8°C, respectively (Othello 6 ESE Station (1941–2002), Western Regional Climate Center 2009, http://www.wrcc.dri.edu). General circulation models and paleo-ecological studies have shown that during the Last Glacial Maximum the region was much colder and drier. A strong temperature gradient formed along the southern margin of the CIS and development of a glacial anticyclone caused wind directions to shift from westerly to easterly dominated [Whitlock, 1992; Thompson et al., 1993; Bartlein et al., 1998; Whitlock et al., 2001; Sweeney et al., 2004]. Regional winter precipitation decreased, up to 2 mm/day lower than present, and regional MAT was approximately 4–8°C lower than modern MAT [Kutzbach et al., 1993; Whitlock et al., 2001].
3. Late Pleistocene and Missoula Flood Events
 There were at least 25 glacial Lake Missoula floods with discharges of greater than 1.0 × 106 m3/s routed through the Channeled Scablands between 19 to 13 k yrs BP as evidenced in the Columbia River Valley. The largest flood occurred post-19,015 + 165 14C yr BP with peak discharges of at least 10 × 106 m3/s [Benito and O'Connor, 2003]. Low salinity anomalies determined from freshwater diatoms in marine sediments off the coast of southern Oregon reported glacial Lake Missoula flood events ranging between 14 to 16 k yrs BP, closely matching glacial Lake Missoula terrestrial flood dates as well as the age of the youngest Missoula flood associated turbidites in the Cascadia Basin [Lopes and Mix, 2009].
4. Geologic and Hydrologic Setting
 The Columbia River Basalts (CRB) formed in the Miocene between 17 and 6 million years ago when lava extruded from north–northwest trending fissures, primarily in northeastern Oregon and southeastern Washington [Hooper, 1982]. Basalt flows within the CRB are columnar, dense in the interior with fractures and joints, and vesicular and porous at the top and bottom [Tolan et al., 2007]. Basalt flows are inter-bedded with Miocene sedimentary deposits typically consisting of clay, silt, sand, and gravels, and overlain by Pliocene to Holocene fluvial, glaciofluvial, eolian, and volcaniclastic sediments (collectively termed “overburden”), ranging in thickness from 15 to ∼180 m in the study area [Vaccaro, 1999; Bauer and Hanson, 2000].
 Together the basalt flows comprise the CRBAs, which consists of three principal hydrogeologic formations (from youngest to oldest: Saddle Mountains, Wanapum, and Grande Ronde Basalts) overlain by the Overburden Aquifer [Whiteman et al., 1994] (Figure 1b). Regional groundwater flow through high permeability interflow zones within the CRBAs occurs dominantly between successive flow units (i.e., parallel to stratiform), while localized flow may occur along vertically oriented fractures and through faults [U.S. Department of Energy (DOE), 1988; Tolan et al., 2007]. The incision of waters into intraflow zones can cause the formation of “erosional windows” into the deeper basalt aquifers (i.e., Grande Ronde), which can create potential recharge and discharge areas into and from the CRBAs [Tolan et al., 2007].
 Previous studies of groundwater in the Saddle Mountains Basalt aquifer beneath the Hanford Nuclear Reserve (up to ∼250 m depth, and within flood pathways of the late Pleistocene Missoula flood events) report 14C values from 4 to 45 percent modern carbon (pmc) and δ18O values from −19.5 to −16.8‰ [DOE, 1988; Hearn et al., 1989; Spane and Webber, 1995]. The DOE  suggests that these groundwaters are late Pleistocene in age and may have been extensively recharged by cataclysmic flood events. Groundwater in the underlying Wanapum and Grand Ronde Basalt aquifers (>250 m depth) contain less than ∼5 pmc 14C and have δ18O values from −20 to −11‰ [DOE, 1988; Hearn et al., 1989]. These deeper groundwaters were interpreted to represent much older pre-Pleistocene groundwater that was recharged under different climatic conditions.
 Sixty-eight groundwater samples were collected from domestic, irrigation, and United States Geological Survey monitoring wells located in Miocene CRBAs and overburden deposits in the Columbia Plateau (see auxiliary material for detailed methods, sample locations and data).1 Oxygen and hydrogen isotope values for groundwaters in the CRBAs and Overburden Aquifer plot along the Local Meteoric Water Line (δD = 6.9*δ18O – 18.5) [Larson et al., 2000] and lie parallel to the Global Meteoric Water Line [Craig, 1961] (Figure 2a). Weighted mean annual precipitation for the central Columbia Plateau has a δ18O value of −15.9‰ [Robertson and Gazis, 2006]. Two distinct groups of groundwaters in the CRBAs were observed: 1) modern agricultural irrigation waters with high tritium (2.8–14.4 TU), high amounts of anthropogenic NO3− (11–116 mg/l), high δ18O values (−16.9‰ to −13.5‰), CFC ages between 20 to 40 yrs., and ∼100 percent modern carbon (pmc); and 2) paleogroundwaters unaffected by agricultural practices with minimal to no detectable tritium (<0.8 TU), low amounts of NO3− (0–5 mg/l), low δ18O values (−18.9 to −16.7‰), and 14C ages ranging from ∼6 k to 33 k yrs BP. The δ18O values of these groundwaters are 0.8 to 3‰ lower than modern weighted mean annual precipitation (Tables S1 and S2). Agricultural irrigation waters and CFC ages are discussed by Brown  and Brown et al. .
 NETPATH modeling [Plummer et al., 1994] of groundwaters (discussed in detail in the auxiliary material) demonstrated that the modeled and observed δ13CDIC values differed by less than 1.0‰ and corrected radiocarbon ages differed from uncorrected 14C ages by less than 1% as expected in basalt aquifers, which contain low amounts of inorganic and organic carbon [e.g., Hinkle, 1995; Douglas et al., 2007]. Radiocarbon age ranges for the Saddle Mountains, Wanapum, and Grande Ronde Basalt Aquifers were 33,300 to 15,800 yrs BP, 24,000 to 749 yrs BP, and 27,400 to 6,520 yrs BP, respectively (Figure 2b).
 Paleogroundwaters in the CRBAs have radiocarbon ages from 6.5 k to 33.3 k yrs BP and low δ18O values (−18.9 to −16.7‰; Figure 2), compared to modern mean annual precipitation (−15.9‰) [Robertson and Gazis, 2006]. These Late Pleistocene waters may have been sourced from paleoprecipitation or Missoula flood recharge.
6.1. Source of recharge to CRBAs
 Recent studies of pedogenic carbonates in the region estimated that δ18O values for late Pleistocene soil waters ranged from −13.4 to −16.1‰ [Takeuchi et al., 2009]. The relatively positive δ18O values of late Pleistocene soil waters were likely due to decreased amounts of isotopically-depleted winter precipitation [Stevenson, 1997; Takeuchi et al., 2009]. These results are consistent with other studies that have reported relatively positive values for late Pleistocene soil water compared to modern values from measurements of pedogenic carbonates, δ18O values of groundwater in conjunction with dissolved noble gas concentrations, and δD values of tree wood cellulose across North America [Yapp and Epstein, 1977; Siegel, 1991; Amundson et al., 1996; Aeschbach-Hertig et al., 2002; Ma et al., 2004]. Modern groundwaters in shallow loess and basalt aquifers in the region have δ18O values (−15.2 to −14.5‰) within the range of modern soil waters (−14.7‰) and winter precipitation (−14.3‰); the majority of recharge occurs in the winter [Larson et al., 2000; Moravec et al., 2010]. Thus, we would expect that late Pleistocene recharge waters derived from paleoprecipitation would have similar δ18O values to late Pleistocene soil waters. Late Pleistocene groundwaters, measured as part of this study, have δ18O values that are much lower than late Pleistocene soil waters and modern precipitation. Therefore, these isotopically-depleted waters were not recharged by paleoprecipitation, but rather recharged by late Pleistocene CIS-related recharge waters (e.g., Missoula flood events), where individual events occurred within days to weeks over a period of thousands of years [Baker and Bunker, 1985]. Hendy  noted that the δ18O value of the CIS was likely similar to modern glacial ice in coastal areas, such as the Harvard Glacier (−22.6‰) and Columbia Glacier (−17.8‰) in southern Alaska [Kipphut, 1990]. These values are consistent with a CIS-source of recharge for late Pleistocene groundwaters in our study.
 It is important to note that Douglas et al.  found late Pleistocene (3.3 to 20.5 k yrs BP) groundwater in the Palouse Basin, outside of the Missoula flood pathways, in the Wanapum and Grande Ronde aquifers that have δ18O values between −18.7 to −15.2‰, also within the range of our study.
6.2. Timing of Recharge to CRBAs
 If isotopically-depleted late Pleistocene groundwater in the CRBAs were recharged by Missoula flood waters, radiocarbon ages may provide information on the timing of flood events. Six samples in the CRBAs display 14C ages between ∼19–13 k yrs BP corresponding to the timing of at least 12 floods with discharges of >1.0 × 106 m3/s [Benito and O'Connor, 2003] (Figure 2b). Two of these six samples also correspond to Missoula flood event ages reported from 14 to 16 k yrs BP based on low salinity anomalies determined from freshwater diatoms in marine sediments off the coast of southern Oregon [Lopes and Mix, 2009] (Figure 2b). Twelve samples from the CRBAs demonstrate 14C ages between 19 and 34 k yrs BP which correspond to pre late Pleistocene CIS-related flood events obtained from low salinity anomalies [Lopes and Mix, 2009], as well as radiocarbon age dates of Quaternary deposits in the Pasco Basin [Baker et al., 1991a]. The source of these flooding events prior to the onset of glacial Lake Missoula floods has been attributed to intermittent releases of subglacial meltwaters from the CIS and surge behavior during advances of the CIS [Shaw et al., 1999; Lopes and Mix, 2009].
 It has been demonstrated that during late Pleistocene Missoula flood events significant volumes of floodwaters converged in the Pasco Basin and were constricted at the Wallula Gap (Figure 1), ponding up to hundreds of meters over several days [Baker and Bunker, 1985; Benito and O'Connor, 2003]. The largest late Pleistocene Missoula flood of 10 × 106 m3/s took place over ∼5 days and has been shown to have reached a maximum ponding height of 366 m in the Pasco Basin, creating 1210 km3 of ponded water or greater than half the total volume of maximum glacial Lake Missoula (2184 km3) [Clarke et al., 1984; O'Connor and Baker, 1992].
 The presence of groundwaters in the CRBAs with anomalously low δ18O and δD values and radiocarbon ages between 15.7 k to 19.6 k yrs BP may suggest these waters were recharged from multiple pulses of late Pleistocene Missoula flood events. Interestingly, several groundwaters have glacial isotopic signatures, but older radiocarbon ages (21.5 k to 33.3 k yrs BP). These paleowaters may have been sourced from earlier (undocumented?) Missoula flood events and/or discharge from the CIS. Results from this study have important implications for our understanding of megaflood phenomena on earth and Mars, and potential recharge mechanisms to groundwater.
 Funding was provided by Science Foundation Arizona, UA Space Grant Program, and NSF (EAR-0635685). Bob Black and Sandy Embrey (USGS) helped with field sampling logistics and Candice Adkins helped with sample collection.