quaternary Article Chronology of Missoula Flood Deposits at the Coyote Canyon Mammoth Site, Washington State, USA George V. Last 1,2,* and Tammy M. Rittenour 3 1 Mid-Columbia Basin Old Natural Education Sciences (MCBONES) Research Center Foundation, Kennewick, WA 99338, USA 2 School of the Environment, Washington State University—Tri-Cities, Richland, WA 99354, USA 3 Department of Geosciences, Utah State University, Logan, UT 84322, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-509-946-8050 Abstract: Late Pleistocene outburst megafloods, mostly from glacial Lake Missoula, hydraulically ponded behind downstream constrictions in the Columbia River in southeastern Washington State, USA. Optically stimulated luminescence (OSL) ages for flood deposits from the Coyote Canyon Mammoth Site, located in a high (315–320 m asl) distal portion of the transient lake, indicate that at least seven floods ponded high enough to inundate the area during the period 20.9 ± 2.6 ka to 16.3 ± 2.8 ka. This is consistent with a radiocarbon age of about 17.4 ± 0.2 ka cal BP from the middle of the flood sequence. OSL ages from loess deposits overlying a paleosol at the top of the flood sequence range from 14.0 ± 2.3 ka to 10.9 ± 2.0 ka, suggesting a hiatus of about 2.3 thousand years. These datasets are consistent with current understanding that multiple late Pleistocene megafloods occurred between 20 ka and 14 ka and that earlier floods produced higher flood stages than later ones. The lack of flood deposits in the Coyote Canyon area younger than 16 ka supports the hypothesis that younger megafloods did not hydraulically pond in the Pasco Basin above about 230 m asl. Citation: Last, G.V.; Rittenour, T.M. Keywords: Missoula floods; Lake Lewis; ice age floods; mammoth; Mammuthus; OSL; geochronology; Chronology of Missoula Flood late Pleistocene Deposits at the Coyote Canyon Mammoth Site, Washington State, USA. Quaternary 2021, 4, 20. https://doi.org/10.3390/quat4030020 1. Introduction Floodwaters from Late Pleistocene outburst megafloods, mostly from glacial Lake Academic Editor: David Bridgland Missoula, hydraulically ponded behind downstream constrictions (such as Wallula Gap), forming a large transient body of water (lasting only weeks) in southeastern Washington Received: 27 April 2021 State, USA. These hydraulically ponded flood stages inundated much of the Pasco Basin Accepted: 29 June 2021 and backflooded its tributary valleys including the Yakima and Walla Walla valleys. This Published: 6 July 2021 repeatedly formed a transient large body of water has been referred to as Lake Lewis, and sometimes confusedly referred to a “glacial Lake Lewis” (Figure1). Rhythmically Publisher’s Note: MDPI stays neutral bedded slackwater sediments deposited in the quiet margins and tributary valleys of the with regard to jurisdictional claims in published maps and institutional affil- hydraulically ponded lakes provide evidence for repeated flood episodes with lengthy time iations. (decades) between each flood episode [1–3]. These floods led to the demise of numerous mammoths and other animals occupying the fertile valleys of southeastern Washington between flood episodes, sweeping their carcasses into the locally quiet waters of Lake Lewis [4]. In fact, most of the mammoth remains discovered in eastern Washington have been found in slackwater flood deposits [5]. Copyright: © 2021 by the authors. Here, we present the stratigraphy associated with seven optically stimulated lumi- Licensee MDPI, Basel, Switzerland. nescence (OSL) ages for Missoula-flood rhythmites that entomb Columbian mammoth This article is an open access article distributed under the terms and (Mammuthus columbi) remains and overlying loess deposits from the Coyote Canyon Mam- conditions of the Creative Commons moth Site (CCMS) and another nearby excavation. These data not only provide key Attribution (CC BY) license (https:// constraints on the timing of geologic events at CCMS, but also expand our understanding creativecommons.org/licenses/by/ of the dynamic nature and chronology of Lake Lewis flood stages [6]. 4.0/). Quaternary 2021, 4, 20. https://doi.org/10.3390/quat4030020 https://www.mdpi.com/journal/quaternary Quaternary 2021 4 Quaternary 2021, 4, x 20 FOR PEER REVIEW 22 of of 17 17 Figure 1. 1. SchematicSchematic of of the the hypothetical hypothetical extent extent of ofhydraulically hydraulically ponded ponded floodwaters, floodwaters, assuming assuming a static a static flood flood stage stage of 366 of m asl, forming a large transient body of water, referred to as Lake Lewis in Southeastern Washington State (based on a 366 m asl, forming a large transient body of water, referred to as Lake Lewis in Southeastern Washington State (based on a map available from the Ice Age Floods Institute (https://iafi.org/product/ice-age-floods-in-the-pacific-northwest-map/) ac- map available from the Ice Age Floods Institute (https://iafi.org/product/ice-age-floods-in-the-pacific-northwest-map/) cessed on 4 April 2021). Darker shading indicates areas impacted by Late Pleistocene outburst megafloods. accessed on 4 April 2021). Darker shading indicates areas impacted by Late Pleistocene outburst megafloods. Current understanding suggests that dozens (perhaps as many as 100) of late Pleis- Current understanding suggests that dozens (perhaps as many as 100) of late Pleis- tocene megafloods occurred between 20 and 14 ka, with large uncertainties in this age tocene megafloods occurred between 20 and 14 ka, with large uncertainties in this age rangerange [2,7–11]. [2,7–11]. The The decreased decreased thickness, thickness, grain-size, grain-size, and and erosiveness erosiveness of of younger flood flood beds,beds, as as well well as as the the reduced reduced number number of of young young flood flood beds at higher altitude, suggests that earlierearlier floods floods were were larger in maximum discha dischargerge than later ones [[1,2,7,10–15].1,2,7,10–15]. Reduced floodflood magnitude magnitude appears appears to to coincide coincide with with increased increased flood flood frequency (as expected for a self-dumpingself-dumping lake) resultingresulting fromfrom late-glacial late-glacial thinning thinning of of the the ice ice dam dam [1,2 [1,2,11,13].,11,13]. However, How- ever,there there are limited are limited data ondata the on age the of age specific of spec floodific flood events events and the and corresponding the corresponding flood stagesflood stagesof Lake of Lewis. Lake Lewis. TheThe maximum maximum elevation elevation of of ice-rafted ice-rafted erratics erratics,, divide divide crossings, crossings, and and loess loess scarps scarps in- dicatesindicates that that the theflood flood stages stages of some of some of these of these transient transient lakes lakesmay have may reached have reached an eleva- an tionelevation of at least of at 366 least m 366asl and m asl perhaps and perhaps as highas as high380 m as asl 380 [11,16]. m asl However, [11,16]. However, the prepon- the derancepreponderance of ice-rafted of ice-rafted erratics, erratics, mammoth mammoth remains, remains, and other and otherdebris debris left stranded left stranded on the on shorelinethe shoreline of Lake of Lake Lewis Lewis suggests suggests that most that most of the of flood the flood stages stages reached reached elevations elevations of only of 180–300only 180–300 m asl m [17]. asl [Rhythmites17]. Rhythmites containing containing Mount Mount St. Helens St. Helens Set S Set (MSH-S) S (MSH-S) tephra tephra are generallyare generally restricted restricted to elevations to elevations below below 230– 230–275275 m m[1,18]. [1,18 ].Thinning Thinning and and fining fining of of these upperupper rhythmites rhythmites suggest suggest the the last last several several (p (perhapserhaps tens tens of) of) floods floods were smaller and may havehave been been more more frequent frequent [1,18,19]. [1,18,19]. Figure Figure 22 illustratesillustrates thethe extentextent ofof LakeLake LewisLewis atat differentdifferent floodflood stages stages [17]. [17]. CosmogenicCosmogenic 3636Cl-exposureCl-exposure ages ages have have been been reported reported for for four four erratics erratics located located along along the southwesternthe southwestern perimeter perimeter of Lake of Lake Lewis Lewis within within the thePasco Pasco Basin Basin [15] [ 15(Figure] (Figure 3).3 Three). Three of theseof these erratics erratics are are located located on on the the northeastern northeastern flank flank of of Rattlesnake Rattlesnake Mountain Mountain in in an an area wherewhere over 2,100 ice-rafted erratics, sometimes found in clusters and bergmounds have beenbeen documented [[16].16]. Located atat elevationselevations of of 209, 209, 225, 225, and and 310 310 m m asl asl they they yielded yielded ages ages of of16.2 16.2± 1.3± 1.3 ka, ka, 16.7 16.7± 2.7± 2.7 ka, ka, and and 16.9 16.9± 3.4± 3.4 ka, ka, respectively respectively (Table (Table1). The 1). The fourth fourth erratic erratic was wasin a sidein a canyonside canyon (Badger (Badger Canyon) Canyon) southeast southe of theast main of the Lake main Lewis Lake body. Lewis Itwas body. located It was at locatedan elevation at anof elevation 268 m asl of and 268 yielded m asl and an anomalously yielded an anomalously old age of 35.6 old± 1.2age ka of (Table35.6 ±1 ).1.2 The ka (Tableunexpected 1). The old unexpected age of this old erratic age isof currentlythis erratic unexplained, is currently butunexplained, is likely due but to is inheritance. likely due Quaternary 2021, 4, 20 3 of 17 Quaternary 2021, 4, x FOR PEER REVIEW 3 of 17 toAn inheritance. OSL age of 21An± OSL2 ka age was of reported 21 ± 2 ka for wa thes topreported of a sequence for the oftop ~15 of rhythmitesa sequence locatedof ~15 rhythmitesalong While located Bluffs along at an While elevation Bluffs of at ~185 an elevation m asl in theof ~185 central m asl portion in the ofcentral former portion Lake ofLewis, former and Lake an OSLLewis, age and of 12an± OSL2 ka age for of the 12 top ± 2 ofka a for younger the top sequence of a younger of sediments sequence in of a sedimentspaleochannel in a incised paleochannel into the incised rhythmite into sequencethe rhythmite [15] (Figure sequence3, Table [15] 1(Figure).