Potholes Coulee Area
hkcFairl3mm Geography and Land Studies Department CeaM Washington University Ellensburg, Washington ~~amQcwu.~
Faculty Advisor Dr. Karl LiDqukt @mgragBy and Land Studies Depammt Central Washington University Ellmburg, Washington [email protected] Figure I. Location rnajfiof%dy area 2. Physiofiaphic map
3. Inundation map of Quincy Basin *.> 4. Oblique ground view %f~eor~eGravels, , 7. 5. George Grapels 4 6. An example of weathered scbid$& : Gravels
7. Map of George channel*f;owrrrs IUL~UUIIVL Seorge Gravels 8. Annotated rn, of bash and butte topography 9 0bliqrp8grouna vlew of stripped structural bedrock terrace n, 10. Map view of stripped structural bedrock terrace I 1. Map view of kolks 12. Oblique ground view of kolk 13. Map view of plunge pools 14. Mosaic oblique ground view of plunge pools 15. Map view of crescent-point bar 16. Map view of pendant bars 17 Mosaic oblique ground view of pendant bars 18. Map view of expansion longitudinal bar and pendant bar 19. Oblique ground view of current dunes from southwest 20. Oblique Ground view of spalling basalt corestone 21. Physical and chemical weathering on interfluve surface 22. Talus mantling fosse side of crescent-point bar 23. Large block rock fall on Babcock bench 24. Oblique ground view of rotational landslide block 25. Map view of undifferentiated landslide scarp 26. 1962 U.S,D.A. airphoto of the Potholes Coulee Tables
Table 1. Giant current dune frequency Abstract ]introduction Study area Background Methods Geomorphology Fluvial Features Potholes Coulee George Gravels Basin and Butte Topography Kolks Plunge Pools Bars Current Dunes Weathering Features Physical Weathering Chemical Weathering Composite Weathering Eolian Deposition Mass Wasting Features Rockfall Landslides Solifluction Biotic Geomorphic Features Flora Fauna Humans Conclusion Literatures Cited Abstract
The Potholes Coulee is located on the western margin of Quincy Basin,
Washington about 15 miles southwest of Quincy . Potholes Coulee has been shaped by various geornorphic processes and agents since the Miocene epoch creating a unique landscape. The bedrock of the basin is Miocene Columbia River flood GasaIt folded by
Pliocene tectonic forces and shaped by catastrophic flooding events of Pleistocene era
(Bretz, 1923). Quincy Basin was a depositionaI (fluvial and eolian) and erosional region of the Missoula floods during the late Pleistocene epoch (Bretz, 1928). Miocene
Columbia River Basalts were folded, and then eroded by Pleistocene glacial outburst flood events. The western margin of Quincy Basin has three eroded outlets through
Babcock/Evergreen Ridge. The Babcock and Evergreen Ridges are separated by Potholes
Coulee. Potholes Coulee is a relict landform characterized as a headwardly eroded cataract. Grolier (1965) describes the coulees as existing prior to the Missoula Floods based on the flood gravels indicating a western source in Quincy Basin. The cataracts contain late Pleistocene Missoula flood crystalline and basalt gravels as various landfoms. The flood sediments are capped by a weathering mantle, eolian deposits, and mass wasting debris. The Columbia Basin Irrigation Project during the 1950's enabled increased settlement in the area adding humans as key geomorphic agents. In the early
19503, Ancient Lake cataract was flooded, filling the cataract with a temporary Iake, leaving a rime indicating the past shoreline. Introduction
Potholes Coulee has been shaped by various geomorphic processes and agents
since the Miocene epoch creating a unique landscape. The bedrock of the basin is
Miocene Columbia River flood basalt folded by Pliocene tectonic forces and shaped by catastrophic flooding events of Pleistocene era (Bretz, 1923). Quincy Basin was a
depositional and erosional region of the Missoula floods during the late Pleistocene epoch
(Bretz 1928). After nearly a century of research, many questions remain regarding the
geomorphology of the Quincy Basin, and specifically the Potholes Coulee (Figure 1).
The primary focus of this paper is to describe the geomorphology of the Potholes
Coulee and adjacent Babcock Bench, as well as to provide geomorphic context to late
Pleistocene sloth remains unearthed at Bishop Springs (Figure 2). Secondly, I will
describe future research possibilities in the area. The overall intention of this study is to
provide further insight to the geomorphology of Quincy Basin and the Channeled
Scablands.
Study Area
The Potholes Coulee is located on the western margin of Quincy Basin about 15
miles southwest of Quincy (Figure 1). The Potholes Coulee is situated 134 meters above
and 1.5 kilometers east of the Columbia River in sections 7, 8,9, 18, 17, 16, T. 19 N., R
23 E., W.M., of the Babcock Ridge, Washington United States Geological Survey
(U.S.G.S.) 7.5 minute quadrangle. The rim elevation of the coulee varies from 41 1 to 320
meters elevation with the floor of the cataract varying from 250 to 308 meters elevation.
The Potholes Coulee consists of double horseshoe cataracts bisecting Babcock Ridge and Evergmn Ridge along the western margin of the Quincy Basin (Figure 2). The rim of the
cataracts opens to the west onto the BhkBench above the Columbia River.
]Figure 1. Loration map, the Myama is bated -15h wlrthwezt of George aad -Mkm sonthw& of Qdncy. (Map derived from US.GA DEM) The bedrock consists of tJw, CoIumbia River Basalt Group (CRBG's) with diatomite and sandstone interbeds. The Rosa flow, Babcock Bench flow and two Ginkgo flows of the Frenchman Spring member of the Wampum basalt pupand Vantage
I Sandstone are exposed (R.D.IBentIey, personal communication, May 2003) to the level of ;I1 &L.IL. - 4I the Babcoek Bench. The Babcock Bench consists of the Gnrnde Ron& N2 group of the
CRBG' s. Loess and cover sand caps glacio-fluvial deposits flooring the cataracts. Brown tuffaceous sand and platy caiiche capped by Iws cover the Babcmk Ridge and
Evergreen Ridge (Grolier, 1 965). The region is characte* a semi-arid, continental setting. At Quincy, from
194 1-2002, the maximum average annual temperature was 17" C and the minimum average mud temperature 3' C; with July behg 30.1' C, and January king -7' C.
The average annual precipitation was 22.3 centimeters, with the majority of precipitation occurring in September, November, December and January. The region receives an annual average of 33 centimeters of snow (www .wnx.dri.edu, March 2003).
Rgum 2. Physiographic map sbw& Potholes CoPrlee, locsted on the whmmrgin d my Basin,errstoitber ' " River. No&dmblebo~~with~veseparatingthtwo catamcta (Map derived Rom U.S.G.S. DEM) Five welldrained Aridisols formed on the diverse terrains of Potholes Coulee.
Gentry (1984) describes the soils as follows:
I. Ephrata sandy loam: a sandy-skeletal, mixed, mesic, Xerollic Camborthid soil;
found on 5- 1W slopes, forrned on subfluvial gravels at the mouth of the Ancient Lakes
cataract.
2, macobb1y sandy loam: a sandy-skeletal, mixed, mesic XemIlic
Cmborthid soil; found on 0-15% slopes, fmmd on the sub-fluvial bm deposits within
the Ancient Lakes cataract.
3. Mdaga very stony smay 1oam: a sandy-skeletal, mixed, mesic XeroUic
Camborthid soit; fwnd on 0.35% slopes, formed on sub-fluvial bar depusits widin the
Dusty Lake cataract.
4. Starbuck-Bakeoven-Rock outcrop complex: a loamy, mixed, mesic, Lithic
XeroIlic bborthid soil; found on 045% dopes, fonned on basalt bedrock along
Ba-k bench and Evergreen Ridge.
5. Starbuck-Presser complex: a loamy. mixed mesic, Utbc Xerollic Cambortbid
soil; found on 0-258 slops, formed on basalt dong the northlnorth east rim of Potholes
CouIee.
The vegetation is primarily bluebunch wheatgrass, cheat grass, with big sage brush, rabbit brush, hapweed, and herbaceous shrubs.
Historic laud uses include human habitation, cattle range, and limited agriculture.
After the implementation of the Columbia Basin Irrigation project in the early 1930's, bay farming and subsequently archads and various crops became a major land use on
Babcock Bench. At present, the Potholes Coulee is a Washington State Fish and WildIife managed area and the majority of land use withthe study area is recreational
(equestrian, biking, hiking, and fishing).
Miocene-Pliocene Emhs
Between 6 ma. to 17 IxLa many flows of Columbia River Basalt Group magmas flooded eastern Oregon and sou~ternWashington. They originated from fusures in eastern mgon and southatem Wasbingtoh The flows are charactebd as having a blocky entablature cap on colonnade with a pillow structured base. The flows are inkmittenfly bedded with Ellemburg Formation detritus and diatomaceous sediments
-kin, 1%1: Toh et a1989). The CRBGs wathen folded from the west then south by Phacene tectonic form crearing ridges and basins. Subsidence of the plateau, uplift of the Cascade Rmge, and plunge associated with regional folding are three possible causes of deformation. The Columbia River continued flowing through the region but was diverted at difhnt locations and times, The stwtudQuincy Basin was formed through these events and is bound to the north by the Beezly Hills, the west by
Babcock Ridge and the south by the F~nchmanHills (Mackin, 196 1; Watters, 1989).
The Columbia River flowed along the western margin of the Columbia basin in its present course eroding through the CRBGs creating Balm& Bench. Two separate occasions of rising anticlines from the tectonic forces from the south were thought to temporady dam the Snake River and Columbia River creating hkeRingold (Newcomb
1958). The hpowded Columbia River Ieft fluvio-lacustrine deposits, known as the
Ringold Formation, from sauth in the Pasco Basin to north along the rim of the
Babcock/Evergreen Ridge (Grolier ztnd Binglum, 1978). Pleistacene Emch (The Channeled Scablaads)
The Chamled Sc&1ands of the Columbia Basin are relict landforms of outburst floods from Late Pleistmne glacially dammed Lake Missoula (Bretz, Smith and Nee
1956). Many huge catastrophic oatburst floods (jakulhhps) from Lake Missoula exWthe capcity ofthe existing drainage system of the Columbia River with thr: flood water flowing up to 2,500,000 m3through the Columbia Bash (Wgtt, 1985). More then 30 £loadsd between 12,700 and 15,300 yem before present (y.b.p) based on tephrichnology and radiocarbon dating (Waitt, 1985). The flood waters flowed across north Wminto north Washington kfoe enwuntering the Ohogan lobe of the
Cordilleran Xce Sheet which diverted the waters south into the Grand Coulee and into the
Quincy Basin.
The water pooIed in Quincy Basin before exiting through four outlets. There were three detson the western margin of Quincy Basin (Cmr Wee,Potholes
Coulee, and Fmchman Code) and one to the southeast (Dmnheller Chmls) at the eastern margin of tb Fbnchman HiUs (Figure 3) (Bretz, 1930). When the Okanogan lobe was not in place, the flood waters flowed down the Columbia River -age and into
Quincy Basin via Crater Coulee and Potholes Coulee (Bretz, Smith and NeR 1956).
During intervals of lower magnitude floods, the waters flowed out the more hydmhcally favorable Dmdeller Channels to the east (Bretz, 1928). I 1 I I 1 I I I 1 0 22.5 25 SO Kilometers Legend s Elwath wedm High : 2870 metsrs Direction of flood waters F.C. - Frenchman CouIee r- L.QC. - Lower Grand Coulee D.C. - IhmhelIer Channels C.C. - Crater CouIee B.R. - Babcock Ridge - : m P.C. - Potholes Coulee E.R. - Evergreen Ridge f -3. ~kmhuadalonddQlrlncyBaslbn~the~d~~test~b ta%te Mhmh Tlw --water mrrrlt b Qldncg Basin was 409 m adapted hmBaker (1973). (Map derived fmm USGA DEM) The catastrophic floodiog crated an amazing anastomising cumglex in the bedrock mretz, 1923). The abandoned catarads are described as resulting from sub fluvial plucking of the columnar basalt by vortices and -porting the blwks downstream by the extremely competent flow during the l&gst flood events (Bretz f 930; Baker 1974). The eroded bedmk features are divide crossings, coulees, bum and mesas of basalt and kolks (i.e. eroded hollows). bswas eroded creating smadhed "loess islands". The floods are further evidenced by subfluvial mega-bed form, such as various bars and giant lcurrent dunes (Bretz, 1927). After flowing through Quincy Basin, the waters pond4 behind a hydraulic constriction formed at Wdda Gap in the Horse
Heaven Hills forming temporary Lake Lewis with a depth of 350 m. O'Connor and Baker
( 1992) calculated the flow through Wallulzt Gap at 10 Wonm 3 /s*1 , a discharge greater than any rivers or flaading events previously or since. Waitt (1995) notes smaller flooding events down the Columbia post dating late Pldstocem Lake Missoula Floods. m€hwls
This resmh was accomplished thmugb document review, interviews,
Geographic ~rmzttimSystem (GIs) Wques, airphoto analysis, and field wok I fmt reviewed past literature that describes regional and local pmo'phic processes and f-rs, landforms, historic use, and recent occapatim to better understand the parameten of the stcldy area 1interviewed a resident of mare than 50 years, Dave Bishop, to understand the role of recent human occupatim at the site. The spatid and ternpard parameters for the project were decided with understanding the past research.
1: l2,OOO 1949 and 196 1 U.S. Department of Agriculhm and 1996 1:24,000
U.S .G.S. digital orthophoto quads were compared to gauge the rate of environmental change since the Columbia Basin Irrigation Roject . I then used U.S.G.S. 1:24,000 topographic maps, airphotos, and ArcGIS by Environmental Systems Research Institute
(E-SIU)to identify and analyze key Imdfom and plan field trips for field checking of those features.
Eeld observations and field checking of observed remotely sensed data was comp1ete.d from July, 2002 - May, 2003. GPS techniques wete used to locate various attributes to better interpret hdfoms in the field such as chod length on possible sub- fluvial Worms. Once identified, the various geomorphic agents and dtingland- forms were mapped onto copied airphotos and topographic maps in the field. In the
C.W.U. GIs lab the measured observations were entered into tbe computer by downloading GPS coordinates into ArcGIS and digitizing field observations and
Mbutes as poiygons in AEGIS. The polygons were then used in slope process modeling using ESRI Spatial Analyst to better understand the pmxsses that may have occurred on the sIopes since initid deposition. Comparisons between the computer's modeled pdictiom versus field obsedons were made. Obsewd historic and pmat pmorphic processes and factors were mapped using ESRI AEGIS.
The various geomorphic processes and factors were categmd in a relative temporal and spatial hierarchy beginning with the formation of the primary landform
(Potholes Coulee) of the study area and finishing with the merit advent of humans to the region. This classificdon is bast4 on the order of landform origins hived from an application of Steno's Principles and observed physical evidence.
Geomorphology
Fluvial Features
Potholes Coulcg Grolier (1965) describes Pothales Coulee as a sag in the
BabwM3~ergnxnanticline that was subsequently m&ed by Pleistocene flood waters flawing down Grand Coulee into Quincy Basin. These ftoad waters then fl~wedout through the sag rejdning the Columbia River. Potholes Coulee, shhto Nagam Faj formed by head-ward erosion by subaqueous undercutting then twwport of sediment down flow as waters from the Quincy Basin flowed though the bmxh to njoh the Columbia River (Figure 3) (BE&, 1930; Grolier, 1965; Baker* 1973). The fluvial event, that formed Potholes Coulee, appears to be ephemeral as there are no visible channels coanecsting the mouth of lower Grand Coulee (northeast) to Potholes Coulee in the southwest. The events attributed to the f~mtionof the Iandform m the late Pmtocene
Lrrk~Migsoda Hoods (12,7@15,300 ybp) @re@, 1930).
Georere Gravels and Gtwrp:e Chamel: Potholes Coulee appears to have an origin much earlier then vtbe late PIeistocene as older, extwbsind, flood sediments (C*seorge Gravels) are found in a channel (George Channel) ~~to the southst from Potholes Coulee into Quiicy Bash. Thou&, it is evident the catafact formed as waters flowed down
Grand Coulee into Quincy Bash adout the Potholes Coulee from the nmtbst, pbab1J mmmhg as arIy Pleistocene flood waters, Gmlier (1965) Mbesthe coulees as existing prior to the late Pleistocene Miamla Floods based on the flood gravels indicating a western source in Quincy Basin (area, Smith, and Neff, 1456).
Bretz (1969) noted the massive capped flood sediments (the George GraveIs) Bowing up the Crater coulee and Po€hoIesCaul= as pm-w~wnsin(80,W ybp) flood deposits
Figure 4). ~+UWpe~~d~Q~&~~pi€mm~~~~: l)~Gra~&~~~a~rn~lg~r~at~~~~~oflO% Ha. 2) Gmt~pbrCedGeage- The George Graoda slre a ~)nglummtccooaposed of extremely weathered basalts with pdagonite, pigs, =hist, qdteand granite cobbles with imbridom indicating a we&em source (F~gruep5 & 6). The gravels are ~appedby a 1.5-m of MCW(Baker,
1973; Penand Nummdd, 1978). BjmW, Frecht and Huhar (2001) m@btbe
George Gravels w nimilrt in age to the Old Maid Coulee gravels, having a reverse magnetic palaity indicating an early Pleistocene (Brunhes-Matum 7gQ,OOD ybp) w.
Though the George Gravels have imbrications hdiclitlq a wstetn source it is evident they post dstc the cmt'sfodon by early Plehne flood watus issuiog 6om
Gtand Coulee.
The George Channel has a northwest to southeast tmd coming from the head of the
Potboles Corrlee to the southeast and contains the George Gavels (Figure 7). The camacts had to be formed by wmflowing from the norbat prior to genesis of the channel and deposition of the George Gravels. The implications are intriguing but are in need of detailed resear~hon the George Gm~elsand the sediments within the Potholes
CQU~.A some for flood waters mqeknt enough to crate the landform is in question as it predates the late Pleistocene Glacial Lake Missmla Alkfnate wakr sowcould be pal80-gkial lakes since obliterated by subsequent gIacia.1 advances, or multiple johhhups (sub-glacial outburst flood) from Olranogaa, region of British
Columbia (Shaw et. al1999; Lasemum and Shaw 2000% 2000b; Mate and Levson 2000).
4 u ~~ F~taI~ m:- @ GmQeQmvak* ~hfO~ Im: If* F'igm?.~dew3Geo~~~~~~~rmdtSe~h~M* the dIrectIon ddmpmd hdbgnwthw& to sw- and btIonof the George Gmv& (Map Wedfrom U.&G& DEM) Basin and Butte Tmmohv: Basin and bum topography is a tern describing generic scabland topography consisting of various scales of eroded hndforms resulting from regional catastrophic flcgd activities. This topography was created by flood waters abrading and plucking,1s mistant khxkIeaving terrace, channel, bk,and butte lmdfm that range in she from ~1 meter to humbtds of meters (Egure 8). Buttes are localized areas of sha~~&f that dduatein a flat Wle" top surface tkt ranges from a fbw metas to thousands of.metmin volrune. Basins are eroded ~to~gchannels, terraces or koh.
Stripped structural bedroclr temm .amfa& at the heads awl margins of the mtmcts by turbulent waters rhrough &aqueous undetz:ming, vortices plwkhg then mrtof the sediment down flow by the exlmmdy competent flow of catastrophic flood waters.
The do&s of the basalt are plucked leaving the more resistant entablature cap of the lower flow to form long, broad, planar terraces (Figures 9 & 10). Striped structural bedrock tenaces indieate an extremely high magnitude and competent idiluvid om.
The featms vary in area from 33,150m2- 529,200m2, and are 15 m - 67 m above the cataract floor and are up to 53 m deep. They are cappi by a shallow tu well developed weathdug mantle and 1- (5 cm to > 2 meters), Some terraces are well shelterd from wind creating eolian deposition hollows which support a variety of flora and fauna. Someofthe~donotappearcohavebn~~~dybythel~
Pleistmxne fi& as idhkdby thi: devel~tof the wtathering mantle md lichen growth on exposed hdmk "it
Kolks: Kok are formed in turbulent waters by sub-aqueousvortices exploithg localized
wedmesses in the entablature cap dg"pitsM (Balrer, 1973). Kolks are similar in
morphology to plunge pools, save in the study site, they are located in areas up d down
flow from the cataracts heads. Koh fondin bedrock indicate an extremely high magnitude and competent flow at time of origin (Baker, 1973). Some of the larger kob
(diameter IOm, depth > 1Om) cut through the e~1~cap, tbea into ttte colonnade
Wow and have potential to be depositional hollows tbat can provide datable materid in form of tephra, organic material atrd laws pigum 11 & 12). The kok on the interfluve between the cataractg are formxi on different surf- with some king breechad dhg c~.fomation. Figure11. MaprlmofLdh~tedmtbe~elAw.cm~aL.LcdhtyWrc cataracts. MotatBom: 1) the same kdk as in F'i12. 2) B& kolk on Atldent Lake side of iuteduv& (1996 U.S.G.S. digitaI ortho- qd) Pluage Po&: Plunge pisformed down flow at the heads of the cataracts as a result of sub-fluvial erosion and transport of sdhntor bedrock at the base of waterfalls (Figures
13 & 14). The plunge pools within the heads of the Ancient Lake and Dusty Lake cataracts are formed in bedrock though there is a '"mispked" plunge pool near the mouth of the Ancient Lake cataract that fondearly jn tbe cataract formation. The plunge pwls generally have bars mantle the plunge pool floor, as well as, extending down stream from them. The plunge pols in the hertds af the catarm contain water from the CoIumbia Irrigation Project; having an elongated to circular morphology with the long axis indicating direction of flow, and no distinct volume. Plunge pools
Nustrate the fluvial origin by which Pleistocene flood waters once flowed through these cataracts before rejoining the Columbia River.
~13.~bofphursepoolbformedkMsediment&ahe~oftbebeadsJ Ancient hkeand DClsty Ldw ~~ Nhr P)Rmge pod (1% US.G& digital claad) Figure 14. Mdobl e gromtd view of p- PIS Iacated at the bd~d Amdent lake ad
Bars: The cataracts are fldby glacio-fluvid deposition with bar morphology. A bar is a streamlined deposition pointing in the direction of flow. A fosse, an elongate, eroded hollow next to the cataract wall, forms where water depth, channel topography and hydraulics allow for deposition of sediment. . The bars are classified based on morpho~ogyand depositional location. The bars of the study site are categorized as: 1) crescent-point bar, 2) pendant bar, and 3) expansion-longitudind bar (Waitt, 1994).The bars consist of a mixed lithology of sub-rounded to angular sand and cobble sized basalt, quartzite, schist, gneiss, rip-up clasts of calcrete, calcmte coated basalt, petdied wood, palagonite clasts with the mode being basalt, There are also angular basalt boulders with an intermdate diameter >1 m.
Crescent-point bars, sidar to point bars in morphology and deposition, form on the lee side of a bend in the channel, with or without an obstruction or a slowing of flow
(Waitt 1994). Great crescent bars rising more than 30 meters above the Babcock Bench and more then 720m long formed on the southeast edges of the mouths of both cataracts of Potholes Coulee (Figure 15). ~15*Msp~oP~-~~barscw~~dthemo*of~and WMWEB~,~~bp - (Waitt 1994). Pendant bsus have formed along the north and south in the Ancient Lake Cataract and north side of the Dusty Lalre cataract. The bar on the north side of the Ancient hkecatamct rises >30m above the cataract floor and is more then 2.25 km long with tdus partialiy to comp1etety fillirrn the fosw side (Figures 16 & 17). Fwth, the bar was mworked to form plunge pooh at the head of the eatmct. The bar dong the south wall of the Ancient Mecatatact experienced 3 bar budding events evidenced by bee pronounced ?errace like" fmon the north side of tha bar. This bar complex rises >3Om above the ccataract flaor and is more then 1.25 Zrm long with a mass wasting block and talus Hhgthe fom side of the bar and is mwoW to form plunge plsat the head of the cataract. The pendant bar on the north side of the Dusty Lake cataract formed on the down current side of a channeI spur and rises >15m above the cataract floor and is more then 1 km long with talus partially fiIling the fosm side of the bar. Figwe 17. Mmak oblique ground view of pendant bar along north side of the Ancient Lake cataract taken from hierfbeto the south. Note cammt &me train on the sdside of tbe bar, evaporite '~,andplmgeporSintltebersdofthecataract to &east. l%[email protected] axe formed in side channels or mid-channe1s with or without obstruction or chamel widening (Waitt I*). The expmsiodon@tudinal bar in Dusty We Camma fondat a widend section mid-channel, Tbis bar rises >10m hvethe c&m& floor and is more then 2 km Ibng with talus partidy Wgthe fosse sides and a plunge pool awedin to it at the hehe of the catarmt (Egw 18). Cuxrent dunes: Current dune features have ripple morphology and atse formed by wata flowing over workable sdbmt at a specific depth and velocity. The current dune features are lwated on the south side of the pendarat bar dong the north wdI of the Ancient Lakes cataract (Figure 19). The dune train is >1.5 km long forming on slopes < 1s" trending east-west with the dunes muding north- south lOOm long and have an average chord of 23 m and a mode of 17 m with surface amplitude < 1 m (Table 1). The rnorph010gy, wruate shnpe and sub-cxiticaily climbing dunes of the feature indicate direction of flow hrnthe eat. The coarse sediments on the surface and in the mnt dunes indicate st fluviaI origin. Giant Current Dunes Giant Current Dunes Dune Chord (m) Dune Chord (m) Statisties 1 35 22 20 Sum 948 2 41 23 27 Average 23 3 41 24 12 Made 17 4 41 25 28 Median 19 5 43 26 30 Minimum 12 6 30 27 17 Maximum 43 7 19 28 17 Std. Dev. 8.647Q74 8 1S 9 18 10 17 r- r- , ye- --- - .;. - - , - ,I, ..- \ . . C.. , -. rl -r - J .. .a 3, :,. ., -, LA .I . .- '9. - . , ' ' .,, ,Vi' 1. - ?-p ;X -: , A. 5 %. -8, :I . 43fd *? ' . > f$h-r # .# --,. :--* , y,,. *.-- .l I--_., .. ---. .- *. .L.. *'#, .;C?'+ L .. '--- r- r- . I.. - -. . A - "' . , .. Chemical and physical weathering both occurs within the study site &pW being discretely cat ego^ they often occur simahanesusly. Physical and &eW-d weathering enable e&h other: for example, physical weatbring mates more sH&@ area, thereby enabling chemical weathering to mur. Furtherpchemical weathering weakens the substrate enabling physical weathezing to wuf. Bath forms of d&& occur at present. , .. 'A I : ' , , I ,-I in i .. 11 . .b.. t PhvsicaI Weathering: Physical weatbering is the degradation of rock via mechanical means, often with more hen om mechanism occurring at once. The domimat form of physical weahzing at the site is a composite af thermal expansion-contraction (freeze- thaw) and fbtwedging. This weatkhg wcurs at locations of bedrock exposed by fldwakrs and on cliffs and horizontal surfhces with maximum surface area exposed to the environment. Fmm-thaw shattahg and wedging enables mkfd and talus coIIBction at the base of the cliffs as talus sIapes, Freeze-thaw shattering occurs at rn abandoned plunge poolkolk inside the eastern mgin of the mouth, more than 250 m away from CWSof the Ancient Lake catam$. The exposed bedrock is discontinuously mantled by a layer angular to sub-angular rock fragments, 5 to 25 cm (intermdate diameter) resembling colluvium and talus. The rock entshave a we&, < lmm weathering rind, little lichen growth with some baving an evaporite coating fkom the flooding of the cataract in the early 1950s. Similar weathering has formed a weII developed weathering mantle on the long plateau surface of the interfluve, A ks common form of physical weathering in the area is root wedging, the wedging apart of rock by shrubs growing the joints of rock. Tbough this is not common in the study site, it dms accur on cliff faces. Spalling occurs with isolated boulders creating a round corestone. An example of spalling is found on the fosse side of the pendant bar dong the south wall of the Ancient Lake camtmeasuring 0.75 m (Figure 20). . .. I - ' ... . I I"' I Figure 20. Oblique ground view of spalling, bdrcorestune resting in fosse of pendant bar 1-ted on the swth side of the Ancient lake a&ract. Note the joinling paraltet to tbe surface on the rock creating a round corastone. Also note the red-brown weathe* rlrrd and white evaporite coating. (Seth Ma- for scale) Chemical weathering: Chemical weathering is the in situ decomposition of rock via chemical reactions resulting in the formation of new minerals. The dominant chemical weathering in this area is oxidation; the process leads to the collapse of the crystal structure of the. iron in the basalt enabling weathering by other means, Oxidation of iron minerals is shown as a red weathering rind on exposed basalt be it a cliff or rock fragment. The development of the oxidation weathering rind varies with the amount of time the rock is exposed to the environment; thus a freshly broken piece of basalt will show little or no weathering rind in contrast to a fragment of basalt with long exposure to the environment will show a deeper weathering rind. The basalt rock fragments on the interfluve have resulting from a combination of oxidation and chelation (chemical weathering resulting from the production of organic acids by biota such as lichen). The basalts are subject to hydrolysis, replacement of Potassium cation (0with Hydrogen ion 0with the pmence of warn to break down the mineral to clay. Comoodte Wtxtk~As mentioned previously physical and chemical weathering rmly mnr independent of each other, so the two will dso be addressed simd~6ously, The long inkdive and cataract rim surf& exbibit signs of composite chemical and physical weahring occ~gtogether in a feedback system. During the fodanof the Potholes Coulee and subsequent floods, the surface was stripped of available sediment* leaving the Wkbare. After thh time physical and chemical weathering ocamd Wga &&ow weathering mantle. A combination of frost action, freeze- thaw Euad oxidation fragmented rock on the surface dngmore surface area enabling chemical watherhg, The rock fragment were then subjected to further physical and chemical weahring decomposing and degradhg the rock frsrgment into angular to sub- angular very coarse oxidized basalt sand and pebbles (figure 21). Tbe composite weathering regime on the interfluve has culminated in forming a shallow, well-developed weathering prome capped by loess. The stability and location of the sediment has enabled various biota and landforms to form such as patterned ground resulting fmm shrink-swell and lichen. ~re2~~and~wenU&&gwthe~e~Notetbebasaltweatheredto pagaiar~affl~-~&,andpebbhthem~rfnd,andbb~~ Eolian Demsition Eolian proeesscs in past and present affected the landscape with loess and cover sand blmketing the flwial depositions and weathering mantles. The loess forms as very fine sediments mmported as mspded bad then settles from the air in areas of decreased velocity and is tbe dominate land cover within the Columbia Basin. The depth of loess ranges from 20 cm at the mouths d rims ~f the cataracts to more then 1.S rn on striped bedrock terraces, damct flm, and dong the cliff bases of Babcock Bench, The loess consists of buff colored silt-sized particles and varies in compaction thoughout the year based on available moistwe and bioturbation. A thin, discontinuous cover-sandnag resulting from deflation is found thmugh out the study site, mostly occurring at the mouth of the catam& dong Babcock Bench or areas of consistent bioturbation. Eolian processes still occur, deflating disturbed surfaces as well as depositing loess. Mas Wasting Features Mass wasting is the dowaslope movement of sediment andfor bedrock as a result of gravity and is affected by fluvial undercutting, weatking, slope, aspect, climate, and vegetation. Mass wasting occurs in the study site as tskFa& Wand slides, undifferentiated slides and wlifluction ranging in magnitude of events from sd rockfall ( Rockfall: Rockfall rxcurs when a rock is pulled hma vertical. or newly vertical surface by gravity enabled by frost wedging and Ereeze-thaw action. Rockfall in the area occm at d aspects ofexpasure with the most occurring along cliffs with a northern aspect and ranges hn3cm to >5 m in diameter. For this report, roclcfrtll is classified as two categories baed on siztalus and discrete large block Tdus is class5ed in this report as coarse angular rock fragments, less than 1 m intermediate diameter, that collects at the base of the cliff it fell from The majority of the &all is classified as talus and forms 20'-30" slopes, more than 25 m tall: The majority of the chsts, in talus, mge hm5 - 50 cm diameter with some cIasts up to 1 m in diamter. The talus cowcoaIesce into one continuous apron at the bases of the cliffs in the study area, partially to completdy £ilhgthe fosse side of the bars. The talus slopes show no pressure ridges or a pronounced be= at the toe of the slope @gure 22). M- 22. Oblique pondview of talm slopmdtling fasse dde of crewmtlpht bar sodof Potholes Codee on Babeaek Bench. Discrete large black rocHall is classified as angular blocks of rock that collects at the base of the cliff it fe11 from occurring as large blocks greater than 1 meter intermediate diameter and as Iarge as 10 meters intermediate diameter. It consists of entablature or colonnades that succumb to the forces of gravity enabled by the jointed nature of the bastsalt bedrock, physical weathering, seismic events and sometimes extreme weather. In the study site, rockfall commonly occurs from cliffs with a northern aspect in both the Ancient Lake cataract and the Dusty Lake cataract as we11 as the along the Babcock Bench (Figure 23). Much of the large bIock rockfall, after rolling, comes to rest mantling the fosse side of bars and the cataract floors. :''.*IF, Lf.7 -' ': 1 ; Figure 23. Oblique pd view of large bloc% rock Mon Bhock Ben&. TbiF rock is more than 5 meters in intermdate diameter. Landslides: Landslides result from the downslope sliding of an area or the slope pulled by gravity enabled by bedrock structure, fluvial undercutting, weathering, seismic events, ox extreme weather. Landslides occurring at the study site incIude rotational slides and undifferentiated slides and range in width from 50 m to greater than 300 ra. The landslides occur on slopes with northern aspects in Ancient Lake, and Dusty Lake cataracts; and western aspects along the slope between the Babcock-Evergreen Ridge and Babcock Bench. Rotational landslides are mass movements in which the bottom of the slope is cantilevered away from the slope allowing the top of the slope rotated downslope leaving a scallop-shaped scarp in the cliff above the slide. The morphology of a rotational landslide includes: a sag, hallow, depression like W,behiud the main body with a toe at the: terminus of the slide. Rotational slides in the study site are often easily identified by the offset of the once vertical colonnade after the slide has corn to rest. A textbook xample of a bedrock rotational slide ~~in middle of the south side of the Ancient Aecataract (Figure 24). The slide dafter bar deposition as it and asso~htd lebris rest on the fosse side. of a pendant bar. Und8fprentiate.d lrtndslides are landslides that the mechanics offbe slide cannot le determined by visual, ohations dtber due to the nature of the slide or past event nodifidon. These landslides omin the southwest extent of the study site on the Babcock Bench along the Evergreen Ridge south of the mouth of the Dusty Lake cataract. The slides we recopkd by the scallop-shaped scarp left in the Evergreen ridge above the slide. The diff~cultyin determining the mechanism of the slide is that the main body of the siide has been reworked by hePleis-ne flooding down the Columbia fiver coupled with bar building. Two of the scarps have channels leading to them indicating fluvial flow prior or subsequent to tbe mass wasting event. Two hn south of the cmca-point bar is an undiEerentiated lmdsfide with the scarp width greater than Solifluction: Solifluction h the down dope movement of a subs-, in this case flood gavels on a bar, by graw enabled by saturation ofthe substme. Sulifluction cmmd on the channel side of the pendant bar dong the north waIl in the head of the Ancient Mecataract, The movement occnrred on a slope than 20 degrees providing ttae energy necessary to compellsate for the weIJ drahhg rmbstrab of ahe bar. The feature is 10 m wide 20 m long and 0.5 m deep with a small tm on the down slope side of the Reatwe. Biotic Geonmmhic Fbatws Bafa (flora, faun& and htmm) can be compet%ntpmphic agents. The study site has wikgom change by £bra,fa- and humans discreetly and in composite, brirrging various forms of change. Biota can act as ether a slrrface stabilizer or a source of &ace disruption. Flora: A well developed flora egime indicates a stable surface. Grasses and shrubs stab& the surface by covering the mufpbce making it resistant to eolian and slup erosion. Lichen acts as a chemical weathexing geomorphic agent by secreting organic acids that decompose the minerals of the rock it's growing on, thereby enabling other forms of weathering and erosion. Lichen grows at a slow rate and the presence of well developed lichen indicates a long duntion of surface stability. Faux Coyotes, badgers, mice and oher small mammolIs canse biohvbation of the suffwe and subsurface mixing the stratum of the sail pmfde by burrowing and fomghg. Worms chum the soil as they work their way through creating a homogenous soil profile ;as well as adding humto the sail, Eoms and deer create trails and disrupt the surface as they walk, enable and prepare the substrate for eolian processes (transportlde:£Mion) and slope erosion. This is especially noticeable during transition from winter to spring whe~surface &grades from a compact surface to an uncolasolidated hose dust that billows mmd ones feet. flumans - Humans, though in the area for a short time, have been very effective geomrphic agents. Humans have occupied the area for several thousand years prior to modern upa at ion which began in early 1920s with homeskdng, The land use includad range land tellring advantage of the natural corrals created by the atamcts (personal communication, Dave Bishop, June 2002). The mawas without irrigation save one pmdd spring on Babcock Bmch outside of tbe Potholes Coulee. In the 1950s the Cp1mnbia Basin higation Project brought warto the region for @cultme. In the early 1950's, the Aneient Lake Cataract was flooded intentionally to create a lake, however, due to a fault in the cataract the water slowly leaked out (Grolier 1965). There were sevrerd attempts to maintain the lake's level, however, this was abandoned in subsequent yews. Despite the ephemeral, nature of this lake, it had an impact on the cataract mating mcidbiotic barriers and creating new landforms. SaIts precipitated from the ponded lake water, Ieaving an evaporite. The shomline mated a barrier blocking the native flora that once occupied the area from re-inhabiting the area (Figure 261, Bacteria and more alkali tolerate plants could prwes the salts in the soil enabling re- colonization by the local shntbs. kt present, more than 40 yeas since the lake was abaudoned, a few sagebrush shrubs are beginning to occupy the area. The daminant flora is cheat grass and Russian thistle. The Iake did nat only impact the local biota, it also mat4 unique landforms. The lake left temwfks and a %bat tub" ring of evapite indicating the past sho~line 17,20, & 26). The tewacettes are continuous pdelsmall terrace features similar to shorelineg that can be fouod along resavois shores at present. It is unlikely that they are of a faunal om(cattle &.ail temttes), m they do not braid hga one another. Also, the pheno~nembe observed at modern memoirs. is: a 0 500 1000 Figure 26: 1962 US.D.A. airphoto of tbe Pothole8 showing waning lake waters m the Ancient Lake cataract. Note the wbite evapite deposition showing dmumlake Ievet. The Potholes Coulee has been affected by volcanism, kctonism, fluvial, eolian, weathering, mass wasting, humans, and biotic geomorphic factors from the Miocene epwh to present creating the Potholes Coulee, an amazing landform The Miocene- Pliocene CaBGs created unique mktbat was folded by Pliocene tectonic forces, setting the stage for future modification by Pleistocene catastrophic fldgevents. Th~ughthe events setting the stage for future moMcation occurrsd over millions of years, they culmin&e in it msmndo of glaci6fluvial processes which sculpted amazing landforms in the stark CRBGs. Throughout tbe Pleistwene, episodic catastrophic flooding even& operated as competent erosional forces, plucking basalt along structural weaknesses crdngthe Potholes Coulee, and then deposited mvid mega-Worms flooring the excavated cataracts. Eolian deposition blanketed the bedforms, wdherhg mantles, and mass wasting deposits. Rockfall detritus mantles the base of the catatact's walls. The combined geomorphic pracesses create a unique landscap. Potholes Coulee is a steep walled double horsedm cataract with sinuous elongate kolk takes above the heads of the catam&. Lakes fill concentric elongate plunge pools a the base of the cataract's W. Mega-bars and bedforms of flood sediment discontinuousiy floor the cataracts. Tdus and landslides mafltle the steep cataract walls. Varied depths of loess blanket tbBe flood ssdiments and flora has st&W the surface. Anthropogenic agents expbited the setting through itrigation and Mebuilding events, as well as comtructing trails and introducing exotic fauna (cattle and Modern hams) to the area. After nearly a century of research, many questions main regarding the geomorphblogy of the Quincy Basin, and specifically the Potholes Coulee: 1) The George Gravels in the George Chamel mmecttd to Potholes Coulee imply it was fodhundreds of thousands of years prior to the late Pleistocene catastrophic floods. Dbtresearch addmsing the age of the gravels and linking them to the Potholes Coulee would 'be a valwb1e project in explaining the genesis of the Channeled Scabland; 2) Exploring the blks on the interfluve in the Poth01es Coulee would yield further understanding in the temporal origin of the glacio-fluvial activity that flowed through the study site as well as reconcile the dramatic difference in weathering between the elevated interftuve surface and lower cataract surfaces; and 3) Quantifying the current dunes by understanding the sub-loess topography would allow description of the current dunes and calculation of flow through the cataract at time of origin. Acknow~ts Tbis research was made possible by the financial suppart of the Evolving Earth Foundati~n.I acknowledge David Bishop for excavating the sloth remains, historic site description, use of his personal library, allowing access to the site, and bring his discovery to the scientific c~mrnunity.I acknowledge Steven Hacbnberger for incorporating this work into his fmal report, access to his personal study site photos, and permitting access to the sloth site. I acknowledge Karl LilIquist for his guidance through out the project, assistance in the field, and pf-readhg earlier versions of tbis manuscript. I appreciate Robert Hickey, for guidance with GIS technologies and for proof-readiug earlier versions, and Seth Mattos, for assistance in the field Literatures Cited Baker, Victor R., 1973, Paleohydrology and Sedinentology of Lake Missoula Flooding in Eastern Washington: Special Paperl44, Geological Society of America, Boulder, Colo* Baker, V. R., and D. Nummedrll. 1978. 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Scom of gigantic, successively sderLake Missuda floods through CWledScabland and Columbia Valley, in Swansun, D. A., and Haugerud, R. A., &. Cieo1ogic field trips in the Pacific Northwest. Wo@cal Society of America Annual Meeting, Boulder, Co., 1K Watters, Thomas, B.,1989, Periodically spaced anticlines of the central Columbia Phu,in V&Wm and Tectonism in the Columbia River Basalt Province: Geologicd Society of America, Special Paper 239, edited by Stephen R. Reidel aad Peter R,Hopper, P, 283-292, Geological Society of America, Inc., Boulder, Colorado Western RegidClimate Center, Quincy Is, Washington, http:fhtww.wrcc.dri.&cgi- bin/cmAIN.pI?wquin