Stratigraphy and Hydrology of the Jackson-Frazier Wetland, Oregon

Stratigraphy and Hydrology of the Jackson-Frazier Wetland, OregonDavid V. D'Amore,* Scott R. Stewart, J. Herbert Huddleston, and J. Reed Glasmann

ABSTRACTThe relationship between wetland soils and hydrology can be better

understood by linking soil geomorphological features to the measure-ment of groundwater depths in the soil. Soil stratigraphic analysis andlong-term measurements of soil water levels in piezometers were usedat the Jackson-Frazier wetland in western Oregon to investigate theinteraction between local geomorphological history and the hydrologyof the wetland. Morphological descriptions confirm the presence ofa recent smectitic alluvial deposit (80-180 cm) overlying Malpass clay(=35 cm thick), which overlies Irish Bend Silt. X-ray diffraction andisotope analysis support the conclusion of the presence of the Holo-cene alluvium and Irish Bend Silt, but are inconclusive regarding theMalpass clay. Piezometer data from 1992 to 1996 show that the smec-titic alluvium controls saturation and drying of the wetland surface,and that a separate water table is present below the Malpass clay inthe Irish Bend silt. The recent alluvium and Malpass clay act as anaquitard that restricts the vertical infiltration of surface water andhelps restrict the groundwater table in the Irish Bend silt deposit.These stratigraphic relationships and associated hydrologic responsesprovide a means to identify wetlands and predict hydrologic conditionson similar wetland landscapes.

WETLAND SOILS and wetland hydrology are intri-cately linked in the development and functioning

of wetland ecosystems. Relationships between wetlandsoils and wetland hydrology are best understood whenthere are adequate data to fully characterize soil mor-phology, surface water hydrology, groundwater hydrol-ogy, and transmission or lack of transmission of waterthrough the soil (Mausbach and Richardson, 1994).

Understanding hydrologic processes in relation togeomorphology and soils can provide indications ofwhere wetlands may occur on the landscape, but de-termining these processes requires information on thegeologic and hydrologic setting and their influence onthe flow of water (Winter, 1988). Often, hard hydrologicdata are unavailable for many wetland types, and infer-ences about wetland functions must be based on land-scape features, soils, geomorphology, and vegetationpatterns. Linking geomorphology, soil stratigraphy, andsoil hydrology provides a basis for interpreting satura-tion patterns and wetland extent without intensive mon-itoring in similar areas.

Water movement, or impedance of movement, in thesoil also plays a critical role in soil formation and therelated soil physical and chemical transformations(Richardson et al., 1992). Studies that establish connec-tions among soils, geomorphology, and hydrology can

David D'Amore, Pacific Northwest Research Station, USDA ForestService, 2770 Sherwood Lane, Suite 2A, Juneau, AK 99801; ScottStewart and J.H. Huddleston, Dep. of Crop and Soil Science, OregonState Univ., Corvallis, OR, 97331; and J.R. Glasmann, Dep. of Geosci-ences, Oregon State Univ., Corvallis, OR 97331. Received 2 Nov.1998. * Corresponding author ([email protected]).

Published in Soil Sci. Soc. Am. J. 64:1535-1543 (2000).

provide information about soil development as well aswater movement. The recent hydromorphic soil studyby Thompson et al. (1998) linked topography and soilgeomorphology to water table dynamics to predictwhere saturated soil conditions occurred on the land-scape. Steinwand and Fenton (1995) developed hydro-logic relationships using soil stratigraphy and piezome-ter data for a glaciated landscape in Iowa. Richardsonet al. (1992) used flownet analysis to illustrate watermovement through soils on different landscape posi-tions. All of these studies have illustrated the impor-tance of the interactions of geomorphology, stratigra-phy, and soils in the movement of water through thelandscape. Establishing regional studies of water move-ment in specific landscapes provide information on wa-ter dynamics that can be applied to similar local land-scapes, as well as similar soil geomorphic relationshipsin other regions.

In the Willamette Valley of western Oregon, wetlandsare a common feature on broad, flat Late Pleistoceneterraces and along the geomorphic boundary betweenthe high terrace of the valley floor and the foothills ofthe Coast Range and Western Cascades. Complex soilstratigraphy related to interfingering of Late Pleistoceneglacial flood deposits on older valley geomorphic sur-faces (the Willamette Formation; Balster and Parsons,1968, 1969) often results in abrupt textural differenceswithin the solum that strongly influence pedohydrology(Boersma et al., 1972; Austin and Huddleston, 1999).In addition, deposition of Late Pleistocene lacustrinesediments within the Willamette Valley disruptedstream gradients along minor channels entering the val-ley from adjacent foothills, augmenting clay-rich, allu-vial fan sedimentation. The Jackson-Frazier wetlandrepresents one of these valley marginal, alluvial fan wet-lands. The site represents a Holocene valley margingeomorphic surface underlain by clayey alluvium. Thesite has previously been used as pasture land, but hasreverted to natural vegetation cover since 1980. Thearea is now a Benton County park and is a valuablewetland resource, both for its environmental qualitiesand for educational programs for the surrounding com-munities.

The objectives of this study were to characterize thesoil geomorphic and stratigraphic relationships of theJackson-Frazier wetland and to investigate the influenceof these deposits on wetland hydrology.

MATERIALS AND METHODSSite Description

The Willamette Valley occupies a structural-erosional de-pression in western Oregon that lies between the CascadeRange to the east and the Coast Range to the west. The

Abbreviations: PVC, polyvinyl chloride; XRD, x-ray diffraction.

1535

1536 SOIL SCI. SOC. AM. J., VOL. 64, JULY-AUGUST 2000

Jackson-Frazier | \Creek — — — _

Transect A to A'

— — — — Watercourse66 Elevation (m)48) Mineralogy sample site

N

Fig. 1. General location of Jackson-Frazier wetland and locations of monitoring sites, mineralogy sample sites, and stratigraphic transect.

Jackson-Frazier wetland is an =64-ha area located at the west-ern margin of the valley north of the city of Corvallis at anelevation of =66 m (Fig. 1). The overall configuration of thewetland is an alluvial fan with the Jackson and Frazier Creeksource streams in the northwest corner dispersing water andsediment down the elevation gradient to outlet channels alongthe northeast (Frazier Creek ditch) and southeast edge (Stew-art Slough) of the wetland. The subdued topography of thewetland and the presence of numerous beaver (Castor cana-densis) dams below the confluence of Jackson and FrazierCreeks in the northwest corner of the wetland have createdan area of complex overland flow and seasonal soil saturation.Vegetation includes forested, shrub-scrub, and prairie palus-trine plant communities (Marshall, 1985). The area receives= 1030 mm precipitation annually, most of which falls fromOctober to March. The mean annual temperature of the areais 11.3°C.

Soils and StratigraphyThe predominant soil mapped (Knezevich, 1975) within the

wetland is the Bashaw clay (very-fine, smectitic, mesic XericEndoaquert). The landscape associated with this soil consistsof a backfilled, low-relief, broad swale that was eroded intothe Calapooia surface (Balster and Parsons, 1968) by Jackson-Frazier Creeks. Neighboring soils on the Calapooia surfaceinclude the Dayton (fine, smectitic, mesic Typic Albaqualf)and Amity (fine-silty, mixed, superactive, mesic ArgiaquicXeric Argialboll) soils developed in Late Pleistocene lacus-trine silts and clays of the Willamette Formation. The Daytonseries has at least three major Late Pleistocene soil strati-graphic units, including the Greenback Member (surficial siltloam), the Malpass Member (clay), and the Irish Bend Mem-ber (silt loam to silty clay loam; Balster and Parsons, 1969).The erosive episode that developed the wetland swale scouredaway the Greenback unit and may have altered some or allof the Malpass unit. The Bashaw soils are located on the

Ingram surface (Balster and Parsons, 1968), which has alluvialdeposits in place of the Greenback member.

The pedons selected for detailed soil description and sam-pling in this study lie within the Holocene alluvial fan thatwas deposited on the eroded Calapooia surface. Three soilpits were excavated to a depth of 2 m (Fig. 1; Sites 1, 2, and3), subdivided into major horizons, and described (Soil SurveyDivision Staff, 1993). Although the soils at all three sites areclassified as Bashaw, the vegetation at each site is different.Site 1 occurs at an interface between wet meadow and shrub-scrub cover types and is dominated by water parsley (Oenanthesarmentosa K. Presl ex DC.). Site 2 lies in a shrub-scrub areaand is dominated by slough sedge (Carex obnupta Bailey).Site 3 occurs in a forested area with a dominant overstoryof Oregon ash (Fraxinus latifolia Benth.) and a dominantunderstory of reed canary grass (Phalaris arundinacea L.).

Five roughly north-south sampling transects were estab-lished across the wetland to observe soil stratigraphic relation-ships. Auger holes 2 m deep were hand-augered 50 m apart,but thick vegetation often made it necessary to lay out thetransects in a zig-zag pattern. The location of each bore holewas determined from compass bearings and measured dis-tance. The elevation of each site was determined with a transitand stadia rod from a reference benchmark. Soil color, texture,redoximorphic features, and horizon boundaries were re-corded for subsequent stratigraphic characterization of thewetland (Soil Survey Division Staff, 1993).

Clay MineralogySamples for clay mineral characterization were obtained

from the 2Bt horizon at Site 1 and at depths of 50, 100, and180 cm at four points along a northwest-southeast transectacross the wetland (Fig. 1, Transect AA', Sites 48, 85, 109,and 129). Each sample was placed in a sealed plastic bagto retain field moisture content until analyzed. Clays weredispersed by mixing 5 to 10 g of moist soil with 150 mL of

D'AMORE ET AL.: JACKSON-FRAZIER WETLAND STRATIGRAPHY AND HYDROLOGY 1537

Table 1. Morphological properties of soils described at three sites at Jackson-Frazier wetland.

Site

1111

1

1112222222333

333

Horizon

AlA2A3Bssl

Bss2

2Btl2Bt2SBCAlBsslBss2Bss32Bt3BCtl3BCt2AlA2BA

BwBsslBss2

Depth

cm0-S8-15

15-̂ tl41-66

66-104

104-119119-135135-155

0-1818-3434-5050-8282-100

100-118118-140

0-1111-2222-39

39-7474-114

114-174

Moist color

10YR 3/210YR 3/110YR 2/160% 10YR 2/140% 10YR 4/260% 10YR 4/240% 10YR 2/12.5Y 4/22.5Y 5/2 2.5Y 4/22.5Y 4/410YR 2/2N3/0N2/02.5Y 4/2 N 3/02.5Y 5/3 N 3/02.5Y 5/42.5Y 4/210YR 3/210YR 3/110YR 3/1

10YR 3/12.5Y 3/260% 10YR 3/140% 2.5Y 3/2

Texturet

SiCSiCCC

C

SiCSiCLSiLCCCCSiCSiClSiLSiCSiCSiC

SiCSiCSiC

Black Fe/Mnconcretions^:

_IfIf2f

-

-_-

1m1m2m2m, IcIf, Ic1mIf (stains)

__

1m

1m2m2m, 2f

Soft masses?

_Ifp 7.5YR 5/6Ifd 7.5YR 5/4

-

Ifd 7.5YR 4/4

Ifd 7.5YR 4/4Ifd 7.5YR 4/4

-_

Iffa SYR 4/4Iffa SYR 4/4

-Iffa 7.5YR 4/62fd 7.5YR 6/42fd 7.5YR 5/6

_3fd 2.5YR 4/43md 2.5YR 2.5/23md SYR 3/32fd 10YR 4/4

_--

Organic C

%7.983.421.321.10

0.58

0.220.150.084.561.361.020.730.230.120.095.252.882.01

1.280.660.32

Slickensidesi

_-Id3p

3p

---_2d2d3p

--_--

-2d2d

t Texture abbreviations: C = clay, SiC = silty clay, SiCl = silty clay loam, SiL = silt loam, Si = silt.t Concretions, soft Fe masses, and slickenside abbreviations: 1 = few, 2 = common, 3 = many, f = fine, m = medium, c = coarse, fa = faint, d = distinct,

p = prominent.

distilled water and 5 mL of 0.5 % (w/v) Na-hexametaphosphatein 250-mL polyethylene bottles and shaking for 8 h. Afterdispersion, the clay fraction (<2 mm) was separated by centrif-ugation and was saturated with Mg by washing three timeswith 0.5 M MgCl2 followed by three rinses with distilled water.Oriented slides of the Mg-saturated clay were prepared forx-ray powder diffraction analysis (XRD) by a paste method(Thiessen and Harward, 1962). The remaining clay was then K-saturated with 1 M KC1 using a similar procedure and orientedslides were prepared for XRD. The slides were treated ac-cording to the method outlined by Glasmann and Simonson(1985), excluding the glycerol solvation and high temperaturetreatments. Slides were analyzed using a computer-automatedPhillips XRG 3100 (Phillips, Eindhoven, the Netherlands)equipped with compensating slits and a focusing monochroma-tor. Copper ka radiation was used (40 KV, 35 mA) and slideswere step scanned from 2 to 34° 26, using a step size of 0.02°20 and a count time of 1 s. Semiquantitative interpretation ofthe clay mineralogy of each sample was facilitated by compari-son of sample XRD spectra to computer-generated clay mix-tures calculated using NEWMOD (R.C. Reynolds, Jr., 1985,Hanover, NH).

Isotope AnalysisThe fine clay fraction (<0.2 |xm) of several soil samples

was separated by centrifugation after dispersing the clays asdescribed above. The samples included soil from the Bss2,2Bt, and 3BC horizons at Site 1 and a reference sample ofthe Malpass Member of the Willamette Formation, whichcomprises the restrictive clayey subsurface horizon of the Day-ton series. Dayton soils do not occur within the Jackson-Fra-zier wetland, but the 2Bt horizon has the color and strati-graphic position of the Malpass clay and we wanted todetermine if Malpass clay did extend into the wetland andinfluence subsurface hydrology.

Organic matter was removed from the <0.2-jjim clay frac-tion using three alternating treatments with 5% (w/v) Na-hypochlorite and 20% (v/v) H2O2. Metal oxides were removed

using dithionate-citrate-bicarbonate extraction as outlined byJackson et al. (1986). The clays were then Mg-saturated asdescribed above, except following the second rinse with dis-tilled water, each sample was rinsed an additional three timeswith ethanol to facilitate complete removal of excess salt.Oriented slides of the Mg-saturated clays were prepared forXRD analysis, and the remaining clay was dried at 50°C andsent to an outside lab for O18 isotope analysis by mass spec-trometry (analyses performed by Dr. F.J. Longstaffe, Uni-versity of Western Ontario, London, ON, Canada). Reproduc-ibility is ± 0.3%o for 8 O (F.J. Longstaffe, 1996, personal com-munication).

HydrologyPiezometers were installed in triplicate at each of the three

detailed soil sampling sites (Fig. 1, Sites 1, 2, and 3) at depthsof 25, 50, and 100 cm. A single 200-cm piezometer also wasinstalled at each site. Piezometers were constructed from 1.9-cm o.d. schedule 200 polyvinyl chloride (PVC) pipe. Eachpiezometer was placed in a hole bored to the required depth.Then the hole was backfilled with fine sand to cover theopenings in the PVC tube, and the remainder of the hole wasbackfilled with bentonite powder to seal the tube from surfaceleakage. The piezometric surface at each site was recordedweekly by measuring the depth from the ground surface toan indicator float.

RESULTS AND DISCUSSIONSoil Stratigraphy

Detailed morphological descriptions at each of thethree soil pits (Table 1) indicate the presence of threedistinct stratigraphic units in these wetland soils. Thesurface unit consists of black clay (dominantly 0 or 1chroma) in which a Bss horizon has developed beneaththe A horizon. The Bss horizon displays both slicken-


350 650Distance (m)

1150

Irish Bend Silt (Unit 3) Malpass Clay (Unit 2) ^H Holocene Alluvium (Unit 1)Fig. 2. Southeast (A) to northwest (A') stratigraphic transect in Jackson-Frazier wetland developed with data from Sites 48, 85,129, and 109.

sides and redoximorphic features as either soft Fe-Mnaccumulations or small, hard Fe-Mn concretions (oftenboth occur together). This surficial unit averages =100cm thick across the wetland, but is as thick as 180 cmin the northwest corner and thins to =80 cm toward thesoutheast, suggesting proximal thickening of the alluvialfan deposits towards the Coast Range sediment source(Fig. 2). In spite of these thickness variations, the textureof Unit 1 remains clayey throughout its areal occurrence.The clay percentages remain consistent across the sitewith 50 to 55% clay in the surface horizon, and 60 to66% clay in the Bss horizons.

The second stratigraphic unit consists of a gray toolive brown silty clay (dominantly 2 or 3 chroma) 2Bthorizon that contains a diverse assemblage of redoxi-morphic features (Table 1). The boundary between thisunit and the overlying Holocene alluvium is a gradual,wavy zone where the underlying unit has been partiallyreworked and enriched with material from the upperdeposit. The second unit is distinguished by colorchanges from black (10YR 2/1) to dark grayish brown(2.5Y 4/2) at Site 1 and to light olive brown (2.5Y 5/3)at Site 2. The texture changes from clay in the overlyingBss to silty clay in the 2Bt horizons. The thickness ofthe 2Bt silty clay unit averages =35 cm across the wet-land (Fig. 2). Unit 2 has the morphology and strati-graphic position of the Malpass clay of the Willa-mette Formation.

The third stratigraphic unit consists of olive brown(dominantly 2.5Y 4/3, 4/4, and 5/4) micaceous silty clayloam 3BC horizons that grade with depth to silt loam.This unit also contains a diverse assemblage of redoxi-morphic features similar to those found in overlyinghorizons. The color, texture, and micaceous mineralogyof this unit strongly suggest that it is related to theIrish Bend Member of the Willamette Formation. TheGreenback member of the Willamette Formation (Bal-ster and Parsons, 1969) is missing at this location, im-plying partial erosion of this younger lacustrine strata

in the wetland, with deposition of Holocene clayey allu-vium over Malpass clay (Fig. 2).

Clay MineralogyThe clay mineral assemblage of Unit 1 (Holocene

alluvium) is dominated by smectite, with minor amountsof dehydrated halloysite, vermiculite, and possibly inter-stratified smectite and kaolinite (Fig. 3). The characterof XRD patterns of the Mg-glycol treated <2-mm clayfrom Unit 1 is nearly identical from site to site alongtransect AA'. This widespread lateral homogeneity insoil mineralogical composition is consistent with a pointsource for the sediments (i.e., Jackson-Frazier Creeks).Noticeably absent from the clay assemblage of the allu-vium are micaceous clays, chlorite or chloritic in-tergrades, and more significant amounts of kaolinite.These minerals generally occur in paleosols that mantlefoothill landscapes of western Oregon to the north ofJackson-Frazier wetland (Glasmann and Kling, 1980).Their exclusion in the sediment discharge of Jackson-Frazier Creeks suggests a process of preferential erosionof smectitic clays, derived from volcanic parent material,along low-order Coast Range streams.

The smectite-dominated clay mineral assemblage thatcharacterizes the 50-cm depth across the wetland is alsopresent in deeper samples (100-cm samples at Sites 129and 109, Fig. 3); however, several samples show an in-crease in the mica component, which coincides withthe stratigraphic break between Units 2 and 3 (weakindication of mica in the 100-cm samples 85 and 48,Fig. 3). The 100-cm sample from Site 48 is below thestratigraphic break between Units 2 and 3, while thesample from 85 is above the stratigraphic break betweenUnits 2 and 3. The mica component increases with depthfrom Unit 2 to Unit 3 (180-cm samples at Sites 85 and48 are in Unit 3; Fig. 3). Since the recent alluvium isnon-micaceous, the increase in the mica component ofthe clay assemblage lower in the profile does not reflect


5 10 15 20 25 30

10 15 20 25Z-Theta(deg)

20 25 302-Theta(deg)

Fig. 3. X-ray diffraction patterns from <2-(j,m, Mg-glycol treated clay at 50-, 100-, and ISO-cm depths at auger sample Sites 109, 129, 48, and85. 1 = Mg dismectite-glycol, 2 = di-vermiculite, 3 = illite, 4 = dehydrated halloysite, 5 = Mg-chlorite, 6 = kaolinite.

pedogenic destruction of mica in Unit 1. Instead, thepresence of mica probably indicates clay minerals de-rived from the Willamette Formation.

The Irish Bend Member of the Willamette Formationhas a clay mineral assemblage characterized by the pres-ence of smectite, mica, chlorite, and kaolinite (Glas-mann and Kling, 1980). This unit was deposited by aseries of catastrophic glacial floods from glacial lakeMissoula and includes micaceous glacial flour scouredfrom loess deposits of the Channeled Scablands ofWashington that were ultimately sourced from the Ca-nadian Rockies and other metamorphic terranes. In con-trast, Jackson-Frazier Creek drains an area underlainby deeply weathered sea-floor basalts and tufaceoussediments of Cascade origin (Orr et al., 1992), rock typesthat are deficient in mica. The micaceous character ofthe olive brown silts of Unit 3 is strong evidence thatthis unit represents the Irish Bend Member. Comparisonof the XRD patterns of micaceous Unit 3 samples withpatterns obtained from the clay fraction of the IrishBend Member from the type locality at Irish Bend andother exposures (Fig. 4) suggests many similarities andindicates that the Irish Bend member has a fairly homo-geneous clay mineral assemblage across the southernWillamette Valley. The increased smectite componentof the Irish Bend Member at Jackson Frazier wetlandmay reflect illuviation of Holocene smectitic alluviumand/or additional postdepositional pedogenic formationof smectite in the wetland environment.

In contrast to the somewhat micaceous and chloriticmineralogy of the Irish Bend silts in Unit 3, the mineral-ogy of Unit 2 clays is very similar to that of the Holocenealluvium in Unit 1 as shown in the 100-cm sample atSite 85 (Fig. 3), and in a sample from known Malpass

clay in the Dayton soil at Corvallis airport (Fig. 5).Although Unit 2 has the color, texture, and stratigraphicposition of the Malpass clay, there is very little mineral-ogical reason to distinguish it from the smectitic allu-vium deposited by Jackson-Frazier Creeks. Comparisonof Unit 2 clay mineralogy with examples of known Mal-pass clay (Fig. 5) suggests that Malpass mineralogy isnot as distinct from the Holocene alluvial material asthat of the Irish Bend member in the Jackson-Fraziersediments (Fig. 3). The origin of the Malpass clay isunclear, but it may have been deposited in depressionalareas of the valley floor as overbank deposits from localstreams. The period following deposition of the IrishBend member was characterized by disrupted drainagesdue to filling of former stream courses by lacustrinesilts. Widespread, frequent flooding of lowlands wasprobably an annual event, resulting in overbank claydeposition from a multitude of sources around the valley(Balster and Parsons, 1968; Parsons and Herriman,1970). Such a model would help explain the mineralogi-cal diversity of the Malpass clay and the weakly ex-pressed micaceous character of the deposit, reflectingslight reworking of the underlying Irish Bend silts(Fig. 3).

Further support for the Malpass-like character of Unit2 clays in the Jackson Frazier wetland is found in thestable isotopic composition of the fine clay fraction (Ta-ble 2). Fine clay from the bulk soil matrix has a veryuniform 818O composition, averaging =21.8%o. By con-trast, the fine clay fraction liberated from Fe-Mn soilnodules of the 2Bt and 3BC horizons (presumed Mal-pass and Irish Bend units) is depleted in 18O relativeto the matrix clay (19.3 vs. 21.8%o, Table 2). Similarcontrasting isotopic compositions have been noted for


10 15 20

2-Theta (deg)30

Irish Bend type section

\ Calapooia River

15 202-Theta (deg)

Fig. 4. Irish Bend x-ray diffraction patterns: (a) sample from Jackson-Frazier Site 48 at 180 cm (upper pattern) compared with a sample fromthe Irish Bend type section at Irish Bend, OR (lower pattern), (b) sample from the Irish Bend type section at Irish Bend, Oregon (upperpattern), compared with a sample from the Calapooyia river near Turner, OR (lower pattern). 1 = Mg-dismectite-glycol, 2 = di-vermiculite,3 = illite, 5 = Mg-chlorite, 6 = kaolinite, 7 = albite.

fine clays from the soil matrix from recent smectiticalluvial soils of the Malabon (fine, mixed, superactive,mesic, Pachic Ultic Argixeroll) series on the Mary'sRiver flood plain near Corvallis, OR, and Irish Bendsilts from Woodburn soils (fine-silty, mixed, superactive,mesic, Aquultic Argixeroll) of the adjacent Calapooiaterrace (Table 2, data provided by Jeff Schatz, Geosci-ences Dep., Oregon State Univ.). These observationssuggest that the O isotopic composition of smectiticclays of the Irish Bend Member of the Willamette For-mation (Woodburn soils, Table 2) is an average of 1.9to 3.3%o lighter (depleted of heavy isotopes) than thatof smectitic clays (Malabon soils, Table 2) derived fromCoast Range soils. The lower isotopic composition ofIrish Bend smectite suggests the extra-valley formationof smectite from isotopically depleted water related toLate Pleistocene glaciation and geographic influenceson the isotopic composition of precipitation (rainfallgenerally becomes depleted of heavy isotopes landwardfrom the ocean). The presence of an isotopically lightsmectite component within redoximorphic nodules of

Units 2 and 3 suggests that the nodules may preservelocal "islands" of clay with Irish Bend-like isotopic com-position due to a somewhat protected environment fromweathering, whereas the surrounding matrix has ac-quired the isotopic character of clay equilibrated withlocal Willamette Valley meteoric water. This indicatesthat the matrix of both Units 2 and 3 have been influ-enced either by illuviation of locally formed smectiticclay or by the in situ genesis of smectite in equilibriumwith local meteoric water. Doser et at. (1998) found thesame type of meteoric water enrichment in fine claydeposits in Louisiana. The deposition of the clayey, Mal-pass-like Unit 2 and overlying Holocene clay stratawithin Jackson Frazier wetland has a profound impacton the hydrology of this wetland system.

HydrologyWater levels in the piezometers and local weekly pre-

cipitation amounts at Site 1 are presented in Fig. 6. Datafrom monitoring Sites 2 and 3 are similar to those from


10000

8000

6000

4000

2000

Table 2. Oxygen isotope data for matrices, nodules, and knownMalpass member and for Irish Bend/Holocene sedimentsf forthe <2-(jLin fraction in Willamette Valley, Oregon.

25 3015 202-Theta(deg)

Fig. 5. Comparison of Malpass x-ray diffraction patterns. Sample frompresumed Malpass deposit at Jackson-Frazier wetland (upper pat-tern) and Malpass sample from Corvallis airport (lower pattern).1 = Mg-dismectite-glycol, 2 = di-vermiculite, 3 = illite, 5 = Mg-chlorite, 6 = kaolinite.

Site 1, but are not shown. Precipitation was below aver-age during the 1992 to 1994 monitoring period. During1994 to 1996, the Willamette Valley had record rainfall,including a 100-yr flood event in 1996. Water levelsmeasured in piezometers placed in the Holocene allu-vium at 25- and 50-cm depths indicate that the soil issaturated from the soil surface to a depth of 50 cm foralmost the entire period from October to June in allyears. The water level in the 25-cm piezometer fluctu-ated from 3 to 10 cm higher than that in the 50-cmpiezometer from the point of initial wetting in Novem-ber 1992 until February 1993 when the water levels at25 and 50 cm showed the same degree of saturation.The same difference of 3 to 10 cm between the 25- and50-cm piezometers occurred from November 1993 toFebruary 1993 as well. The different response was prob-ably due to the swelling of the surface clay and thedecreased infiltration of water to the 50-cm piezometerdue to rainfall and runoff from Jackson and FrazierCreeks after a prolonged, unsaturated period during thesummers of 1992 and 1993. During August and Septem-ber 1993, ponded water disappeared and the upper 50cm of soil was dry. Though there are no data for the1994 season, the data from 1995 and 1996 show that theabundant rainfall during the summer months main-tained surface saturation for much longer periods oftime than the 1992-1993 period when the soil surfacecompletely dried out from July to November. Surfacesaturation increased from a minimum of 245 d during1992-1993, to 297 d in 1994-1995, and 285 d in1995-1996.

The piezometers at the 100-cm depth were located inthe lower portion of the Holocene alluvium, and thepiezometers at 200-cm depth were located in the IrishBend silt. The piezometric surfaces at these two depthsshow water levels rising toward the surface during theearly part of the rainy season in November and Decem-ber in all years (Fig. 6). The water in the 200-cm piezom-eter was close to 200 cm below the surface in earlyNovember 1992 after the summer drought, and climbedrapidly through late January 1993 to a level 40 cm belowthe soil surface in response to higher quantities of rain-fall. The piezometric surface eventually corresponded

Sample Depth 5018

Bashaw Bss matrixBashaw Bss noduleBashaw 2Bt matrixBashaw 2Bt noduleIB (3BC) matrixIB (3BC) noduleDayton 2Bt (Malpass) matrixWoodburn-1 East SalemWoodburn-1 East SalemWoodburn-1 East SalemMalabon @ 53rd St., Mary's River bridgeMalabon @ 53rd St., Mary's River bridgeMalabon @ 53rd St., Mary's River bridgeWoodburn-2, Calapooia surface @ 53rd

St., Mary's River bridgeWoodburn-2, Calapooia surface @ 53rd

St., Mary-s River bridgeWoodburn-2, Calapooia surface @ 53rd

St., Mary's River bridge

m %o0.50 22.20.50 22.01.15 21.71.15 18.91.40 21.81.40 19.6NAt 21.60.46 16.21.83 16.02.44 15.80.61 18.71.22 19.91.98 19.40.55 17.2

1.68 17.5

2.44 17.6

t Data courtesy of Jeff Schatz and J. Reed Glasmann, Oregon State Uni-versity.

t NA = not available.

with the soil surface in April 1993, before beginningto move downward through the profile in spring andsummer. The pattern of an increase in water levels dur-ing the fall, and a water level drop in the spring wasrepeated each year. No data were recorded in the springof 1994, but the measurements taken in the fall indicatethat the water levels dropped sometime between Apriland October.

The piezometric surface in the 100-cm piezometersrose in a manner similar to that observed in the 200-cmpiezometers, but it was below the piezometric surface ofthe 200 cm piezometers during 1992-1993. The patternsof water levels were similar for the piezometers at both100 and 200 cm, and the piezometric surfaces wereroughly equal from 1994 to 1996. The piezometric sur-faces indicated that the soil was completely saturatedin January each year when the piezometric surfaces ofthe 100- and 200-cm piezometers rose above the 50-cmdepth. The pattern of rising and falling water levels inresponse to precipitation repeated during each yearlyrainfall and drought cycle at 100- and 200-cm depthsthroughout the monitoring period.

Rainfall in the 1992 to 1994 monitoring period led toa quick response in the water levels at 50- and 100-cmdepths in response to the infiltration of surface waterdownward into large cracks in the soil formed by thedrying smectitic clay during the summers of 1992 and1993. Water flowing over the surface moved directlydown to the 50-cm level via the cracks. Once the cracksswelled shut, water movement through the fine poresin the slowly permeable clay was very slow, and the soilsurface became saturated from the top downward aspores filled with water ponded on the soil surface. Oncethe pores within the Bss aquitard swelled shut due tothe shrink-swell nature of the clay, the pressure of waterrising from the 3BC silts was the dominant hydrologicfactor at 100 cm. Rainfall throughout the year duringthe 1994 to 1996 seasons maintained surface saturationand prevented the clay from drying and cracking. Theincreased rainfall did not have as significant an impact


50

ND JFMAMJ JASON DJFMAJAODJ FMAM J AONDJ FMAM J SN1992 1993 1994 1995 1996

—+- 200 cm -a - 100 cm -*- 50 cm ^ 2 5 cm |̂ PrecipitationFig. 6. Precipitation (bottom) and water table data (top) as observed in piezometers at Jackson-Frazier Site 1.

on the subsurface water levels in the 100- and 200-cmpiezometers as shown by the consistent drop during thesummer months in all years, though to a lesser extentat 100-cm depth during 1995-1996.

Water levels in the 200-cm piezometers show a higherpressure potential than in the 100-cm piezometers rang-ing from 6 to 52 cm from January to June 1993, indicatingthe presence of a confined aquifer. Water in the siltswas under pressure due to the overlying restrictive layerduring the low rainfall years of 1992-1993 (Fig. 6). Dur-ing the high-rainfall period of 1994 to 1996, the soilbecame completely saturated with water supplied bythe lower silt deposit and the surface. During the 1994to 1996 period when the surface did not dry, soil satura-tion was maintained at the 100-cm level. The presenceof a confined aquifer was noted in similar soils of theWillamette Valley by Boersma et al. (1972).

The slowly permeable smectitic clay impedes watertransmission downward in the profile, leaving an unsatu-rated zone between 50 and 100 cm for several monthsduring periods of lower rainfall, such as during 1992-1993, and probably prevents additional detention andstorage of water in the subsurface. Buffkin-Drost (1985)conducted a study to assess water inflow, outflow, andstorage in the Jackson-Frazier wetland during 1984-1985. This study showed that the peak flow lag timebetween the input at Jackson-Frazier creek and outputat Stewart Slough was longer at the onset of the rainyseason in November than later in the rainy season inFebruary, indicating that water was moving across thewetland more rapidly in February. This decrease in lagtime indicates that groundwater detention storage de-creased after the soils were completely saturated andsurface runoff was the dominant flow pathway for in-coming water. In our study, the piezometers at the 100-cm depth indicated that the subsurface (50-100 cm)was partially unsaturated until late winter. Once the

subsurface becomes saturated, the storage capacity ofthe wetland may be very limited. Also, the water con-fined below the clay aquitard may move laterallythrough the Irish Bend silt along the topographic gradi-ent of the wetland.

CONCLUSIONSTwo distinct stratigraphic units are present beneath

the Jackson-Frazier wetland: a Holocene alluvium andthe Irish Bend silt. There is also evidence for a thirddeposit that is similar to the Malpass clay of the Willa-mette formation. The alluvial fan shape, dark organicrich surface, and clayey subsurface confirm that the up-per deposit is recent Holocene alluvial material fromthe Coast Range foothills. Mineralogical compositionof the Holocene alluvium corresponds with the sourcematerial of the foothills as well. Morphological descrip-tions confirm the presence of the Irish Bend silt in thelower profiles of the Bashaw soils at the wetland. Com-parison of known Irish Bend mineralogical samples withJackson-Frazier samples also provides evidence for thepresence of the Irish Bend deposit. The mineralogicalevidence to support the presence of the Malpass claymaterial is unclear, but the deposit has the appropriatemorphology and stratigraphic position, relative to theIrish Bend silt, of the Malpass clay.

Hydrology data for the 4-yr monitoring cycle revealtwo important stratigraphic influences on water move-ment in the wetland: (i) Holocene smectitic (shrink-swell) clays are so slowly permeable that water isperched above them, and (ii) there is a separate watertable in the Irish Bend silt, which is confined by thevery slowly permeable Malpass-like deposit and Holo-cene clay strata. In drier years, water in the Irish Bendis under greater pressure than in the clay aquitard andacts as an artesian source. In wetter years, the water in


the Irish Bend, Malpass, and lower Holocene alluviumis under the same pressure and the soil becomes continu-ously saturated from both above and below. The slowlypermeable Holocene alluvium and the low relief of thewetland allow ponded water to spread laterally and de-crease in velocity as it moves away from the wetland.The retention of surface water may help to attenuatepeak flow conditions in Jackson-Frazier Creek and cre-ates conditions conducive to growth of hydrophyticplants. The Holocene alluvium is an indicator of appro-priate conditions for formation of wetland areas in simi-lar geomorphic positions in the Willamette Valley. TheIrish Bend silt indicates the presence of an independentaquifer below the Holocene alluvium, which helps main-tain saturation of the lower portions of the soil profile.

Floodplain surfaces, such as the one described here,are not extensive in the Willamette Valley because ofwidespread suburban and agricultural development.The remaining intact floodplain wetland areas on similargeomorphic positions provide important examples ofnatural wetland conditions, which can serve as examplesfor restoration projects and are possible sites for wetlandpreservation. The interaction of the stratigraphic depos-its and hydrology described here also provides an exam-ple of the influence of stratified sediments of variablepermeability on the hydrology of floodplain wetlandsfor similar areas in other regions.

ACKNOWLEDGMENTSWe would like to acknowledge the Natural Resources Con-

servation Service, National Wet Soil Monitoring Program forfunding assistance with this project. We also wish to thankWill Austin (Oregon State University) for logistical supportand the journal reviewers for offering many helpful commentsthat improved the manuscript.

Documents

Stratigraphy and Hydrology of the Jackson-Frazier Wetland, Oregon