SOILS OF THE CARIBOU-POKER CREEKS RESEARCH

WATERSHED, INTERIOR ALASKA

by

Samuel Rieger, Clarence E. Furbush, Dale E. Schoephorster,

Harry Summerfield, Jr. and Luther C. Geiger

Setting

The Caribou-Poker Creeks Research Watershed, an area of 26,550 acres about 25 miles north of Fairbanks, Alaska, includes the entire watershed of Poker Creek and its principal tributary, Caribou Creek, above the floodplain of the Chatanilca River. It is representative of the forested hills of lower elevations in the Yukon-Tanana Upland and includes, on the ridges bordering the watershed, small areas above the tree line. Elevations range from about 700 feet near the mouth of Poker Creek to more than 2600 feet.

The underlying rock in the area is mica schist of the Birch Creek formation. A thin cap of loess mantles the area but, because the loess is derived from the floodplains of streams draining areas of the same formation, there is no sharp boundary between it and the weathered schist below it. The region has never been covered by glaciers, and has a mature dendritic drainage pattern. Slopes are generally convex, with the steepest slopes bordering secondary drainageways, but slopes are concave adjacent to the lower segments of Poker and Caribou Creeks.

The area has a cold continental climate, with long, cold winters and short, warm summers. No long-term climatic data exist for the immediate watershed area, but it is likely that the mean annual temperature at the lower elevations is about 25°F, with mean July temperatures of 60°F and mean January temperatures of -12°F. The mean annual precipitation is probably between 12 and 14 inches. Almost half of this amount falls in the three summer months (June-August), much of it in the form of heavy showers. Winter precipitation falls almost entirely as snow, most of which runs off in the sprieg before the soil has thawed.

Several different vegetation types occur in the area. On southerly slopes, the dominant trees are paper birch, quaking aspen and white speuce. On north-facing slopes and in the bottoms of drainageways, the principal tree is black spruce and there is a ground cover of willows, low-growing shrubs, mosses and sedges. High ridges are covered with dwarf birch and willows, with grasses, sedges, mosses and, especially in areas close to the tree line, thick patches of alder.

How soils are named, mapped and classified

Soils are made up of a series of layers, or horizons, that are generally parallel to the soil surface. A soil profile is the sequence of these horizons from the surface down to the underlying material which has not been altered by plant roots or soil-forming processes. Soils that have protilea almost alike make up a soil series. All soils of one series have major horizons that are similar in important characteristics. These include 1) color; 2) texture, or relative propoetions of gravel, sand, silt and clay; 3) structure, or arrangement of soil particles into aggregates or clusters; 4) consistence, or degree of compaction and plasticity; 5) aeration and drainage conditions; 6) reaction, or degree of acidity or alkalinity; 7) thickness; and 8) arrangement in the profile. Each soil series is named for a town or other geographic feature near where it was first observed.

Soil series may be subdivided on the besis of other features that are significant to users of the survey. These subdivisions are called phases. In this area, most of the soil series have been divided into slope phases.

In using the soilsemap (Appendix A) it must be recognized that it is not possible, even in a detailed map, to show very small areas of a soil. Most mapping units contain patches up to 5 acres in size of soil of some other kind.

Classification of the soil series of the area according to the Soil Taxonomy of the National Cooperative Soil Survey is given at the end of this report.

Seven soil series are recognized in the area.* The location and distribution of the soils are shown in Appendix A. Their acreages and proportionate extent are given in Table I.

Table I. Acreage and proportionate extent of soils of the

Approx. Proportionate

Map area extent

symbol Mapping unit (acres) (%)

Br Bradway silt loam 500 1.89

EsD Ester silt loam, 12 to 20% slopes 695 2.62

EsE Ester silt loam, 20 to 30% slopes 2000 7.53

EsF Ester silt loam, 30 to 45% slopes 2375 8.94

FpB Fairplay silt loam, 3 to 7% slopes 1385 5.22

FpC Fairplay silt loam, 7 to 12% slopes 1675 6.31

FpD Fairplay silt loam, 12 to 20% slopes 2575 9.70

FpE Fairplay silt loam, 20 to 30% slopes 190 .71

GmD Gilmore silt loam, 12 to 20% slopes 215 .82

GmE Gilmore silt loam, 20 to 30% slopes 1505 5.68

GmF Gilmore silt loam, 30 to 45% slopes 1335 5.02

KaB Karshner silt loam, 3 to 7% slopes 445 1.68

KaO Karshner silt loam, 7 to 12% slopes 10 .03

OnB Olnes silt loam, 3 to 7% slopes 20 .08

OnC Olnes silt loam, 7 to 12% slopes 350 1.31

OnD Olnes silt loam, 12 to 20% slopes 3810 14.33

OnE Olnes silt loam, 20 to 30% slopes 5380 20.25

OnF Olnes silt loam, 30 to 45% slopes 930 3.51

SuB Saulich silt loam, 3 to 7% slopes 60 .23

SuC Saulich silt loam, 7 to 12% slopes 955 3.60

SuD Saulich silt loam, 12 to 20% slopes 140 .54

TOTAL 26,550 100.00

Bradway series

The Bradway series consists of poorly drained, stratified silty and sandy soils with shallow permafrost. These soils are formed in alluvial materials in the floodplains of Poker and Caribou Creeks. A typical profile has a thin mat of organic materials at the surface over layers of mottled and streaked dark gray, very dark gray, and olive gray silt loam and fine sand. The layers vary in thickness and sequence; some profiles are dominantly silty and others are dominantly sandy. The soil is perennially frozen at a depth of 15 to 24 inches.

The soils are almost always wet, with free water perched above the permafrost. Some espe

cially wet areas are the result of springs and seepage water from the adjacent uplands. In some places, gravelly or sandy stream overflow channels cut through areas of Bradway soils.

The natural vegetation consists of willows, low shrubs, sedge tussocks, and black spruce. In a small area above the confluence of Poker Creek and the Chatanika River, paper birch grows on very small, well drained spots included with the Bradway soils in mapping.

Representative profile of Bradway silt loam, in Caribou Creek floodplain about 1.8 miles west of confluence with Poker Creek.

Horizon Depth Description

01 2-0" Black (5YR 2/1)* mat of organic matter with silt admix

ture; charcoal fragments; many roots; abrupt smooth

boundary.

B21g 0-4" Dark gray (BY 4/1) silt loam; dark reddish brown (5YR

3/4) stains along root channels; streaks of very dark

gray (5Y 4/1) and few fine distinct mottles of dark red

dish brown (5YR 2/2); weak thin platy structure; friable;

roots common; medium acid; abrupt smooth boundary.

B22g 4-8" Very dark gray (5Y 3/1) silt loam; thin dark reddish brown

(5YR 2/2) lenses of organic matter; weak thin platy

structure; friable; few roots; medium acid; abrupt smooth

boundary.

B23g 8-10" Olive gray (BY 4/2) loamy fine sand; single grain; loose;

no roots; medium acid; abrupt smooth boundary.

B24g 10-17" Very dark gray (5Y 3/1) silt loam; streaks of dark red

dish brown (5YR 3/4) and dark gray (1OYR 4/1); weak

thin platy structure; friable; medium acid; frozen at 17

inches in late summer.

Mapping units. The Bradway soils occur only on nearly level slopes. Bradway silt loam is the only mapping unit in the area.

Ester series

The Ester series consists of poorly drained silty soils on steep, north-facing slopes. These soils have shallow permafrost and are less than 20 inches thick over bedrock. A typical profile has aethick mossy mat on the surface, a thin dark horizon immediately beneath the mat, and mottled dark grayish brown and olive gray horizons at greater depths. The upper mineral horizon is generally not gravelly, but gravel occurs within a few inches and increases with depth. Where the organic mat is thinner than normal or where bedrock is very shallow, there may be no permafrost above the rock.

The soils are usually cold and wet, but the upper part of the mat may dry out in midsummer. The principal trees are black spruce and willows, with some alder and stunted paper birch. Sphagnum, hypnum, and polytrichum mosses, lichens, and low-growing shrubs make up the ground cover. Where the organic mat has been destroyed, as by fire, birch trees become dominant.

* Colors are for moist conditions; Munsell color notations are in parentheses.

Representative profile of Ester silt loam, about 0.3 mile north of Haystack Mountain.

Horizon Depth

011 12-8"

Description

Reddish brown (5YR 4/3) undecomposed moss peat; roots common; abrupt smooth boundary.

Dark reddish brown (5YR 3/2) partially decomposed organic material, mostly derived from mosses; many roots; abrupt smooth boundary.

Dark reddish brown (5YR 2/2) finely divided organic matter; many charcoal fragments in upper part; many roots; abrupt wavy boundary.

Very dark grayish brown (1OYR 3/2) silt loam; weak fine granular structure; very friable; many roots; a few schist fragments; many roots; very strongly acid; abrupt wavy boundary.

Dark grayish brown (2.5Y 4/2) silt loam; few fine to medium faint mottles of dark brown (1OYR 3/3); friable; no roots; very strongly acid; abrupt smooth boundary.

Olive gray (5Y 5/2) gravelly silt loam; proportion of gravel increases with depth; massive9 with thin lenses of clear ice; no rooto; strongly acid.

012 8-4"

02 4-0"

A 1 0-1/2''

B2 1/2-4"

Clf 4-10"

Mapping units. Three slope phases are recognized:

Ester silt loam, 12 to 20% slopes

Ester silt loam, 20 to 30% slopes

Ester silt loam, 30 to 45% slopes

In general, the organic mat is thicker and the depth to bedrock decreases on the steeper slopes.

Fairplay series

The Fairplay series consists of moderately well drained to somewhat poorly drained silty soils of high ridges above tree line. The soils are moderately deep over bedrock, and apparently have no permafrost. A typical profile has a thin organic mat over mottled olive brown silt loam. Below about 18 inches, the soil contains schist fragments, which become more numerous with depth. Schist bedrock is at depths of 30 to 40 inches.

The Fairplay soils are usually moist, but are not saturated throughout the summer. They support a brushy vegetation made up mostly of dwarf birch and willows, together with some stunted white spruce. Low shrubs, lichens and some mosses cover the surface. Patches of alder are common immediately above the boundary with the forested Cilmore and Olnes soils.

Included in mapped areas of Fairplay soils are small patches of well-drained soils with thin black or very dark gray horizons at the surface and dark brown lower horizons. These soils occur, commonly, in association with small rock outcrops, and are themselves less than 20 inches thick over bedrock. Also included are poorly drained soils with permafrost, similar to those of the Saulich series, in microdepressions on the ridges. Together, these included soils make up less than 5% of the Fairplay mapping units.

Representative profile of Fairplay silt loam, about .75 mile southwest of northernmost point in the Poker Creek watershed.

Horizon Depth Description

01 3-0" Black (5YR 2/1) partially decomposed organic matter;

roots common; abrupt smooth boundary.

C1 0-18" Olive brown (2.5Y 4/4) Yilt loam; many medium and coarse

faint mottles of dark yellowish brown (1OYR 4/4) and

dark grayish brown (2.5Y 4/23; weak fine subangular

blocky structure; friable; roots common; very strongly acid;

gradual boundary.

C2 18-30" Olive brown (2.5Y 4/4) gravelly silt loam; moderate fine

subangular blocky structure; friable; few roots; many fine

tubular pores; silty coatings on upper surface of pebbles

and rock fragments; strongly acid.

R 30-40" Partially weathered fractured schist; can: be removed with

a spade, but contains only small quantities of sand and

silt.

Mapping units. Four slope phases are recognized. In general, within each slope group, the more gentle gradients are near the top of the ridge and the gradient increases downslope.

Fairplay silt loam, 3 to 7% slopes

Fairplay silt loam, 7 to 12% slopes

Fairplay silt loam, 12 to 20% slopes

Fairplay silt loam, 20 to 30% slopes

Gilmore series

The Gilmore series consists of well drained silty soils that are very shallow over shattered schist bedrock. These soils occur on southerly slopes and are free of permafrost. They differ from the Olnes soils principally in color and in shallower depth to shattered bedrock. A typical profile has a thin surface mat of forest litter and 10 to 20 inches of dark yellowish brown or dark brown silt loam containing rock fragments over shattered bedrock. The proportion of rock fragments in the silt loam horizons increases with depth.

The Gilmore soils, because of their shallowness, have a low moisture supplying capacity. In years with less than normal rainfall they are dry in midsummer. Most areas have been burned over during the last century and are covered mostly with paper birch and quaking aspen, with patches of alder. Young /white spruce trees are also present and, barring new fires, will eventually become dominant. A few remnants of older white spruce forest also exist.opar

About 25% of the mapped areas of Gilmore soils are actually included areas of the closely similar Olnes soils. These occur in an irregular pattern on all slopes. Also included in the Gilmore mapping units are a few outcrops of bare rock.

Representative profile of Gilmore silt loam, about 2 miles north of confluence of Poker and Caribou Creeks.

Horizon Depth

Description

C2 17-30"

011 2-1" Dark reddish brown (5YR 2/2) forest litter; many roots;

abrupt smooth boundary.

012 1-0" Dark reddish brown (5YR 2/2) partially decomposed

forest litter; many roots; abrupt smooth boundary.

Al 0-1" Dark brown (7.5YR 3/4) silt loam; weak fine crumb struc

ture; very friable; roots common; angular schist fragments

make up 6 to 10% of the soil volume; medium acid; abrupt

wavy boundary.

B2 1-6" Dark yellowish brown (1OYR 4/4) silt loam; weak fine

subangular blocky structure; very friable; roots common;

angular schist fragments make up 5 to 10% of the soil

volume; medium acid; clear wavy boundary.

B3 6-11" Dark brown (1OYR 4/3) gravelly silt loam; weak medium

subangular blocky structure; very friable; roots common;

schist fragments make up 15 to 30% of the soil volume;

medium acid; clear wavy boundary.

11-17" Dark grayish brown (2.5Y 4/2) very gravelly silty loam;

weak medium subangular blocky structure; very friable;

few roots; schist fragments make up 35 to 50% of the

soil volume; slightly acid; clear wavy boundary.

Angular schist fragments, averaging less than 10 inches

in long axis; unconsolidated and can be removed with

hand tools; thin silty coatings on some fragments,

especially in upper part of horizon.

Mapping units. Three slope phases are recognized:

Gilmore silt loam, 12 to 20% slopes

Gilmore silt loam, 20 to 30% slopes

Gilmore silt loam, 30 to 45% slopes

Karshner series

The Karshner series consists of poorly drained, stratified silty and very gravelly sandy soils with shallow permafrost. These soils are formed in narrow floodplains bordering the upper courses of streams in the area.

A typical profile includes a few inches of organic matter on the surface and layers of mottled and streaked dark grayish brown silt loam and very gravelly sandy loam or loamy sand. The upper part of the profile commonly has layers of dark reddish brown very gravelly sandy loam. Depth to permafrost is between 20 and 30 inches, but may be less where the surface mat is thicker than normal.

The Karshner soils are nearly always wet, with free water close to the surface. Areas of the soil are dissected by overflow and seepage water channels. The vegetation is mainly sedge tussocks, mosses, willows, black spruce and low-growing shrubs.

Representative profile of Karshner silt loam, Caribou Creek floodplain about 2.5 miles west of confluence with Poker Creek.

Horizon Depth

Description

01 4-2" Dark reddish brown (5YR 3/2) moss peat; many roots;

abrupt smooth boundary.

02 2-0" Black (5YR 2/1), partially decomposed peat; many roots;

abrupt smooth boundary.

B21 0-6" Dark grayish brown (1OYR 4/2) silt loam; patches and

streaks of very dark brown (1OYR 2/2) organic matter and

few medium faint mottles of dark brown (7.5YR 3/2); weak

thin platy structure; very friable; few roots; strongly acid;

abrupt smooth boundary.

B22 6-8'' Dark reddish brown (5YR 3/4) very gravelly sandy loam;

single grain; loose; no roots; medium acid; abrupt smooth

boundary.

B23 8-10" Dark grayish brown (1OYR 4/2) silt loam; streaks of dark

brown (7.5YR 3/2) and very dark gray (5YR 3/1) mottles;

weak thin platy structure; friable; no roots; medium acid;

abrupt smooth boundary.

B24 10-11" Dark reddish brown (5YR 3/4) very gravelly sandy loam;

single grain; loose; no roots; strongly acid; abrupt smooth

boundary.

B25 11-17" Dark grayish brown (2.5Y 4/2, with patches of lOYR 4/2)

very gravelly loamy sand; single grain; loose; no roots;

medium acid; abrupt smooth boundary.

B26 17-24" Dark gray (5Y 4/1) silt loam; weak thin platy structure;

friable; no roots; rests on permafrost.

Mapping units. Two slope phases are recognized

Karshner silt loam, 3 to 7% slopes

Karshner silt loam, 7 to 12% slopes

loam.

In places, the texture of the upper mineral horizon is gravelly sandy loam rather than silt

Olnes series

The Olnes series consists of well drained silty soils that are shallow over shattered bedrock. These soils have not developed the colors of mature soils of the area. They Occur cur on southerly slopes and have no peomafrost. They differ from the Gilmore soils mostly in color and in their greater depth to shattered bedrock.

A typical profile has a thin mat of forest litter over dark grayish brown silt loam, c commonly with schist fragments. The proportion of rock fragments increases with depth. Consolidated bed is deep.

The Olnes soils have a low moisture supplying capacity and, in years with low rainfall, are dry in midsummer. Most areas have been burned over during the last century and are covered mostly with paper birch, quaking aspen and patches of alder. Young white spruce trees are also present and, barring new fiwes, will eventually become dominant. A few patches of older white spruce forest also exist.

About 15% of the area mapped as Olnes consists of included Gilmore soils. These occur in an irregular pattern, especially on the steeper slopes.

Representative profile of Olnes silt loam, about I mile north at confluence of Poker and

Caribou Creeks.

horizon Depth

Description

01 1/2-O'' Dark reddish brown (5YR 3/2) forest litter; many ifs20 roots;

charcoal fragments; abrupt smooth boundary.

C1 0-1/2" Grayish brown (1OYR 5/2) silt loam; weak, very thin

platy structure; friable; roots common; medium acid;

abrupt broken boundary.

C2 1/2-19" Dark grayish brown (2.5Y 4/2) silt loam; weak fine platy

to weak fine subangular blocky structure; friable; few roots; contains 5 to 15% schist fragments by volume, with thin coating of silt on upper surface of fragments; medium acid; gradual boundary.

C3 19-40" Dark grayish brown (2.5Y 4/2) very gravelly silt loam;

weak fine subangular blocky structure; very friable; few

to no roots; contains 35 to 50% schist fragments by

volume; medium acid.

Mapping units. The Olnes soils occur on a wide range of slopes. Five slope phases are recognized:

Olnes silt loam, 3 to 7% slopes

Olnes silt loam, 7 to 12% slopes

Olnes silt loam, 12 to 20% slopes

Olnes silt loam, 20 to 30% slopes

Olnes silt loam, 30 to 45% slopes

Saulich series

The Saulich series consists of poorly drained soils on the foot slopes of hills. The soils have shallow permafrost. They differ from the Ester soils in that they are deeper over bedrock and are not as acid.

A typical profile has a thick mat of moss peat over a thin layer of mixed mineral and organic matter and, beneath that, dark grayish brown silt loam. Depth to permafrost ranges from a few inches to 30 inches, depending largely on the thickness of the most ~ mat on the surface. A few "fwost scars" with little or no vegetation occur; the depth to perter is greater under the scars. Shattered bedrock is at depths of 20 to 40 Inches.

The Saulich soils are almost always wet. The vegetation is a sparse stand of black spruce and willows, with a ground cover of mosses, sedges and low-growing shrubs.

Representative profile of Saulich silt loam, about 1.3 miles west of the confluence of Poker and Caribou Creeks.

Horizon Depth Description

01 13-3" Relatively undecomposed sphagnum moss peat; abrupt

smooth boundary.

02 3-0'' Dark reddish brown (5YR 3/2) finely divided peat; many

roots; charcoal fragments; abrupt wavy boundary.

A1 Very dark grayish brown (2.5Y 3/2) silt loam mixed with

lenses of moss peat and charcoal fragments; massive;

roots common; strongly acid; abrupt wavy boundary.

C2f 3-12" Dark grayish brown (2.5Y 4/2) silt loam; frozen, with

clear ice lenses; medium acid; no roots.

Mapping units. Three slope phases are recognized:

Saulich silt loam, 3 to 7% slopes

Saulich silt loam, 7 to 12% slopes

Saulich silt loam, 12 to 20% slopes

Estimated physical properties of soils

The estimated textural and engineering classifications of the soils of the area, and their estimated permeabilities, available water capacities, and reaction range are presented in Table II. These estimates are based on limited data from areas outside the Caribou-Poker Creeks Watershed, and are subject to revision as more data are accumulated from research within the watershed.

Permeability, as used here, refers to the rate at which the soil will transmit water under saturated conditions. It is equivalent to the saturated hydraulic conductivity of the soil. Available water capacity refers to water in the range between the permanent wilting point and field capacity (roughly water held against tensions between 15 and 1/3 atmospheres).

Soil classification

The classification of soils according to the Soil Taxonomy of the U.S. Cooperative Soil Survey, as revised in 1971, is shown in Table III. A complete discussion of this classffication scheme, including definitions of all taxa, will be published by the U.Se Department of Agriculture within the next year.

Table 11. Estimated physical properties of soils of

the Caribou-Poker Creeks Research Watershed.

Depth from Available

surface water

(typical capacity"

profile) USDA Permeabilityt (in./in. Reaction

Soil series (in.) texture Unified AASHO (in./hr) of soil) pH

Bradway 0-8 sit ML A-4 0.6 to 2.0 .18 to .23 5.6 to 6.0

8-10 If s SM A-2 2.0 to 6.0 .04 to toe 5.6 to 6.0

10-17 sil ML A-4 0.6 to 2.0 .18 to .23 5.6 to 6.0

17+ Silty or sandy permafrost

Ester 0-4 sil ML A-4 0.6 to 2.0 .18 to .23 4.5 to 5.0

4 10 Gravelly permafrost

10+ Fractured bedrock

Fairplay 0-18 sit ML A-4 0.6 to 2.0 .18 to .23 4.5 to 5.0

18-30 gsil ML A-4 0.6 to 2.0 .13 to .17 5.1 to 5.5

30-40 Fractured bedrock

Gilmore 0-6 sil ML A-4 0.6 to 2.0 .18 to .23 5.6 to 6.0 6-11 gsil ML A-4 0.6 to 2.0 .13 to .17 5.6 to 6.0

11-17 vg sil GM A-1 0.6 to 2.0 toe to .08 6.1 to 6.5

17-30 Fractured bedrock

Karshner 0-6 sil ML A-4 0.6 to 2.0 .18 to .23 5.1 to 5.5

e-24 Stratified sil Stratified Stratified 5.1 to 6.0

vgsl, vgls ML, GM, GW A-4, A-1

24+ Gravelly

permafrost

Olnes 0-19 sil ML A-4 0.6 to 2.0 .18 to .23 5.6 to 6.0

19 40 vg sil GM A-1 0.6 to 2.0 toe to .08 5.8 to 6.0

Saulich 0 3 sil ML A-4 0.6 to 2.0

3 12+ Silty permafrost

* sil - silt loam If s - loamy sand

gsil - gravelly silt loam vgsl - very gravelly sandy loam

vg sil- very gravelly silt loam vgls - very gravelly loamy sand

.18 to .23 5.1 to 5.5

t Estimates subject to verification, at completion of ongoing research at University of Alaska.

Table 111. Classification of soils of the Caribou-Pokor Creels Research Watershed

according to the Soil Taxonomy of the U.S. Cooperative Soil Study

Soil series Family Subgroup Order

Bradway Loamy, mixed, nonacid Pergelic cryaquepts Inceptisols

Ester Loamy, mixed, acid Histic lithic cryaquepts Inceptisols

Fairplay C oar se -loamy, mixed , ac id Aquic cryorthents Entisols

Gilmore Loamy-skeletal, mixed Typic cryochrepts Inceptisols

Karshner Loamy-skeletal, mixed, acid Pergelic cryaquepts Inceptisols

Olnes Loamy-skeletal, mixed' nonacid Typic cryorthents Entisols

Saulich Loamy, mixed, nonacid Histic Pergelic cryaquepts Inceptisols