Tab

le 2

.11—

Geo

clim

atic

cha

ract

eris

tics

of

ecol

ogic

al r

epor

ting

uni

ts.

ER

U

Nor

th C

asca

des

Sou

th C

asca

des

00 o'

en o'

91 HK

Upp

er K

lam

ath

Nor

ther

n G

reat

Bas

in

Col

umbi

a P

late

au

Blu

e M

ount

ains

Nor

ther

nG

laci

ated

Mou

ntai

ns

Landfo

rms

Gla

ciat

ed a

ndun

glac

iate

d m

ount

ains

;w

ith s

ome

hills

, pl

ains

,bu

ttes

and

plat

eaus

.

Mou

ntai

ns,

plat

eaus

,la

va p

lain

s, b

utte

s,sh

ield

vol

cano

s, a

ndso

me

high

ele

vatio

ngl

acia

tion.

Mou

ntai

ns,

plat

eaus

,ro

lling

hills

, flo

odpl

ains

,an

d t

erra

ces.

Bas

in a

nd r

ange

.

Pla

teau

s, h

ills, a

ndpl

ains

.

Dis

sect

ed p

late

aus,

foot

hills

, m

ount

ains

,ca

nyon

s, a

nd v

alle

ys.

Gla

ciat

ed m

ount

ains

,fo

othi

lls,

basi

ns,

and

valle

ys.

Bed

rock

& S

urf

icia

l Mat

eria

l

Cry

stal

line

rock

s:

gran

itic,

gnei

ss, s

chis

t, se

rpen

tine,

volc

anic

roc

ks, s

edim

enta

ryro

cks;

gla

cial

, flu

vial

and

mas

sw

astin

g de

posi

ts.

Vol

cani

c an

d se

dim

enta

ryro

cks;

ash

, and

pum

ice.

Met

amor

phic

and

vol

cani

cro

cks,

and

allu

vium

.

Vol

cani

c ro

cks.

Bas

alts

and

vol

cani

c ro

cks;

loes

s, g

laci

al o

utw

ash,

and

flood

dep

osits

.

Gra

nitic

s, m

etam

orph

ics,

volc

anic

s, a

nd s

erpe

nten

ite.

Gra

nitic

, gn

eiss

, sc

hist

, si

ltite

,sh

ale,

qua

rtzite

, ca

rbon

ate;

glac

ial t

ill,

and

out

was

h.

Ele

vatio

n

Mea

n A

nnual

Ran

ge (

m)

Pre

cip. &

Tem

p.

600-

2,89

6 25

0 to

4,0

00 m

m1 to

9°

C.

91-2

,743

25

0 to

3,0

00 m

m2

to 1

0° C

.

1219

-2,4

38

300

to 8

75 m

m4 to

10°

C.

1 ,2

00-2

,200

1 0

0 to

790

mm

5 to

10°

C.

61-1

,220

18

0 to

450

mm

4 to

14°

C.

300-

3,30

0 23

0 to

460

mm

-2 to

11°

C.

244-

3,08

1 41

0 to

2,5

40 m

m-1

to 1

4° C

.

Maj

or P

ote

ntia

lV

egeta

tion G

roups

Pon

dero

sa p

ine,

Dou

glas

-fir

and

sage

brus

h; w

hite

fir

and

wes

tern

red

ced

ar;

silv

erfir

and

mou

ntai

n he

mlo

ck.

Pon

dero

sa p

ine,

Dou

glas

-fir

, sa

gebr

ush,

whi

te fi

r,an

d si

lver

fir.

Sag

ebru

sh,

pond

eros

api

ne,

Dou

glas

-fir,

whi

tefir

, re

d fi

r, a

nd s

ilver

fir.

Sal

t de

sert

shr

ub,

sage

brus

h, a

nd ju

nipe

r.

Sag

ebru

sh,

and

blue

bunc

h w

heat

gras

sId

aho

fesc

ue.

Sag

ebru

sh,

gras

slan

ds,

pond

eros

a pi

ne,

Dou

glas

-fir,

gra

nd fi

r,an

d m

inor

sub

alpi

ne f

ir.

Dou

glas

-fir,

pon

dero

sapi

ne,

gran

d fir

, wes

tern

hem

lock

, an

d su

balp

ine

fir.

*"* CD o" o Q)

Tab

le 2

.11

(con

tinu

ed).

ER

ULa

ndfo

rms

Bed

rock

& S

urf

icia

l Mat

eria

lE

leva

tion

M

ean

An

nu

al

Maj

or P

ote

ntia

lR

ange

(m

) P

reci

p. &

Tem

p.

Veg

etat

ion

Gro

up

s

Low

er C

lark

For

k

Upp

er C

lark

For

k

Ow

yhee

Upl

ands

Upp

er S

nake

Sna

ke H

ead

Wat

ers

Cen

tral I

daho

Mou

ntai

ns

Dis

sect

ed m

ount

ains

,br

eakl

ands

, foo

thill

s,an

d va

lleys

; som

eal

pine

gla

catio

n.

Gla

ciat

ed a

ndun

glac

iate

d m

ount

ains

;va

lleys

, an

d c

anyo

ns;

glac

ial an

d la

cust

rine

basi

ns.

Dis

sect

ed m

ount

ains

,pl

ains

, pla

teau

s, a

ndfo

othi

lls.

Bas

ins,

val

leys

,m

ount

ains

, pla

teau

s an

dpl

ains

.

Ove

rthru

st m

ount

ains

,va

lleys

, fo

othi

lls, a

ndba

sins

.

Dis

sect

ed m

ount

ains

,br

eakl

ands

, ca

nyon

s,ba

sins

, fo

othi

lls,

and

valle

ys,

and

som

e a

lpin

egl

acia

tion.

Met

ased

imen

tary

, gr

aniti

cm

etam

orph

ic c

arbo

nate

rock

s.

Gra

nite

, gne

iss,

sch

ist,

sand

ston

e, s

hale

, ca

rbon

ate,

and

rock

s.

Vol

cani

c ba

salti

c flo

ws

and

pyro

clas

tic r

ocks

.

Vol

cani

c- b

asal

t to

rhy

olite

; an

dca

rbon

ate,

pho

spha

te,

clas

ticse

dim

enta

ry r

ocks

.

Vol

cani

c- b

asal

t to

rhy

olite

; an

dca

rbon

ate,

pho

spha

te,

clas

ticse

dim

enta

ry r

ocks

.

Gra

nitic

s, g

neis

s, s

chis

t, sh

ale,

carb

onat

e r

ocks

, and

vol

cani

cro

cks.

366-

2,13

5

1,02

0 to

2,0

30 m

m

Dou

glas

-fir,

pon

dero

sa2

to 7

° C

. pi

ne,

gran

d fi

r, w

este

rnre

d ce

dar,

and

sub

alpi

nefir

.

915-

3,11

1 36

0 to

2,0

30 m

m

Gra

ssla

nd,

sage

brus

h,2

to 8

° C

. D

ougl

as-fi

r, p

onde

rosa

pine

, and

sub

alpi

ne fi

r.

641

-2,5

01

200

to 4

00 m

m

Sal

t de

sert

shr

ub,

2 to

8°

C.

sage

brus

h, a

nd ju

nipe

r.

397-

2,28

8

100

to 7

90 m

m

Sal

t des

ert

shru

b,4 to

13°

C.

sage

brus

h, a

nd ju

nipe

r.

1,52

4-4,

202

400

to 1

,015

mm

D

ougl

as-fi

r, l

odge

pole

2 to

7°

C.

pine

, sag

ebru

sh,

and

suba

lpin

e f

ir.

427-

3,86

1

250

to 2

,030

mm

D

ougl

as-fi

r, g

rand

fir,

3 to

10°

C.

sage

brus

h, g

rass

land

s,an

d su

balp

ine fi

r.

Plate tectonics—The oldest rocks of the Basininclude granite and gneisses in western Montanaand Wyoming that cooled and crystallized deepwithin the earth as long as 2.7 billion years ago(Hoffman 1989). The Belt basin, in the easternportion of the Northern Glaciated Mountains(ERU 7), formed by rifting and subsidence of theolder granitic and gneissic rocks of the continentalcrust over 1.5 billion years ago. The sediments thatfilled the resulting basin formed a thick sequenceof siltstones, quartzites, and carbonates that arenow exposed at the surface in part of ERU 7.During the Late Cretaceous Period, about 100million years ago, continental and oceanic crustalfragments were joined to the western edge of theNorth American continent; the trace of this junc-tion now lies in western Idaho (Oldow and others1989). This junction is buried under youngerrocks in southern Oregon and Washington andhas been deformed by subsequent tectonic events.All rocks west of this junction (that is, rocks lo-cated in ERUs 1-4, 6, and parts of 5, 7, and 10)have been added to the continent by plate tectonicprocesses in the last 100 million years.

Volcanic processes and hazards—Volcanismhas been and continues to be a profound agent ofchange for the landscapes of the Pacific Northwest.Volcanic rocks underlie most of the assessmentarea in southern Washington, Oregon, and south-ern Idaho (ERUs 1-6 and 10-12).

Arc volcanism, which is the process that leads toconstruction of linear chains of cones and flowssuch as the volcanic Cascade Mountains (ERUs 1-3), has occurred continually for at least 25 millionyears. Arc volcanoes form in response to subduc-tion, which is the overriding of oceanic crust bythe edge of the continent. The physiography of theCascade Mountains (ERUs 1-3) is the result of theeruption of lava flows, domes, and cones from thehundreds of volcanoes in the Cascade arc. Erup-tions of Glacier Peak, Mt. Mazama, and otherCascade mountain volcanoes have depositedextensive blankets of volcanic ash throughout theBasin (Sarna-Wojcicki and Davis 1991).

The eruption of Mt. Mazama about 7,000 yearsago dispersed about 120 cubic kilometers of te-phra over the entire Basin (Bacon 1983). Com-pacted unconsolidated volcanic ash (tephra)deposits from the Mazama eruption are as thick as50 millimeters (mm) at 1,000 kilometers east ofthe crater and as thick as 4 meters at 20 kilometersfrom the crater (Hoblitt and others 1987). Lahars(volcanic mud flows) may also affect large areasand may be caused by very small eruptions. Melt-ing of large volumes of glacial ice from volcaniceruptions may produce deposits that travel manykilometers downstream and can blanket large areaswith deposits of mud.

During the last 4,000 years, volcanoes of theCascade mountains have erupted, on average,twice per century (Dzurizin and others 1994).Such return frequencies are also similar to thoseassociated with very large forest fire events. Thetephra deposits of these volcanoes are easilyeroded. Management practices in areas underlainby such ash deposits may greatly affect erosion andsedimentation rates. Tephra deposits also modifythe water-holding capacity of soils into which theyare incorporated. Additionally, tephra depositstypically provide a readily available source ofmineral nutrients to terrestrial and aquatic ecosys-tems. The glassy nature and fine-grain size ofvolcanic ash permits rapid chemical weathering ofits mineral components and release of containednutrient elements for uptake by plants (Goughand others 1981).

About 1 km3 of tephra was ejected from the volca-nic eruption of Mt. St. Helens on May 18, 1980(Sarna-Wojcicki and others 1981). Areas nearestthe volcano were most affected by this eruptionand currently provide a natural laboratory for thestudy of ecosystem response to disturbance. Thepyroclastic surge of the eruption of Mt. St. Helensleveled trees as far as 28 kilometers from the craterand affected an area of 600 square kilometers(Moore and Sission 1981). Ash from the maineruption of Mt. St. Helens and subsequent erup-tions was deposited over most of eastern Washing-ton and much of central Idaho and westernMontana (ERUs 1,5, and 8). Ash deposits in

Biophysical

Ritzville, Washington, 175 kilometers northeast ofthe volcano, were as thick as 50 millimeters(Sarna-Wojcicki and others 1981). The landslideand proximal deposits from this eruption filledmost of the Spirit Lake Basin on the north side ofthe mountain and raised the lakes surface about61 meters (Voight and others 1981).

Lahars and floods caused by the Mt. St. Helenseruption and by rapid melting of glaciers andsnow pack on the mountain affected both aquaticand riparian environments and the human popula-tions living in the floodplains on the north side ofthe mountain (Janda and others 1982). The re-establishment and development of both aquaticand terrestrial communities following this erup-tion has been rapid. An excellent compendium ofpapers on the volcanic processes and short-termeffects of the 1980 eruption of Mt. St. Helens iscontained in Lipman and Mullineaux (1981).Although the site, size, and time of the next volca-nic eruption from the volcanoes of the Cascademountains cannot be predicted, such events willcontinue to occur sporadically in the foreseeablefuture.

Estimated likelihoods of tephra deposition of 1centimeter thicknesses in a single year vary from 1in 100 in the Cascades ERUs (1,2, and 3), to lessthan 1 in 1,000 in the eastern part of the Basin.The likelihood of thicker deposits in a single yearare correspondingly lower (Hoblitt and others1987). Although there is little likelihood for thickdeposits of ash in any given location in the westernpart of the Basin in a given year, it is certain thatwithin a 200-year timeframe there will be a sub-stantial eruptions within or adjacent to the Basin.The eruption from Mt. St. Helens in 1980-1985deposited ash over an area equal in size to Wash-ington State; furthermore, eruptions of the size ofMt. Mazama blanketed the entire Basin and be-yond. Although the probability of very large erup-tions in any given year is low, the potential effectsof such an eruption would be substantial to allphysical, biological, and social components andprocesses of the Basin.

The Columbia River basalts form the cliffs of thecentral Columbia plateau (ERUs 5 and 6) insouthern Washington, northern Oregon, andwestern Idaho. These flood basalts erupted be-tween 17 and 6 million years ago; however, ap-proximately 95 percent of the 174,000 cubickilometers of basalt were erupted within a 2-million year time period (Swanson and others1979, Tolan and others 1989). These basaltsrepresent gigantic outpourings of lava, which arewidespread between Lewiston, Idaho and Port-land, Oregon as well as from Wenatchee, Washing-ton to Walla Walla, Washington. Individual flowunits exceed 100 meters in thickness and 1,000cubic kilometers in volume. Some traveled manyhundreds of kilometers from their vents (Reideland others 1989). Erosion of these basalts byPleistocene floods produced many of the cliffsfound within the Columbia Gorge, Dry Falls, andthe Channeled Scablands.

Volcanism began about 16 million years ago ineastern Oregon and northern Nevada (Pierce andMorgan 1992) and has shifted systematicallynortheast at about 3 centimeters/year to its presentlocation in the Yellowstone area. This hot spot hasbeen a major source of silicic volcanic rocks and isresponsible for much of the present topography inthe Snake River plain (ERUs 10 and 12) (Pierceand Morgan 1992). The topographic depression ofthe eastern Snake River plain is supported bysilicic calderas that are covered by basalt flows. Thelargest known caldera eruption in or near theBasin occurred about 2 million years ago in theYellowstone area (Christiansen 1979). About620,000 years ago, eruption of approximately1,000 cubic kilometers of tephra led to collapse ofthe Yellowstone caldera (Christiansen 1979). Thiseruption deposited volcanic ash over much of thepresent-day United States and Canada. The basaltflows of the eastern Snake River Plain representthe youngest volcanic activity following passage ofthe Yellowstone hot spot (that is, a few thousandyears ago).

Biophysical

Earthquake hazards—Earthquake hazard risk isnot uniformly distributed within the Basin. Themap presented by Algermissen and others (1990)displays the maximum potential horizontal accel-eration due to an earthquake not likely to beexceeded in 50 years. This map provides a reason-able regional assessment of earthquake hazard risk.Although urbanized areas west of the Basin havethe highest probability for occurrence of substan-tial earthquakes, other parts of the Basin haveexperienced substantial earthquakes in recenttimes. In 1872, an earthquake of estimated Rich-ter magnitude 7.4 occurred near Lake Chelan inWashington (Yelin and others 1994). In Montana,the Hebgen Lake earthquake of 1959 (Richtermagnitude 7.3) produced fault scarps as high as 3meters and triggered a landslide that dammed theflow of the Madison River, creating Quake Lake.The Borah Peak earthquake of 1983 (Richtermagnitude 7.0) in Idaho produced a 36-kilometerlong fault scarp along which the Lost River Rangewas uplifted between 1 and 2 meters (Stein andBarrientos 1985). Southeastern Idaho, theYellowstone Park area, and the overthrust belt ofMontana (parts of ERUs 7, 9, and 11-13) are alsosubject to earthquake risk (Algermissen and others1990).

The effects of major earthquakes on human popu-lations and social infrastructures are beyond thescope of this discussion. Less obvious is the likeli-hood for substantial change in aquatic ecosystemsdue to landslides caused by earthquakes. Despitethe fact that earthquakes cannot be accuratelypredicted, the probability for future earthquakes inparts of the Basin are high.

Pleistocene Epoch Glacial andFlood Processes

At the watershed scale, much of the present land-scape of the Basin was shaped by processes andevents during the Pleistocene epoch (1.6 millionyears to 10,000 years ago). The Pleistocene epoch

was a time of multiple cycles of major climatevariation, ranging from ice ages to warm intergla-cial periods. The climax of the last cold period was20,000 to 14,000 years ago when average summertemperatures in the Pacific Northwest were 5° to7° C cooler and winter temperatures were about10° to 15° C less than today (Barry 1983). Duringthese cooler and moister times, large ice sheetsformed in the northern hemisphere and coveredmost of Canada and the northern tier states of theUnited States (map 2.25) (Mickelson and others1983, Waitt and Thorson 1983). The Pleistocenelobes of the ice sheet originating in Canada ad-vanced into and retreated several times from theUnited States and excavated and molded valleys inERUs 1 and 7 (Waitt and Thorson 1983).

Alpine glaciers shaped valleys along the entire eastflank of the Cascade Range, the Klamath Moun-tains, the central Idaho Mountains, the mountainsof western Montana, the Blue Mountains, SteensMountain, and in the Yellowstone area of theSnake River headwaters (Porter and others 1983).Much of these glaciated landscapes are now cov-ered with a mantle of glacial till. Downstream ofglacier termini (and in the wake of retreatingglacier termini), thick sedimentary sequences ofsilt, sand, and gravel outwash were left as valley filland terraces that now flank most rivers with glaci-ated headwaters. Valley bottoms that were inun-dated by glacier-dammed lakes, such as much ofthe Clark Fork River drainage (ERUs 8 and 9) andthe Columbia River upstream of Grand Coulee(ERUs 5 and 7), as well as pluvial lakes developedin closed basins (ERUs 4,10, and 11) during wetperiods, now have thick mantles of fine-grainlacustrine deposits (Benson and Thompson 1987).

During the Pleistocene ice ages, silt and fine-sandoutwash from alpine and continental glaciers andglacial floods were redeposited by wind as thickblankets of loess. These loess deposits locallydominate many of the landscapes of the Basin. Forexample, the rolling hills of the Palouse in easternWashington are entirely composed of loess that hasbeen deposited over the last 2 million years and is

Biophysical 181

Co

rdil

leQ

tn

Ice

C

ap

Lake

Mis

soul

aC

hann

eled

Sca

blan

ds

«3S

traw

berr

y M

tns.

Lost

Sal

mon

R

iver

Mtn

s.B

ois

e

Riv

er M

tns.

Bon

nevi

lleF

lood

Rou

te

EX

PLA

NA

TIO

PLE

ISTO

CE

NE

FLO

OD

ING

Ass

essm

ent A

rea B

ound

ary

0

100

200

Roo

d E

xten

t

Late

Wis

cons

in Ic

e

Mod

em r

iver

s an

d la

kes

locally over 75 meters thick (Busacca 1991,Busacca and McDonald 1994). Sequences of loessmantle much of the Columbia Plateau (ERU 5)and Snake River plain (ERUs 10 and 11) (Malde1991). These loess deposits are highly productive,and most agriculture in these ERUs are dependenton soils developed from loess.

The wetter climate of the last ice age led to theformation and expansion of large freshwater lakesin closed basins throughout the western UnitedStates (Benson and Thompson 1987). The pres-ence of these lakes resulted in deposition of siltand clay on inundated valley floors and substan-tially altered the patterns of hydrologic connectiv-ity between now-separated basins. This situationstrongly affected the distribution of many aquaticspecies within the Basin. In the Basin and Rangeprovince of Oregon, there were nine major pluviallakes, the largest of which was pluvial Lake Modocthat covered 2,800 square kilometers and includedUpper and Lower Klamath lakes. Additionally,Summer Lake and Lake Abert were connected intothe 1,200-square-kilometer, pluvial ChewaucanLake. Goose Lake expanded and fully integratedinto the Sacramento River system. In a similarmanner, the Warner Lakes are remnants of pluvialColeman Lake, the Harney-Malheur Lakes are theremnant of pluvial Malheur Lake, and AlvordLake (in the Alvord Desert) is a remnant of pluvialLake Alvord (Allison 1982, Orr and others 1994).Each of these pluvial lakes covered about 1,300square kilometers at their maximum extent, be-tween 15,000 and 13,500 years ago (Benson andothers 1990). All of these lake systems rapidlyevaporated at the end of the Pleistocene, and by10,000 years ago, most lakes approximated theirpresent size.

During the last ice age, the drainage systems of BigLost River, Little Lost River, Birch Creek, Medi-cine Lodge Creek, Beaver Creek, and CamasCreek (which are all now influent into the basaltsthat underlie the Snake River Plain) combined toform pluvial Lake Terreton, a large shallow lake onthe Snake River Plain (Pierce and Scott 1982).Between 14,500 and 13,000 years ago, an even

more profound drainage integration occurredwhen much of the eastern Great Basin was tempo-rarily connected to the Columbia River drainageas a result of pluvial Lake Bonneville overtoppingits lowest divide between the closed Bonnevillebasin and die Portneuf River drainage in south-eastern Idaho (Gilbert 1980, O'Conner 1993).Rapid breaching of ice-dammed lakes and thepluvial Lake Bonneville spillover led to cataclysmicflooding that dramatically affected the physiogra-phy of the Columbia Plateau and Snake RiverPlain.

The Bonneville Flood resulted from the filling andconsequent spillover of Pleistocene LakeBonneville into the Snake River drainage about14,500 years ago. About 4,750 cubic kilometers ofwater was released from this flood within a periodof a few weeks near Red Rock Pass, Idaho andtraveled down the present courses of Marsh Creekand the lower Portneuf River before entering theSnake River Plain north of Pocatello. For the mostpart, this flood route followed the present courseof the Snake River, locally widening and deepen-ing the canyon. Some of the large falls, such asShoshone and Twin Falls (anadromous fish block-ages), are parts of large cataract complexes thatformed during the flood. The Bonneville Floodsubstantially modified the valley of the SnakeRiver as far downstream as Lewiston, Idaho.Downstream of Lewiston, Bonneville Flood fea-tures are buried by subsequent Missoula Flooddeposits (Malde 1968, O'Conner 1993).

Between 15,000 and 12,000 years ago, a lobe ofthe Cordilleran Ice Sheet advanced down thePurcell Trench and blocked the Clark Fork Rivernear the present site of Lake Pend Oreille. At itsmaximum extent, the resulting glacial LakeMissoula inundated as much as 10,000 squarekilometers with 2,500 cubic kilometers of water(Craig 1987). Repeated formation and failure ofthe ice dam led to flooding that overwhelmednormal drainage routes in northern Idaho andeastern Washington. As the Missoula floods spread

, x>5

Biophysical II

through eastern Washington, they carved the"channeled scabland" by stripping loess off basaltsurfaces and eroding river courses and large fallsand depositing immense gravel bars (map 2.25).

Dry Falls in central Washington, now abandonedby the Columbia River, was the site of an immensewaterfall created by Lake Missoula flooding. Fallscreated by eroded basalts on the Spokane and thePalouse Rivers from this flooding created fishmigration blockages. Tributary valleys such as theSnake, Yakima, Walla Walla, Tucannon, John Day,Klickitat, and Hood Rivers (ERU 5) were mantledby sand and silt carried by floodwater backed-upinto these side valleys. Large basins such as theQuincy, Pasco, and Umatilla Basins were filledwith immense volumes of sand and gravel. Therewere as many as 100 Lake Missoula flood eventsbetween 15,000 and 12,000 years ago as the icedam repeatedly formed and failed (Atwater 1986,Baker 1983, Baker and Bunker 1985, Bretz 1923,Waittl 980, Waitt 1985).

Minerals and Mining Impacts

Mineral resource needs have contributed to theexploration and development of metallic andindustrial minerals within the Basin. Importantmetallic and industrial mineral resources withinthe Basin include gold, silver, lead, zinc, molybde-num, copper, sand, gravel, and stone. Even thoughthere are thousands of mineral deposits, prospects,and occurrences in the Basin, a small number ofworld-class deposits account for most of the pro-duction and resource base. (Refer to the EconomicAssessment for a discussion of mineral develop-ment in the Basin.)

Mineral deposits are not distributed uniformlythrough the Basin and tend to occur in clusters orbelts, which ultimately reflect the underlyinggeological processes that control their distribution.The locations of substantial metallic deposits andmines (defined as having past production and/orknown reserves of greater than a threshold value)within the Basin and adjacent regions have beendescribed by Bookstrom and others (in press b).

The USBM estimated that there are nearly 14,000inactive or abandoned mining sites in the Basin.Although environmental effects from mining maybe substantial, they tend to be restricted in arealextent. Mining activities have also resulted inlocally important landscape, aquatic, atmospheric,and visual disturbances.

In undisturbed areas, aquatic and terrestrial eco-systems have adapted to the fluxes of metal andmineral deposits exposed at the earth's surface.Mining, however, can expose large volumes ofrock to physical and chemical weathering,which may increase the rates of introducedmetals, acid drainage, or sediment to the envi-ronment if unmitigated.

Past mining activities within the Basin have hadprofound effects on some aquatic and riparianecosystems. The Clark Fork River in Montana, forexample, has four distinct but contiguous Envi-ronmental Protection Agency (EPA) SuperfundNational Priority List sites covering 8,100 hectaresalong 235 kilometers of river. As a result of miningdischarges at the Clark Fork River's headwatersnear Butte and Anaconda, Montana, this river isconsidered to be of concern for metal exceedencesunder section 303(d) of the Clean Water Act for200 kilometers downstream to where it is joinedby the Flathead River (EPA 1980). The SouthFork of the Coeur d'Alene River basin in Idaho ishost to a world-class lead-zinc-silver mining dis-trict. Mining began in this area in the 1800s; fromthat time to as recently as 1968 spent tailings fromsome mills were discharged directly into the river(Horowitz and others 1993). Mining of placerdeposits has also locally modified aquatic habitatin parts of the Basin. The extraction of mostresources, including gold and industrial minerals,was processed by gravitational separation from thewaste rock, although mercury was used to amal-gamate gold in some historical placers. Most placermining has been for gold, although there arenumerous garnet sand placers in parts of ERUs 5,7, and 8. Exploitation of sand and gravel resourceshas also locally modified aquatic condition insome drainages.

184 Biophysical

In addition to potential increases in the release ofmetals such as zinc, lead, cobalt, copper, arsenic,or other metals, some mineral deposit types aremore prone to produce acid rock drainage thanothers (Plumlee and others 1994). Deposit typeshigh in pyrite and other acid-producing sulfideminerals and low in acid-buffering minerals suchas calcite, aragonite, or dolomite have a highpotential for producing acid rock drainage (Kwong1993, Plumlee and others 1994). When minedand exposed to air and water, weathering of natu-rally acid-producing minerals is accelerated andappropriate remediation technologies must beemployed to prevent acid-rock drainage problems.Acid-rock drainage for a given deposit type willalso have a greater or lesser probability, dependingon the amount of water available, to react with thecontained minerals.

The USGS has classified the areas that are favor-able for mineral development according to theirrisk of acid rock drainage if remediation technolo-gies are not employed (Zientek and others, in pressb). In this classification, areas are subdivided(Kwong 1993; Plumlee and others 1994; Zientekand others, in press b) into three groups:

• Those deposit types with a high risk of generat-ing acid rock drainage (that is, they have a highsulfide mineral content and a low bufferingcapacity of the characteristic host and alterationmineralogies).

» Those that may generate acid rock drainagedepending on local climatic, hydrologic, andsite-specific deposit characteristics.

» Those that have low potential for acid rockdrainage due to low sulfide mineral contentand/or high buffering capacity of the character-istic mineralogies.

Potential impacts from unmitigated resourceextraction and processing of ores include release ofmetals, acid-rock drainage, atmospheric emissions,and increased sediment loads to the environment.Map 2.26 shows a compilation of the areas favor-able for metallic mineral deposit types in the

Basin, ranked according to the worst-case inherentpotential for deposits in those areas to generateacid rock drainage (Zientek and others, in press b).Individual site characteristics must be evaluatedbefore a given prospect or deposit can be classifiedas having high, moderate, or low potential forgenerating acid rock drainage.

Climate FeaturesThe Basin is in a transition-type climate zone. It isinfluenced by three distinct air masses:

» Moist, marine air from the west that moderatesseasonal temperatures.

» Continental air from the east and south, whichis dry and cold in winter and hot with convec-tive precipitation and lightning in summer.

» Dry, arctic air from the north that brings coldair to the Basin in winter and helps cool theBasin in summer.

These air mass types interact with each other in aregion of complex topography. Most precipitationaccumulates during winter (75 to 125 cm in theEastern Cascades, 25 to 95 cm in the NorthernRockies, and 20 to 40 cm in the Central Colum-bia and Snake River Plateaus)(map 2.27). Themountain snowpack acts like a natural reservoirand supplies the Basin with most of its useablewater. Only the eastern and southern parts of theBasin have summer maximum precipitation,which is associated with significant thunderstormactivity. Summer precipitation throughout theBasin ranges from about 20 to 50 centimeters.Since the 1970s, winter precipitation generally hasdecreased to a level comparable to the 100-yearhistorical mean. Summer precipitation has in-creased during the last 30 years, similar to 1910values.

Temperatures are generally mild in the Basinbecause of periodic influxes of moderating Pacificmoisture. Winter mean monthly temperatures

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Oregon Climate Service, 1995George H. Taylor, State Climatologist

Map 2.27—Average annual precipitation (western United States).

Biophysical 187

range from minus 10° to minus 3° C while sum-mer temperatures range from 10° to 15°C. Trendsin the last 100 years indicate a slight increase intemperatures during all seasons.

The timing and extent of competing air masses iscontrolled largely by synoptic weather patternsand local terrain features that vary across theBasin. Prolonged periods of drought occur whenPacific storms are deflected around the region,preventing the intrusion of moist, marine air. Atthese times dry, continental conditions prevail.Damaging frosts and freezing conditions com-monly occur when Arctic air invades the Basinbefore winter hardening in autumn or afterbudbreak in spring. Cold damage also may occurin winter if a warm, marine intrusion is followedby a sweep of Arctic air. In addition, the uniqueinterplay between air mass types results in dra-matic changes during transition. The most uniqueof these transitions is rain-on-snow flooding thatoccurs when warm, wet marine air displaces cold,Arctic conditions in winter. Lightning and gustywinds also occur during transitions between conti-nental and marine air masses, mainly in spring andsummer.

An interesting analysis of temperature found twomajor frontal zones in the Basin (Mitchell 1976).A Pacific air mass boundary, which dominatesduring summer, extends diagonally across theBasin from northwest California to northwesternMontana. Relatively moist, marine air exists northof the boundary and drier, continental air is com-mon south of the boundary. Mitchell noted thatthe Pacific boundary coincides with the eastwardextension of coastal vegetation found in northernIdaho and northwestern Montana. A westerlies-anticyclone boundary, which dominates duringthe winter, was found to stretch east-west alongthe Oregon-Nevada border and across northernUtah. It marks the boundary between a region ofprevailing westerly winds in the north and south-erly winds in the south, caused by circulationaround a persistent center of high pressure (anti-cyclone) over southern Nevada. This frontal zone

also coincides with the northern or southernextent of several tree species.

Bryson and Hare (1974) and Mock (1996) havefound that while large-scale climate controls (suchas the polar jet stream, Pacific subtropical high,and subtropical ridge) play important roles inprecipitation variability, small-scale climate con-trols (such as the complex topography, thermaltroughs, confined mixing heights, and convectivesystems) can dominate.

Weather Data

Recorded weather observations began in the west-ern United States during the late 1800s. Becausepopulation was relatively sparse at that time, onlyeight stations within the Basin have continuous,quality-controlled records approaching 100 years.These are Spokane WSO, Washington; Dufur,Oregon; Fortine, Kalispell WSO, and Haugan,Montana; and Priest River Experiment Station,Caldwell, and Saint Ignatius in Idaho.

High quality measurements of temperature, dewpoint, relative humidity, wind, precipitation, andradiation data are collected from the NationalWeather Service first-order stations that are oper-ated by trained observers. Another source of dailyprecipitation and temperature measurements ismaintained by the National Weather Service,Cooperative Observer Network (COOP) (NationalClimatic Data Center 1991). Data from COOPstations provide the highest spatial resolution ofdaily measurements although consistency andquality can be somewhat lacking. Observationsites for these two types of stations are usuallylocated near population centers or airports awayfrom the wildland areas of forests and mountains.

Significant additions to high elevation data oc-curred in the mid-1930s as snow course observa-tions increased and in the late 1970s with theinstallation of SNOTEL sites and remote auto-mated weather systems (RAWS). Snow course andSNOTEL sites are commonly placed near theheadwaters of major river basins. Snow waterequivalent measurements are available from both

Biophysical

types of stations. SNOTEL sites additionallytransmit precipitation and temperature. RAWSstations are designed to support fire weather fore-casting so they operate mainly during summer andare located in forest clearings on hill slopes andridges. RAWS stations transmit hourly informa-tion on air temperature, precipitation, fuel tem-perature, relative humidity, and wind. There areabout 200 RAWS, 200 SNOTEL, and 200 snowcourse sites in the Interior Columbia River Basin.

A northwest cooperative agricultural weathernetwork (AgriMet) is maintained by the Bureau ofReclamation in Boise, Idaho as part of the PacificNorthwest Hydrometeorological Network for riverand reservoir management.' Historical data since1983 include daily soil moisture, soil and airtemperatures, crop water use, and evapotranspira-tion.

A consistent network of radiosonde observations(RAOBs) began in the mid-1930s (U.S. Depart-ment of Commerce 1964). RAOB sensors measurewind, temperature, dew point, and height atmandatory atmospheric pressure levels (such assurface, 850 mb, 700 mb, 500 mb) and othersignificant levels twice a day. Stations at Spokane,Washington and Boise, Idaho are the only stationswithin the Basin that regularly report these data.

There are over 600 mountain weather and snow-pack observations in the Basin, but the stations arespaced too far apart (50 to 150 km) to representsmall-scale climate caused by complex topographyin the region. Consequently, model-generated dataplay an increasing role in climate analyses of themountainous west. Three sets of model-generateddata were available for this study:

» Historical means of monthly and annual pre-cipitation at 2.5-minute (about 5 km) and 5-minute (about 10 km) latitude/longitudespatial resolution from the PRISM model(Daly and others 1994).

1 Personal communication. 1996. Monte McVay, Bureau ofReclamation, 1150 N. Curtis Road, Boise, ID 83706.

* Daily temperature and precipitation for threecharacteristic years (1982, 1988, and 1989) at2-kilometer resolution from the MTCLIM-3Dmodel (Thornton and Running 1996).

» Monthly mean winds at 5-minute latitude/longitude resolution from the WINFLO model(Ferguson, in review).

Trends in Regional Climate Patterns

Prior to about 1900, climatic trends in the Basincould only be determined using proxy data such asthat evident from tree rings, glacier fluctuations,ice cores, deep sea sediments, lake levels, and fossilpollens. Several major epochs in modern climatehave been manifested globally (IntergovernmentalPanel on Climate Change 1990). (Moderateclimate is considered by many to have begun afterthe last major ice age, about 10,000 years ago.) Athermal maximum occurred between about 9,000and 5,000 years ago (Holocene period) whentemperatures were 1 ° to 2° C greater than today.Another warm period occurred between A.D.1000 and 1250, known as the Medieval ClimaticOptimum, with temperatures about 0.5° Cwarmer than today. A "Little Ice Age" betweenA.D. 1550 and 1850 caused low snow levels andadvancing mountain glaciers.

Regional trends in ancient climate also are note-worthy. For example, fossil-pollen data showvariable responses to alternating warm, dry andwarm, wet climates in the Holocene period(NOAA 1992 and 1993; Whitlock and Bartlein1993) with the spatial heterogeneity controlledprimarily by topography (Davis and others 1986;Whitlock and Bartlein 1993). In more recenttimes, local tree-ring chronologies (for example,Briffa and others 1992) suggest periodic warmperiods in 1630s, between 1640 to 1660, 1790s,and the 1920s, with temperatures about 0.5° Cgreater than the 1881 to 1982 average. Alternatingcold periods also are apparent in the chronologies,with the most significant from 1870 to 1900(about the time of the Little Ice Age) having

Biophysical 189

temperatures 0.2° to 0.5° C less than the 1881 to1982 average.

Historical weather observations within the Basinbegan in the late 1800s around the time that theLittle Ice Age ended. Since then, mountain glaciershave generally retreated (Ferguson 1992, Meier1984), which is a sign of general warming.Drought occurred in the 1930s, 1950s, and 1980s.There appear to be decadal trends in regionalclimate that could be related to the El NifioSouthern Oscillation (ENSO) and Pacific NorthAmerican (PNA) indices (Cayan and Peterson1989, Redmond and Koch 1991, Ropelewski andHalpert 1986, Yarnal and Diaz 1986). Thesetrends are most obvious in streamflow fluctua-tions, which aggregate precipitation and tempera-ture signals over large areas of the catchment anddrainage basin of the stream.

Disturbance Climate Events

Climate patterns that disrupt ecosystem processesare common and often critical components in thenatural cycle of events. Although the Basin is notknown for extreme climate events, many climaticfeatures that are ordinary to the Basin (such assummer cold fronts fanning wildfires or rain-on-snow floods) cause significant disruption of eco-system processes.

Lightning—Cloud-to-ground lightning strikesare the most common cause of wildfires in thewestern United States. Although the number oflightning strikes in the Pacific Northwest is lowcompared to elsewhere in the Nation (Orville1994), their fire ignition potential is no less sig-nificant.

Until 1983, the best lightning occurrence datawere available from fire lookouts. The data frommore that 2,600 storms that were observed from404 fire lookouts in Washington and Oregon from1925 to 1931 were analyzed in a classic study byMorris (1934). He found that most lightningstorms occurred on the south side of the BlueMountains in eastern Oregon and in the Colvilledistrict of northeast Washington. Smaller areas of

high frequency lightning occurred on the eastslopes of the Cascade mountains in the Okanoganand Deschutes forest districts.

With the advent of an automated lightning detec-tion system (Krider and others 1980, Latham1983, Rasch and Mathewson 1984), more detailedand ongoing analysis of lightning strike frequencypatterns became possible. Hill and others (1987)analyzed lightning frequency pattern in Idaho withautomated lightning data from 1985 and 1986.The greatest number of strikes were found tooccur in Idaho's southeast corner.

A summary of lightning frequency during theperiod 1986 to 1990 (map 2.28) shows that thehighest lightning frequencies occur near the pe-rimeter and outside the boundary of the Basin, inthe higher elevations of Nevada, Utah, Wyoming,and Montana. Within the Basin, highest frequen-cies are observed in western Montana, the Idahopanhandle, and eastern Oregon. Very few light-ning strikes were recorded throughout the periodof record in most of Washington and the westernhalf of Oregon, except a relatively high number ofstrikes occurred in the Oregon Cascades in 1989and 1990. It is important to note that higherfrequencies observed in regions where lightning istypically rare are likely the result of one or twoindividual storms, rather than a higher occurrenceof lightning throughout the course of the year.Also, detectors only record about 70 percent oftotal strikes (Orville 1994) so numbers of strikesshould be viewed in relative terms instead of exactmagnitudes.

The number of lightning strikes in a given areadoes not directly correspond to the number ofwildfires. Most lightning-caused fires in the Basinoccur in the Blue Mountains of northeast Oregonand Sawtooth-Bitterroot ranges of central Idahoand far-western Montana (map 2.29), whereasmost lightning strikes occur in southern Idaho andthe Montana Rockies. This is somewhat contraryto the 1934 work of Morris, who found thatlightning-caused fire locations agreed relativelywell with lightning storm locations reported forthe same period in Washington and Oregon.

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|1 - 60

|ei - 120|121 - 180

|181 - 240

| 241 - 300

301 - 360

I 361 - 420

|421 - 480

I > 480

Total Number of Lightning Strikes

1986 - 1990Scale = 1:7000000

0 KM 100 200 300

Map 2.28—Total number of lightning strikes (1986-1990).

Biophysical 191