629
Canyon-Filling
Lavas and Lava Dams on the Boise River,
Idaho, and Their Significance for Evaluating Downcutting
During the Last Two Million Years
K. A. Howard’,
J. W. Shervais*, and E. H. McKee’
ABSTRACT
Basalts that periodically dammed the Boise River
and its South Fork over the last 2 million years reveal
the canyon history and illustrate how lava interacted
with impounded
river water. Intracanyon
basalt
flows record a granite canyon successively filled by
lava and then recut at least five times in the last 2
million years. The most voluminous flow, Steamboat
Rock Basalt, reached 60 kilometers downstream and
spread out on the Snake River Plain just east of
Boise. Lavas that reached the river were erupted from
vents bordering the main canyon and adjacent tributaries. The river canyon was periodically flooded by
basalt from these eruptions, reentrenched to a new
lower level, then flooded again. This succession
resulted in terraces of older flows high above the
river and of younger flows lower on the canyon
walls. The canyon-filling flows dammed the river and
created deltas of pillow basalt and hyaloclastite
overlain by massive subaerial basalt. Foreset beds in
the hyaloclastite deposits and inclined pillows indicate flow into reservoirs behind the lava dams. A
well-documented
example of a lava dam is the one
formed by the Smith Prairie Basalt, which is about
0.2 million years old. Potassium-argon
ages calibrate
a canyon history in which the river was lowered at a
rate between 0.005 and 0.01 centimeter a year over
the last 2 million years. The lava dams interrupted
this lowering but were each rapidly incised in about
a quarter of a million years at rates averaging 0.03 to
0.07 centimeter a year.
INTRODUCTION
Basalt terraces along the granite canyons
‘U. S. Geological
Survey, Menlo Park,
2Department
of Geological
Sciences,
Santa Barbara, California
93106.
California
University
of the
94025.
of California,
Boise River show that successive lava dams up to 60
kilometers long alternated with downcutting.
The
dam and the deltas of foreset pillow basalt formed
by the Smith Prairie Basalt are particularly
well
displayed.
We examine here the building and
incision of the lava dams and the canyon history
over the last 2 million years, based on detailed
mapping of the Smith Prairie area (Howard
and
Shervais, 1973) and reconnaissance elsewhere along
the Boise River (Shervais and Howard, 1975). Three
manmade reservoirs obscure some of the geologic
relationships along the Boise River canyon, but we
have reconnoitered
the basalt terraces exposed
during low water in Arrowrock
Reservoir and have
made use of unpublished engineering reports on the
dam sites and topographic
maps of the reservoir
areas prepared by the Bureau of Reclamation and
the Corps of Engineers. Rates of downcutting
are
calculated from potassium-argon
ages of the basalts.
SETTING
The Boise River and its South Fork drain
highlands of the Idaho batholith just north of the
Snake River Plain (Figure
1). The river flows
through
canyons several hundred
meters deep,
largely in granitic bedrock. Lindgren (1898, 1900)
and Russell (1902) observed that basalt terraces
follow the canyon for 80 kilometers (Figure 2). The
lava flows erupted onto the granitic highlands near
the river, forming outliers related to the basalts of
the Snake River Plain. The river canyon was
periodically flooded by basalt from these eruptions,
reentrenched to a new level deeper than before, and
then flooded again. This history has resulted in
terraces of older intracanyon flows high above the
river and rimrock benches of younger flows lower on
the canyon walls (Crosby, 1911). These relations are
illustrated in cross sections and a longitudinal profile
Cenozoic
630
Geology
of Idaho
4?4
Smith Prairie
M
SAWTOOTH
0
U
N
T A I N
Basalt
Basalt
01 Lava Creek
Basalt
01 Mores
Basalt
01 Smith Creek
Basalt
01 Rock Creek
Basalt
01 Anderson
Steamboat
Creek
Ranch
Rock Basalt
Basalt of Long Gulch
Basalt of Lucky
Figure 1. Generalized
Figure 3
map of intracanyon
basalt flows
along
the Boise River,
along the canyon showing the elevations of flow tops
(Figure 3).
BASALT
FLOWS
A correlation chart (Figure 4) shows ten
recognized basalt units. Older basalt flows probably
once were present, as suggestedby a variety of basalt
cobbles in old gravels beneath the Steamboat Rock
Basalt at Long Gulch. Vent locations are known for
five of the basalts (Figure I). Detailed descriptions
of the basalt of Long Gulch, basalt of Rock Creek,
basalt of Smith Creek, Steamboat Rock Basalt, and
Smith Prairie Basalt are given by Howard and
Shervais (1973). The Steamboat Rock and Smith
Prairie Basalts form the most extensive terraces.
Basalt of Lucky Peak is exposed in cliffs as a
single thick flow unit overlying gravel and forming
terraces downstream from Lucky Peak Dam (Figure
3, section A-A’). We correlate this basalt with a
single hackly jointed basalt flow 40 meters thick that
rims Lucky Peak Reservoir at the mouth of Mores
Creek, against which the younger Steamboat Rock
Idaho.
Star symbols
indicate
vents.
Peak
Cross sections
shown
in
Basalt is juxtaposed (Figure 3, section B-B’).
Basalt of Long Gulch is known only from one
exposure at Long Gulch, where two flows of this
unit, separated by gravel, lie below another gravel
bed, that in turn is covered by the Steamboat Rock
Basalt. The stratigraphy indicates that each of these
flows formed the canyon floor before the canyon
was filled by the Steamboat Rock Basalt.
Steamboat Rock Basalt occurs in numerous
pahoehoe flow units 2 to 20 meters thick, forming a
rimrock high above the Boise River South Fork
from Smith Prairie to Lucky Peak Reservoir (Figure
2a). Its shield-volcano source is a broad tableland
known as Smith Prairie. The basalt fills an ancestral
canyon broader and shallower than the present one
and is as much as 150 to 180 meters thick.
Downstream beyond Lucky Peak Dam, a ledge of
basalt that probably correlates with the Steamboat
Rock Basalt occurs at an elevation lower than the
basalt of Lucky Peak and continues out onto the
Snake River Plain as a layer 25 meters thick near
Boise. The Steamboat Rock Basalt thus flowed at
least 60 kilometers downstream from its source to
the Snake River Plain and had an original volume of
Howard
and others--low
Dams
on Boise
River
Figure 2a. View northward
of high terrace of Steamboat
Rock IUsalt (ST) and a lower terrace formed by the younger Smith Prairie
above the Boise River South Fork. The Smith Prairie Basalt reached the river canyon from the high plateau of Smith Prairie
a lava tube (T) and spillover at Black Canyon (BC).
several cubic kilometers.
It has since been deeply
incised.
Basalt of Anderson
Ranch, which occurs upstream on the South Fork around Anderson Ranch
Reservoir, forms rimrock composed of many flow
units high above the river bed. More than one basalt
flow may be present. Malde and others (1963)
correlated this rock with the Pleistocene Bruneau
Formation, part of the Idaho Group in the Snake
River Plain. The elevation difference between the
top and lowest exposure of the basalt (C. J. Okeson,
U. S. Bureau of Reclamation, written communication, 1969) gives a minimum thickness of about 180
meters near Anderson Ranch Dam. Two outcrops
downstream that possibly represent remnants of this
basalt form a terrace 30 meters lower and probably
younger than the Steamboat Rock Basalt. One
outcrop is on the southwest flank of Smith Prairie
(sec. 10, T. 1 N., R. 7 E.), and the other is nearby
Steamboat
Rock
itself. These were originally
assigned to the Steamboat Rock Basalt by Howard
and Shervais (1973).
Basalt of Rock Creek is exposed over a small
area, where it rests on the Steamboat Rock Basalt; it
is at most 10 meters thick.
Busalr of Smith Creek forms a flow 8 kilometers
long in Smith Prairie, mostly along the margin of
the older and underlying Steamboat Rock Basalt
(Figure 5A). The flow, which does not reach the
river canyon, was erupted from a now-dissected
cinder cone on Smith Creek. The basalt of Smith
Creek is distinctive in that it contains numerous
granitic xenoliths. Rare granitic xenoliths are also
known from the basalts of Lava Creek and Fall
Creek.
631
Basalt (sp)
(right) via
Figure 2b. Terrace (t) underlain
by Smith Prairie Basalt showmg
numerous
flow units in an ancestral canyon that is incised in
light-colored
granodiorite
(gr).
Basalt of Mores Creek forms terraces along the
canyon of Mores Creek, a tributary
to the Boise
River at Lucky Peak Reservoir. The basalt ledges
continue downstream along the river in the reservoir
at a lower elevation than the older basalt of Lucky
Peak and the Steamboat Rock Basalt. Ledges at
elevations appropriate for the basalt of Mores Creek
occur within
the western part of the reservoir,
according to a detailed topographic
map of the
reservoir basin (Corps of Engineers, 1958). A flow
not far above the river bed at Lucky Peak Dam
(Corps of Engineers, 1949, 1951-1955), now excavated, probably was a continuation
of the basalt of
Mores Creek.
Basalt of Lava Creek forms an aa flow 10
kilometers long from a slightly eroded cinder cone
632
Cenozoic
A
Geology
of Idaho
A’
1000
B-B’
Figure 3. Top.
Elevations
meters) of
exaggerated
Anderson
Cross sections of river canyon at true scale at places A-A’, B-B’, C-C’, and D-D’ indicated in Figure 1 and bottom of Figure 3.
in meters. Bartom. Longitudinal
profile, constructed
along the present Boise River and its South Fork, showing elevations (in
tops and (poorly
constrained)
lowest exposed bottoms of basalt flows. Inset shows tributary
Mores Creek. Vertical scale
20X. Basalt units as follows: sp, Smith Prairie Basalt; mc, basalt of Mores Creek, rc, basalt of Rock Creek; ar, basalt of
Ranch; ST, Steamboat
Rock Basalt; Ig, basalt of Long Gulch; Ip, basalt of Lucky Peak.
Figure 4. Correlation
chart of intracanyon
flows along Boise River
from northwest
(left), downstream
to southeast (righr),
upstream.
Potassium-argon
ages reported
in this paper and
reconnaissance
field measurements
of magnetic
polarity
(N,
normal;
or R, reversed) are indicated.
Correlation
with Idaho
and Snake River Groups (Malde and Powers, 1962, Correlation
with Idaho and Snake River Groups (Malde and Powers, 1962,
Armstrong
and others, 1975) is based on surface preservation
(Howard
and Shervais,
1973), polarity,
and potassium-argon
ages.
near Smith Prairie. The flow lapped over the
Steamboat Rock Basalt and basalt of Smith Creek
but did not extend to the river canyon (Figure 5A).
Smith Prairie Basalt is the best known lava flow
of the area (Howard and Shervais, 1973) and
illustrates well the processes of canyon filling and
building of a lava dam. The basalt overlies the
Steamboat Rock Basalt and the basalts of Smith
Creek and Lava Creek. It erupted after the South
Fork of the Boise River had cut through the
Steamboat Rock Basalt and an additional
150
meters of granodiorite. Two of three tongues of the
Smith Prairie Basalt spill through notches over a
high rimrock of eroded Steamboat Rock Basalt
(Figure 2a) and join a fill of basalt up to 150 meters
thick in the river canyon (Figure 2b). Remnants of
this lava fill occur 35 kilometers downstream in
Lucky Peak Reservoir; one remnant forms part of
the foundation
for Arrowrock
Dam. The lava
surface and vents of the Smith Prairie Basalt are
fresh and little eroded except in the river canyon,
where downcutting has exposed nearly the complete
Howard
and others-Lava
Dams
on Boise
633
River
Borolt
of Lava
Creek
il
Steamboo!
Figure 5. Maps of Smith Prairie area showing sequential development of Smith Prairie Basalt (Howard and Shervais, 1973). Vents for each flow
are indicated bv radial svmbol. Heavv line indicates lava tubes. SC, Smith Creek; BC, Black Canyon. Panel D shows the Smith Prairie
Basalt after erosion.
150 meter thickness of the canyon fill. Pillow basalt
in a lava delta associated with the intracanyon flow
will be described in a later section. The eruptive
history can be summarized as follows (Figure 5A-C).
The initial eruption of porphyritic Smith Prairie
Basalt built a cone of cinder and agglutinate 90
meters high near Smith Prairie (Figure 5A). A thin
lava stream from this vent flowed 10 kilometers
through Smith Prairie, around a broad shield of the
Steamboat Rock Basalt and along a creek bed to the
rimrock edge of Smith Prairie, where it cascaded340
meters in elevation through Black Canyon (BC in
Figure 2a) across rimrock of the Steamboat Rock
Basalt to the floor of the ancestral South Fork. Two
flow units of this early porphyritic basalt, with a
combined thickness of 15-25 meters, crop out 8 to 10
kilometers downstream in the thalweg of the lava fill
in the river canyon; the modern Boise River South
Fork has not yet cut through to the base. The
uppermost of these flow units has an aa clinker top,
in contrast to the pahoehoe nature of the rest of the
Smith Prairie Basalt.
Eruption of microporphyritic basalt from a new
shield vent 1.5 kilometers south of the cinder cone
followed the samepath and filled the river canyon to
a depth of 120 meters with ten or more flow units
(Figure 5B) having a total volume of approximately
0.5 cubic kilometer. This huge fill, more than 35
kilometers long, was fed by a lava tube only 10
meters wide (T in Figure 2a) that developed just
upstream from Black Canyon. Stearns and others
(1938) described an analogous lava tube that drained
the Sand Springs Basalt into the Snake River
canyon. Insulated flow in lava tubes allows for long
distance transport of pahoehoe in Hawaii (Peterson
and Swanson, 1974). Surprisingly, we found no
evidence for tubes in the long canyon fill of Smith
Prairie Basalt or in any of the other basalt canyon
fills.
Eventually, another tongue of lava made its way
off Smith Prairie by following the narrow gully of
Smith Creek to the river canyon (Figure 5C). A lava
tube also developed just upstream from this gully.
The tongue deposited a single flow unit of relatively
viscous, pressure-ridged basalt up to 30 meters thick
on the thick basalt fill already in the river canyon.
Pahoehoe festoons in the underlying basalt fill show
that it had reached the river via Black Canyon and
Cenozoic
634
Geology
flowed up Smith Creek from the river canyon.
The final basalt to erupt from the shield vent was
more viscous than earlier flows, as suggested by
numerous pressure ridges and by increased phenotryst content nearer the vent; it sent off a third
tongue of lava 6 kilometers long on Smith Prairie
(Figure SC).
Basalt of Fall Creek originates at a fresh cinder
cone named Red Mountain.
The lava forms a
narrow rough-surfaced
tongue down the canyon of
Fall Creek. Observations
made before Anderson
Ranch Dam was built show the basalt extending
onto the present floor of the reservoir (C. J. Okeson,
U. S. Bureau of Reclamation, written communication, 1969). This position on the reservoir floor
suggests that the basalt of Fall Creek is younger
than the Smith Prairie Basalt downstream,
which is
now deeply dissected by the Boise River South Fork
(Figure 2b).
FLOW
OF LAVA
INTO
of Idaho
it (Figures 6, 7). The basalt pillows (Figures 8 and 9)
have glassy margins, are between 0.1 and 1 meter
across, and are elongate and interconnecting
like
those described by Jones (1968) Moore (1970), and
Moore and others (1971, 1973). The elongate pillows
plunge upstream, in the direction of lava flow, at
angles up to 90 degrees but averaging about 30
degrees. A small amount of hyaloclastic debris fills
the interstices between pillows. The external mold of
a horizontal log 0.3 meter across was found in the
pillows.
The upper, approximately
horizontal contact of
each layer of pillow basalt of the Smith Prairie
Basalt is gradational with overlying massive subaerial
basalt. Elongate pillow fingers connect with and
extend down from massive columnar basalt at the
contact (Figure 9a). This contact, termed the passage
WATER
Many lava flows in the Pacific Northwest
have
blocked stream drainages, often resulting in deltas of
pillow lava and fragmented basaltic glass, and lake
beds deposited on the upstream side of the lava dam
(Russell, 1902; Wright, 1906; Fuller, 1931; Stearns,
1931; Stearns and others, 1938; Peterson and Groh,
1970; Malde, 1965, 1971, 1982 this volume; Waters,
1960; Brown, 1969). The dam formed by the Smith
Prairie Basalt provides an unusually well-displayed
example of pillow-lava deltas. The Steamboat Rock
Basalt also exposes part of its delta.
Pillow basalt alternates with subaerial pahoehoe
in the upstream part of the canyon fill of the Smith
Prairie Basalt, showing that the lava repeatedly built
deltas into a reservoir of river water dammed behind
Figure 6. Foreground
shows interlayered
covered by talus) and subaerial basalt
Basalt upstream
from Black Canyon.
basalt of Cougar
Flat flow unit (cf)
passage zone into underlying
pillow
pillow basalt of a younger flow unit.
Steamboat
Rock Basalt (sr).
pillow basalt (p, typically
in dam of Smith Prairie
Cliff-forming
subaerial
grades down through
a
basalt and is overlain
by
High rimrock
beyond is
LAVA ENTERED
CANYON
HERE
METERS
1100
PRESENT RIVER CUED
0
I
I
I
Figure 7. Reconstructed
longitudinal
section of the lava delta on the upstream
Several flow units of massive basalt grade downward
into pillow basalt (wavy
concealed under talus slopes. The delta is 0.4 kilometer
wide and 6 kilometers
the right.
2
I
KILOMETERS
face of (shaded) dam formed by the Smith Prairie Basalt.
lines). Dashed where inferred.
Much of the pillow basalt is
long. The reservoir backed behind the dam was upriver to
Howard
and others--lava
Dams
on Boise
River
635
lavas at the mouths of Smith and Trail Creeks show
that water backed up these tributaries while the dam
was still building. If the average supply rate of lava
to the dam through its lava tube were comparable to
the tube discharge at the 1969-1971 eruption of
Kilauea Volcano (Swanson,
1973; Swanson and
others, 1971), the dam could have been built in 4-6
years. The copious 1783 eruption at Lakagicar in
Iceland built a dam 100 kilometers long and as much
as 180 meters thick in only a month (Stearns, 1931;
Thorarinsson,
1969).
Figure 8. Pillows in Smith Prairie
fragmental
hyaloclastic
material.
Basalt,
here
nearly
free
of
zone by Jones and Nelson (1970). indicates the water
level at the time of emplacement of a flow unit that
built its own delta of pillow “foresets”
topped by
subaerial pahoehoe “topsets.” A section through the
lava dam, as reconstructed
in Figure 7, shows that
wedges of pillow foresets thicken upstream in the
river canyon from the crest of the lava dam and that
subaerial basalts thin upstream. It can be concluded
from this relationship that successive layers of the
lava dam each built deltas out into the reservoir
backed behind the dam, and that the water level rose
and overtopped the lava between the delta-building
episodes (Figure 10).
Changes in the elevation of the passage zone are
interpreted
to record fluctuations
in water level
(Jones and Nelson, 1970; Furnes and Fridleifsson,
1974). The passage zone in the largest exposed delta
(Cougar Flat flow unit of Smith Prairie Basalt,
Figure 7) drops 40 meters as the delta is traced
upstream, then rises 40 meters as the subaerial part
of the flow unit thins out. This suggests that the lake
lost water, perhaps by evaporation
or leakage,
during the initial building of this delta, then the lava
supply slowed as the lake rose and overtopped the
subaerial lava.
The lake held about 0.4 to 0.5 cubic kilometer of
water when full, a volume approximately equal to the
basalt volume in the dam. The lake may have
overflowed from time to time and was full when the
last lava was emplaced, as indicated by pillow basalt
nearly at the crest of the lava dam. The average
supply of water therefore exceeded the average
supply of lava. At the modern average discharge of
the river (about 30 cubic meters per second as
extrapolated from gauging records in U. S. Geological Survey, 1968), the lake would have filled, if
leakage is discounted, in about six months. Pillow
The Steamboat Rock Basalt also formed deltas
where it flowed into water dammed behind it. An
exposure
on the south side of Smith Prairie
(Howard
and Shervais,
1973) displays subaerial
basalt passing downward across a horizontal passage
zone into foreset-bedded
fragmental basaltic glass,
containing lenses of pahoehoe, pillows, and pillow
fragments, which together dip 30 degrees upriver
(Figure 1 I). The subaqueous interval is more than 50
meters thick measured vertically.
This exposure
strikingly resembles a lava delta formed by the flow
of lava into the sea from Kilauea Volcano in 1971
(Moore and others, 1973, Figure 4). Deltaic lava
that is transitional
between bedded breccia and
pillow basalt is represented
by an exposure of
pillows in hyaloclastite matrix 2.3 kilometers to the
south (Figure 12). The pillows and breccia underlie
claystone lake beds and unconformably
overlie
arkosic colluvium. The claystone appears to underlie
higher flow units of Steamboat Rock Basalt. These
relationships record a complex sequence of canyon
cutting,
colluviation,
erosion
of arkosic
talus,
damming of the river by the Steamboat Rock Basalt,
advance of a lava delta into the reservoir backed
behind the dam, deposition
of lake beds, and
ultimate eruption of final parts of the Steamboat
Rock Basalt. Material for the delta of Steamboat
Rock Basalt probably was supplied across a broad
front, in contrast to the simple point at which the
Smith Prairie Basalt was supplied to its delta owing
to the confined path of the lava.
Lava dams are common in the western United
States, as along the Snake River, Idaho (Malde,
1965, 1971, 1982 this volume),
Deschutes
and
Crooked
Rivers, Oregon (Stearns, 1931; Peterson
and Groh, 1970), Tieton and Naches Valleys,
Washington
(Smith,
1903), and Grand Canyon,
Arizona (McKee and others, 1968; Hamblin, 1969;
Hamblin and Best, 1970). Historic eruptions have
dammed rivers in Iceland (Stearns, 1931; Thorarinsson, 1969) and in British Columbia (Brown, 1969;
Symons, 1975; Wright, 1906). The building of the
deltas formed by the Smith Prairie Basalt may
model many of those lava dams.
636
Cenozoic
Geology
of Idaho
‘igure 9a. Passage zone (overhang)
from
subaerial basalt (s) above pillow basalt
(p) in the Cougar Flat flow unit of the
Smith Prairie Basalt.
:igure 9b. Close-up of the upper part of the
pillow lava showing elongate tubelike
pillows
plunging
down to the right.
Interstices
all filled with fragmental
basaltic glass.
Howard
and others--low
Dams
on Boise
River
637
Figure 1 I. Delta deposits of Steamboat
Rock Basalt. Foresetbedded
hyaloclastite
dips upriver
to left below ledges of
subaerially
deposited part of Steamboat
Rock Basalt. Exposed
combined
thickness of both is 150 meters. View north across
Rock Creek at Smith Prairie tableland.
1
I
c
Figure 10. Schematic section to illustrate how successive lava deltas
form wedges of dipping pillow lavas (foresets) between massive
subaerial lavas (topsets) as the water level rises. Passage zones
record the water level changes.
POTASSIUM-ARGON
AGES
Potassium-argon
age determinations
were made
on samples of basalt from seven flows in the Boise
River canyon (Table 1). Four of the flows were
dated twice to evaluate the accuracy of the age
determinations.
Sample preparation and argon and
potassium analyses were carrried out in the U. S.
Geological Survey laboratories
at Menlo Park,
California. The basalts were crushed, sieved to 60 to
100 mesh size, washed and treated for I minute in 5
percent HF and 30 minutes in 14 percent HNOr
solution,
then loaded into a high-vacuum
gasextraction
system.
This treatment
has proved
advantageous in eliminating atmospheric argon from
whole-rock
basalt samples (Keeling and Naughton,
1974). Potassium
analyses were performed
by a
lithium metaborate flux fusion-flame
photometry
technique,
the lithium
serving as an internal
standard (Ingamells,
1970). Argon analyses were
performed by standard isotope-dilution
procedures,
using a 60 degrees sector, 15.2-centimeter-radius,
Neir-type mass spectrometer,
operated in the static
mode for mass analysis. The error limit shown in
Table 1 as a * value in millions of years is a
weighted value combining the analytical precision at
Figure 12. Pillows of Steamboat
Rock Basalt in breccia matrix,
unconformably
over a west-dipping
erosion surface in eastdipping arkosic colluvial
breccia. Hammer
for scale. Sec. 22,
T. I N., R. 7 E.
one standard deviation with an estimate of the
similarity in analyzed amounts of K20 and Ar from
splits of the same sample. In general the ratios of
radiogenic 40Ar from the sample to total 40Ar is an
index of the f figure. Samples reported here range
from 9.9 to 2.5 percent radiogenic “Ar and have a
weighted -f- value of between about 20 to 50 percent
of the age. Six determinations
yielded less than 2
percent radiogenic NAr and are not listed here
because their i- value is so great that the age is
considered meaningless. Similar difficulties in dating
young basalts of the Snake River Plain were found
by Armstrong
and others (1975) and Kuntz and
Dalrymple (1979).
Cenozoic
638
Geology
of Idaho
Table 1. Potassium-argon ages of basalts in Boise River area.
IHSP-I81 and HSP-186 are from the early porphyritic part of the Smith Prairie Basalt. HSP-13 is from the middle of the Steamboat Rock
Basalt. H79 BOISE-l is the upper flow of basalt of Long Gulch.
2Age considered too young based on stratigraphic relationships with other dated flows.
A + A,. = 0.581
x IO-‘oyi’;
Ap + 4.962
x IO-“‘yr -1; ‘OK/K = 1.167 x I0~4mole/mole
P
The ages in Table 1 suggest the following
chronology consistent with stratigraphic position,
magnetic polarity (Figure 4), geomorphic position,
and degree of lava preservation. Basalt of Lucky
Peak is roughly 2 million years old. The Steamboat
Rock Basalt is roughly 1.8 million years old. Basalt
of Mores Creek is about 0.4 million years old. The
Smith Prairie Basalt is about 0.2 million years old.
Basalt of Lava Creek is older than the Smith Prairie
Basalt and has normal magnetic polarity, so it is
younger than 0.7 million years, the beginning of the
Brunhes normal polarity epoch. Basalt of Smith
Creek is younger than the Steamboat Rock Basalt
and has reverse magnetic polarity, so it is older than
0.7 million years.
CANYON
HISTORY
AND
INCISION
The Boise River drainage was established more
than 2 million years ago before eruption of the basalt
of Lucky Peak. The successivelava dams punctuate a
record of progressive deepening accompanied by
steepening of the canyon walls. A chronology of
incision (Figure 13) demonstrates a long-term rate of
lowering, averaging 0.005 to 0.01 centimeter a year
over the last 2 million years. Upstream reaches have
deepenedfaster than downstream reaches. Each dam
of basalt interrupted this lowering but was rapidly
cut through. In the last 0.2 million years since the
Smith Prairie Basalt partly filled the canyon, the
South Fork has cut down nearly to its former level
before eruption of the Smith Prairie, incising at an
average rate of 0.07 centimeter a year (C-C’ and
D-D’, Figure 13). This compares with the modern
rate of 0.05 centimeter a year estimated for the
Dearborn River in Montana, decreased from an
average rate of 0.20-0.25 centimeter a year over the
last 25,000 years (Foley, 1980). At Mores Creek
(B-B’, Figure 13) the Boise River cut each of its
basalt dams at rates averaging 0.03 centimeter a year
for about a quarter of a million years.
The erosional history of the Smith Prairie Basalt
illustrates how its dam was cut. River-laid cobbles
and boulders on the crest of the dam suggest that
sedimentsfilled its reservoir and overtopped the dam
before significant erosion. Sands in the reservoir
basin are exposed near Danskin bridge. As the river
coursed over the dam, it commonly followed its
margins as described by Crosby (1911):
The lateral contact of the lava terrace and the
granite slopeagainstwhich it was formed is usually
marked by a depression,which must be regardedas
an original feature, the surfaceof the lava, after the
manner of lava streams, being lower along the
marginsthan in the middle of the valley. The River
would naturally follow one or the other of these
lateral depressions,and we thus find a ready
explanation of the fact that lava terraces of
correspondingelevation rarely occur coincidently on
oppositesidesof the river.
Smaller streams also typically followed flow margins
as documented by modern and abandoned stream
courses on Smith Prairie (Howard and Shervais,
1973).
Over time the river entrenched itself partly in the
Smith Prairie Basalt and partly in less resistant
granodiorite at the margins of the basalt fill (Figure
2b). The grade of the river is steepestwhere incision of
resistant lava is incomplete (Figure 3, bottom). The
river presumably incises by abrasion during floods
when its thick bed of gravel (seecaption, Figure 13) is
in motion. On tributary Smith Creek, erosion instead
Howord
and
others-Lava
ELEVATION
Dams
OF
RIVER
on
Boise
BED,
River
639
METERS
Figure 13. Incision history inferred for the Boise River and its South Fork at locations B-B’, C-C’, and D-D’ (Figures 1 and 3) based on basalt
potassium-argon
ages and top and bottom elevations of the lava dams. Large dots are better data points; small dots are less well-constrained
data points. Gravel fdI (shaded) below streambed is estimated from its modern thickness before construction
at Lucky Peak Dam (23 meters,
Corps of Engineers, 1949, 1951-55). at Arrowrock
Dam (20-27 meters, Crosby, 1911) and at a site 5 kilometers
upstream from Arrowrock
Dam (25 meters, Crosby,
1911). Gravel 1 meter thick underlies probable basalt of Mores Creek at Lucky Peak Dam (Corps of Engineers,
1949, 1951-55), and gravel 3 ‘meters thick underlies the Steamboat
Rock Basalt at Long Gulch (C-C’). Bed thickness beneath the other
paleocanyons
is unknown.
proceeds by headward retreat of 30-meter-high Smith
Creek Falls where resistant basalt is underlain by less
resistant pillow basalt.
Whether downcutting
is fastest when the river
begins to cut a basalt dam, or after incision is more
nearly complete, is not clear, for a basalt dam offers
higher gradients but increased resistance to erosion.
After incision of a basalt dam is complete, downcutting
slows to a long-term lowering probably brought about
in part by relative lowering of the base level downstream.
The intracanyon basalts contain no obvious evidence of tectonic lowering of the Snake River Plain
relative to the Idaho batholith to account for the
lowered Boise River. No fault offsets of any of the lava
terraces have been found. Nor are older flows demonstrably tilted, for their gradients (Figure 3) show little
downstream
convergence toward younger flows, if
dome-shaped profiles near each lava source are discounted. The domes resemble shield volcanoes and
may result from the decreased number of flow units
and increased viscosity away from the source. Excluding the domes, lava gradients upriver from Arrowrock
Dam, subparallel to the margin of the Snake River
Plain, compare favorably with those downriver and
along Mores Creek that are more or less perpendicular
to the plain. Downtilting toward the plain would have
been expected to result in higher gradients of lava
canyon fills perpendicular rather than parallel to the
plain,
Cenozoic
Geology
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