FLOOD02685

• County: Larimer
• Basin: South Platte
• Title: Flood Geomorphology of Arthurs Rock Gulch: Paleoflood History
• Date: 1/1/1994
• Prepared For: Larimer County
• Prepared By: Elsevier Science Publishers
• Floodplain - Doc Type: Flood Documentation Report

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C.F. Wayrhomas. R.D. Jarrett / Geomorphology 11 (1994) 15-1()

25
;;JAMursRocltc..lch
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20
mCglCll'lldoF'rorl'RlInp
~itl
,. IS
~
!!;i
0
'" '0
"
~
s

o
o 10 20 30 40 SO 60 70 80 90
IMeRIC~T10N ANGLE. IN oeGREES

Fig. It. Frequency histogram of imbrication angles associ.ued with
the A-8 planes of clOlStS from f'IasJl..tlood deposits in the Colot:l~o
From Range foothills and Arthurs Rod:: Gulch.

Sharp.
imnlis

skewed (mode about 180) and funher indicates that
these deposits probably did not form by water-floods.

5.7. Paleoflood history

range
'abric
epos-
ncen..
gthat
ding.
from
!rkey
brics
'P"r-
than
luire
ade-

We interpret at least five paleofloods from the dis-
tribution of granodiorite boulders. erosional terraces,
and slackwater deposits in Arthurs Rock Gulch. Rela-
tive-age criteria. stratigraphic relations, and the results
of radiocarbon dating and tree coring are used to esti-
mate the chronology of flooding and late Quaternary
fluvial-geomorphic history of the basin (Fig. 12).
Because the weatheringcharacteristicsofFBw.FBO.
and FB I deposits are similar. these deposits may have
been deposited by three or fewer closely-spaced floods.
The age of the flood(s) that deposited FBw, FBO. and
FB I ,is estimated by comparison of boulder weathering
characteristics with montane glacial deposits of known
age in Colorndo. These comparisons suggesl thaI FB w,
FBO. and FB I flood boulders may correlale withdepos-
its formed during the Pinedale glaciation (Birkeland,
1973. 1984; Madole. 1976; Porter el a!., 1983). FBw,
FBO. and FB I granodiorite flood boulders lack the
extreme weathering found on erralics of the penulti-
mate BuULake glaciation that dales to ca. 130,000 yr
BP (Madole, 1976; Shroba el aI.. 1983; Madole'et aI..
1984) and thus the FEw, FBO. and FBI paleofloods
may have happened afler this time. However. if forest
fires swepl the area then the weathering of surface boul-
ders would have been effected by thermal expansion
and spalling thus complicating attempts to use rock
weathering as a chronologie tool. Al present, Sites 3

ybe
lood
itA-
Igles
osits
typ-
:Iass
lbri-
s. in
:lass
n of
it at
vely

35

and 4 are below the lower treeline and only scattered
cottonwood trees are present along the active channel.
For fire to have been a factor in rock weathering. lower
treeline at Sites 3 and 4 would have to have been
depressed at least 10 to 15 m from its present lower
limit. The altitudinal position of the upper treeline in
the Colorado Front Range has fluctuated vertically over
of hundreds of meters during the late Pleistocene and
Holocene (Elias, 1985; Short. 1985; Short and Elias.
1987). However. little is known about the position of
the lower treeline during this same time period.
althol>gh Markgraf and Scott (1981) suggest that in
south central Colorado, montane forests occupied pres-
ent day sagebrush shrub lands in lowland areas from the
late Pleistocene until about 4000 yr BP. Even consid-
ering the effects of fire. it is unlikely that FBw, FBO.
and FB I deposits are younger than the Pinedale glacia-
tion because none of the granodiorite flood boulders
are weathered less than Pinedale erratics.
We did not find any fluviaJ deposits older than TO
and FBD in the lower part of the basin. FBD boulders

Event Stratigraphy and Radiocarbon Dates
o FB4(:lo 1941 AO?)
100
200
300
400
500

I-
Z
W
'"
w
0:
0-
W
0:
f2 1.000
~ 2.000
'"
~ 3.000
~ 4.000
6 5.000
CD
0:
<
8 10.000
c
~

40.000

>100.0001

FB3 (289 yr B.P q 302 yr B.P ,)
Gully erosion in upper basin
AJluviation in lower basin
(Site 1, Unit 8) (297yt6.P.,S17."e.P.l
Oebris~flow deposition, Site 5
(509 yt8.P.)

:'-0.;. _
;:;;"
_ Episodic alluviation. Site 1 (Unit A)
'1 (4.868 yr s.P.)
F82

?

I !

Colluviation and soil development?

~~~
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~.:~~'~-~
..>:;if.,"""",
1"::~~" ....
" ~-1'--

FB1

1
??FBO?
_.F8w' ?

III--- Colluvlation and soil development
1

Fig. 12. Summary of geomorphic events inferred from exposed sur~
ficial deposits in Arthurs Rock Gulch. RadioclU'ban dates on the right
are~ibl'llted:lCtordingto the method ofStuivera.nd Reimer ( (987).

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