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. Jarrm / Geomorphology 11 (J 994) J5-40

31

Ti:lbleJ
Field- and particle size d:uo. for soils developed in termce deposits llnd colluvium

Horizon Soil horizona MunseHb color Percenr= ?:u.nt
thickness or SUbSUllte (moist) material
(em) Sand Silt Cloy
0-13 AlC",- IOYR 4/2 n 21 7 Colluvium
13-35 2Bt 7.5YR 3/4 76 II 13 Alluvium
35-250 2Cox 7.5YR 3.5/4 93 3 3 Alluvium
:>250 bedrock FounUlin Fm.
0-3 A 10YR 4/2 36 49 15 Colluvium
3-20 etl 10YR 3/3 30 45 26 Alluvium
20-40 Ba 7.5YR 3/3 23 42 36 Alluvium
>40 bedrock Fountain Fm.
0-2.5 A 10YR 3/3 59 2S 16 Colluvium
2.5-10 28tl 7.5YR 3/4 43 23 29 Alluvium
10-20 28t2 7.5YR 3/4 63 18 20 Alluvium
20-120 3S, 7.5YR 3/4 46 31 24 Colluvium
> t20 covered
0-10 A 10YR 4/2 33 12 5 Alluvium
10-30 A2 IOYR 2/2 60 27 12 Alluvium
>30 Cn IOYR 2/2 96 3 1 Alluvium
0-5 A 10YR 2/1 47 36 17 Colluvium
5-30 A/Bt IOYR 3/3 <Ill 35 17 Colluvium
30-70 sa 10YR 3/4 2S 37 38 Colluvium
>70 covered

Location

T1 terrace.
Site 3

T t terrace.
Site)

Tlte~.
Site 4

from
nt of
,d on
1St of
than
5[ud-
tidal
that
,"old
::row-
;s for
esti-

D tenuce.
Site4

Colluvial slope.
Silc5

'Soil horizon nomenclm:ure from Soil Survey Staff (1975).
bMunsell Soil Color Chort. Munsell Company. 1954.
~ than 2 mm frnctiOI1.

:FBI
ooul-
n the
1984;
ng of
,.and
:time
fire.
lative
Iders.
,Iders
lSe of
posits

the active channel. FB2 deposits are found mainly
along the break in slope between the terrace tread and
riser (Fig. 5b) and form distinct boulder bars whose
long dimension is generally parallel to the active chan-
nel. FB2 deposits are considerably less weathered than
FBI or FBO deposits (Fig. 7; Table 2). I
FB3 flood deposits are recognized at Site 3. but are
limited in extent (Fig. 5b). These deposits are found
about I m above the active channel (Fig. 6). FB3
deposits are little weathered (Fig. 7; Table 2).
FB4 deposits are the youngest deposits recognized
at Site 3. FB4 deposits are about IS cm above the active
channel and define the channel margin (Fig. 5b). FB4
deposits consist of unweathered, little-modified con-
centrations of rounded to subrounded. imbricate peg-
matite and granodiorite boulders. Matrix sediment is
rarely preserved and probably has been eroded by mod-
eratefloods incapable of mobilizing the coarsest FB4
boulder fraction.

;rano-
lbove

5.4. Site 4

Site 4 is located about 0.96 km upstream from Hor-
setooth Reservoir and about 500 m downstream from
the mouth of Arthurs Rock canyon (Fig. 2). Four allu-
vial terraces and five flood boulder deposits, are rec:
ognized at Site 4 (Fig. 5c).The terraces are similar to
those recognized at Site 3 and the upper two terraces
(TO. TI, Fig. Sc) can be traced continuously from Site
3 to Site 4. The lower terraces and flood deposits (TI,
TI. FB2. FB3. Fig. Sc) are present at about the same
relative elevations above the active channel as are
equivalent deposits at Site 3 (Fig. 6).
Although FBO deposits are close to the mountain
front, we interpret them as fluvial deposits rather than
colluvial or debris,"f1ow deposits. because (1) FBO
deposits are inset within the colluvial slope rather than
being part of it. indicating fluvial erosion of the collu-
vium. (2) a source for granodiorite boulders found in

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