FLOOD06021

• County: Larimer
• Stream Name: Big Thompson
• Basin: South Platte
• Title: Flood History of the Big Thompson River Basin: Flash Floods, Paleofloods, and Dam-Break Floods
• Date: 1/1/1979
• Prepared For: Larimer
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

GEOLOGIC AND GEOMORPHIC EFFECTS. BIG THOMPSON CANYON AREA 113

... FIGURE 77.-HeIicopterview .howingOood damage in part 01 Drake. ft high and 100 ft wide. It was built in the late 1920's
Down.tream (north) is .t top 01 picture. Belore the lIood the chan. to divert water to the hydroelectric powerpiant about 2
nel 01 the Big Thompson River was ju.t to th.lolt of the hou.... mi farther downstream. The flood first overtopped
but it shifted widely during the flood and was far to the lolt out-
side the picture at the time the photograph was taken. Rainfall then rapidly breached the dam, carrying large sections
here wasabout4 in. of concrete and boulders downstream in a wave of
water at least 23 ft above present stream level (fig. 81).
Just upstream from the dam, as much as 10.0 ft of
main stream gravel and at least 8.5 ft of overlying
slack-water sediments were scoured out. Boulders
deposited upstream from the dam were as large as 8.3
ft in longest ,!;mAnsion (table 8, mile 43.3). The south
abutment of the dam remained in place, even though
about 5.5 ft of alluvium was removed from its base.
Below the dam a large newly deposited bouldery gravel
bar extended about 650 ft downstream. The bar stood
about 11 ft above present stream level and contained
blocks of concrete as large as 1,650 ft' (table 9) and
many large boulders. The largest boulder apparently
moved by the flood was 11.8XI2.0X22.9 ft (table 8,
mile 43.2) and weighed an estimated 275 tons.
Within the 0.6-mi reach between the breached dam
and the site of the Covered Wagon Restaurant, the
floodwaters deeply scoured the 100-20o-ft-wide flood
plain. Here, the gradient was about 4 percent. Scour
removed most of the highway on the north side of the
canyon and parts of the aqueduct o!['the south side.

TABLE 8.-Size and lithololfY of large.t boulders transported by the
1976 flood in the Big Thompson Canyon, mile 44.3 to mile 39.5
{River mile 1ocatioaa lbown em plate 1]

HI_
'"'"

Location

s...{feotI Ll<hoIogy
1.9X 2.2X 3.9 Granite.
1.8X 2.0X 3.9 Do.
1.4X 2.3X 3.5 Do.
1.3X 1.3 X 3.9 Do.
4.7X 5.4X 5.8 Pegmatite.
4.1 X 5.2X 5.9 Do.
4.0X 4.2X 4.7 Granite.
3.5X 5.5 X 6.4 Pegmatite.
2.7X 3.2X 6.2 Do.
5.DX 5.3X 8.3 Granite.
4.9X 5.0X 7.3 Do.
3.9X 4.9X 6.0 Do.
2.9X 3.9X 8.0 Do.
2.9X 3.2X 8.2 Do.
2.8X 4.7X 6.0 Do.
ll.8X 12.0 X 22.9 Granite.
8.5X 7.0X 9.5 Do.
6.0X 7.2XI2.5 Do.
5.DX 5.3X 8.0 Do.
4.5X 5.5X 7.0 Do.
4.DX 5.DX 7.0 Do.
5.4X 7.3XI0.5 Granite.
4.5X 4.9X 7.2 Do.
4.3X 5.5X 8.3 Do.
4.0X 4.0X 7.0 Do.
3.9X 6.7XID.0 Do.
3.2X 3.3 X 4.9 Granite.
2.3X 2.7X 4.4 Gneiss.
2.1X 2.8X 3.2 Granite.
1.8X 2.3X 3.1 Do.
0.9X 1.7X 3.6 Gneiss.
2.2X 2.6X 3.7 Granite.
1.7X 2.8 X 3.8 Do.
1.6X 2.3X 3.3 Pegmatite.
1.1X 2.2X 3.2 Gneiss.
0.7X 2.2 X 2.8 Do.

44.3

44.1

43.3

43.2

42.6

41.2

39.5

0.3 mile
down.tream
from Drake.

Up.tream and
of Midway.

Up.tream side
01 hydroelectric
diversion
clam.

Downstream
side 01
bydroelectrlc
diversion
dam.

Covered Wagon
Resteurant
area.

Loveland
power
plant.

Cedar Cove
area.

TABLE 9.-Concrete blocks cruried downstream
from destroyed diversion dam at riuer mile 43.25

Block size
(inft')
720
600
1,650
270
280
30
4

ApprcWmate weight
,....~
54
45
124
20
20
2
.3

DistaDce from dam
,It)
o
65
90
ll5
160
325
2,400

THE COVERED WAGON RESTAURANT AREA

The Covered Wagon Restaurant and parts of an ad-
joining motel had stood on a low terrace on the north
side of the river (mile 42.6). One canyon resident who
watched the flood from high ground reported that ris-
ing water first simply surrounded the buildings and
trapped the occupants inside. Moments later, a surge
of water carried off the restaurant and nearby
buildings. When the flood subsided, a large bouldery
gravel bar occupied the site.
This stretch of canyon (mile 42.8 to mile 42.3) is
about 200 ft wide and is relatively straight, but a
winding channel was cut by the flood as the current
was deflected from side to side, and bou1dery gravel
was deposited along the insides of bends. A gravel bar
about 5 ft thick formed at the site of the restaurant,
and its surface was strewn with boulders as much as
10.5 ft long (table 8, mile 42.6). Opposite the bar on the
south bank, lateral scour on the outside of the bend ex-
posed bouldery alluvium deposited by a previous flood.
From the Covered Wagon Restaurant area
downstream to the Loveland powerplant, a distance of
0.9 mi, about 65 percent of the highway was destroyed.
Along this stretch the canyon is walled by bedrock, ex-
cept for the scoured-out highway embankment, and
deposition was limited to insides of bends and places
where the current was slowed or impeded. A bouldery
gravel bar was deposited JUst downstream from a
highway bridge that remained standing. The bridge
(mile 41.81. although overtopped was spared serious

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