FLOOD02855

• County: Larimer
• Community: Larimer County
• Stream Name: Big Thompson River
• Basin: South Platte
• Title: Big Thompson River Flood Report
• Date: 10/1/1976
• Prepared For: Larimer County
• Prepared By: UDFCD
• Floodplain - Doc Type: Flood Documentation Report

I

I

I

I

I

I

I

Flood runoff in the Big Thompson basin derived
from an area of approximately 60 square miles (155
!an') centered on the Big Thompson River from Lake
Estes to Drake. The topography of the area is charac-
terize,d by steep north-and south-facing slopes with
rugged rock faces along the ridges and a thin soil
mautle at lower elevations which supports a moderate
stand of coniferous trees. Because of the steep slopes
and small storage capacity of the soils, storm runoff
quickly reached nearby surface channels. Based on
records at stream-gaging stations in the area and on in-
formation from individuals who observed the flood,
most streams in the area started to rise about 7 p.m.
The first rcported damage occurred about 7:30 p.m.
along U. S. Highway 34, 7.5 miles (12.1 km) east of
Estes Park where the highway was blocked hy rocks
and trees deposited by water cascading down the can-
yon walls and small gulches alcmg the north side of
the Big Thompson River. This area approximately
corresponds to the areas of maximum rainfall as shown
on figure 1.

I

I

I

I

I

The normally placid Big Thompson River quickly
responded to the large volumes of water entering from
several small tributaries between Lake Estes and Wal-
tonia and became a raging torrent of water, trees,
debris, and boulders moving downstream with a in-
descrihahle force. The reported peak stages on the
Big Thompson Hiver occurrcd as follows: 8:00 p.m. at
Glcn Comfort, 8:30 p.m. at Waltonia, 9:00 p.m. at
Drake, 9:30 p.m. at Loveland powerplant, and about
11:00 p.m. at the mouth of the canyon about 8 miles
(13 km) west of Loveland. Because the river reported-
ly remained extremely high from the first peak stage

I

I

I

I

I

I

I

THE FLOOD

until after midnight, it is apparent that several other
periods of intense rainfall produced secondary rises on
streams in the area during the night. A gaging sta~
tion on Fish Creek, southeast of Estes Park and up.
stream from Lake Estes, recorded a rise which peaked
at 10:00 p.m. and a smaller rise which peaked at 4:00
a.m. Dry Gulch which flows into the Big Thompson
River from the north just uownstream from Lake Estes
peaked at 10:30 p.m., as recorded by the gaging sta-
tion on the Big TllOmpson River near Estes Park.
The flood on the North Fork Big Thompson River
resulted from extremely heavy rainfall withm a few
miles of Glen Haven, The western limit of significant
flood runoff was auout 2 miles (3.2 km) west of Glen
Haven while the eastern limit was about 3 miles (4.8
km) east of Glen Haven. Extreme flood runoff oc-
curred from the basin divide on the south to about 4
miles (6.4 km) northeast of Glen Haven.
Although heavy rainfall was reported to begin at
Glen Haven about 7:30 p.m., the first report of extreme
flooding in that area was on Fox Creek which reached
the peak stage at 9:00 p.m. at Glen Haven. Another
rise almost as high as the first one occllrred at about
11:00 p.m. North of Glen Haven, both Miller Fork and
Black Creek reached peak stages about 11:00 p.m.,
with local residents reporting no flood runoff in the
streams prior to that rise. A stream~gaging station
operated by the Colorado State Engineer's Office on
the North Fork Big Thompson River at the mouth at
Drake operated satisfactorily until about 11:00 p.m.
when the intakes were covered by deposited bed ma-
terial. The record from this station indicates one peak
at 9:15 p.m. (possibly hackwater from the Big Thomp-
son River), a recession of ahout 0.3 foot (0.09 m) until

5

9:30 p.m., then an additional rise of about 0.8 foot
(0.24 m) which pcaked at 9:40 p.m. Flood water from
Miller Fork and Black Crcck reached the Drake area
shortly after 11:00 p.m. but the peak from this rise was
lowcr than the 9:40 p.m. peak.
The major part of flood damage in the North Fork
.Big Thompson River basin occurrcd in the vicinity of
Glen !laven, caused mainly by the combined flow of
Devil's Gulch and Wcst Creek which entered the town
from the southwest. The hasin between Glen Haven
and Drake is sparsely developed; thus. thc main dam-
age in this reach was to the county highway which
generally follows the river.
The relative timing of the peak stage;s was such
that the peak on thc Big Thompson River just down-
stream from Drake occurred hefore the peak from the
North Fork arrived at Drake. The flood peak moved
through the 7.3-mile (11.7-km) length of channel be-
twoen Drake and the canyon mouth in about 2 hours
with no apparent reduction in discharge as indicated
by the computed peak discharges. East of the can-
yon mouth. the Big Thompson River valley widens
rapidly and the flood dischargc was quickly reduced
by valley storage and overflow to numerous reservoirs.
The peak discharge at the Big Thompson River at the
mouth near La Salle was about 2,500 cubic feet per
second (70.8 m 3/S) occurring at 12:00 p.m. on August
1, as compared to 31.200 cfs about 35 miles (56 km) up-
stream at the "mouth of the canyon."
Aerial photographs of 'before" and "after" the
flood at Waltonia and Glen Cove are shown in figures
2 and 3, respectively. Photographs of some of the
destruction caused by the flood are shown in figures
4 through 10.