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

4

LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES. COLORADO

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descends over a steep cliff into Lawn Lake. Downstream
from Lawn Lake, a broad. till-covered valley extends
southeast for about 0.5 mi; then the valley turns south.
Slopes range from 5 to 26 percent, and average 10 per-
cent along the Roaring River (fig. 2). Mean annual
precipitation in the area varies with elevation, ranging
from 20 in. at Estes Park to 40 in. or more on the Con-
tinental Divide above Lawn Lake.
Downstream from Lawn Lake. the Roaring River de-
scends over a series of steep bedrock falls and through
gentle mountain meadows. The Roaring River valley
and the Fall River valley were repeatedly glaciated dur-
ing the Pleistocene Epoch. Landforms and sediments
along the course of the flood bear strong imprints of
this glacial activity (Jones and Quam, 1944; Richmond,
1960). Lawn Lake is at tree line; along the Roaring
River. vegetation consists of spruce. fir, and aspen for-
ests. Where the Roaring River joins the Fall River at
an elevation of 8.550 ft in the west end of Horseshoe
'Park. the Roaring River descends 500 ft in 0.3 mi in a
series of rapids known collectively as Horseshoe Falls
(fig. 1).

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Horseshoe Park is a flat, moraine-rimmed basin that
was occupied by a large glacial lake when terminal
moraines dammed the Fall River at the east end of the
park. The hills surrounding Horseshoe Park are covered
with ponderosa and lodgepole pines and aspen forests.
The floor of Horseshoe Park is 0.5 mi wide and 3 mi
long, and is underlain by ground moraine. outwash, and
lacustrine sediments. Valley slope is relatively flat,
averaging 0.7 percent. The park is covered by meadow
grass; dense willows mark the meandering course of the
Fall River through the park. Sinuosity of the Fall River
in Horseshoe Park downstream from the Roaring River
is 2.2, compared to 1.0 to 1.05 for other steeper moun-
tain streams in Rocky Mountain National Park.
The Fall River flows into Cascade Lake at the east
end of Horseshoe Park. Cascade Lake, located about
5.3 mi west of Estes Park, was an artificial lake private-
ly built in 1908 and obtained by Estes Park for power
generation in 1945. Downstream from Cascade Lake,
the Fall River gradient steepens again to 9 percent
as it flows through a series of Pleistocene terminal
moraines and into Aspenglen Campground (fig. 2).

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I ~BIg Thompson Ria.,.

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4 5 S 7 S 9 10 11 12
DISTANCE DOWNSTREAM FROM LAWN LAKE DAM. IN MILES

13

FIGURE 2.- Long profile of flood path ofthe water from Lawn Lake dam aod Cascade Lake dam failures. Verticsl exaggeration
10.5 times.

4-3