FLOOD02077

• County: Larimer
• Community: Larimer County
• Stream Name: Lawn Lake Dam
• Title: Hydrology, Geomorphology, and Dam-Break Modelling of the July 15, 1982 Lawn Lake Dam and Cascade Lake Dam Failures
• Date: 1/1/1983
• Prepared For: State of Colorado
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

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remote Lawn Lake. Wayne Graham and Curtis Brown of the Bureau of Reclamation
provided data on the flood traveltime and warning and response to the flood.
Charles Huntley and Zenas Blevins, also of the Bureau of Reclamation, provided
computation of the inflow to Lake Estes and hydrologic information.

The U,S. Army Corps of Engineers, Omaha District, provided aerial photo-
graphs that were taken 4 hours after the f1 ood and were used to prepare
topographic maps of the two lakes. Other valuable photographs in this report
were obtained from the town of Estes Park, Federal and State agencies, and
from private individuals, These photograph sources are credited in individual
fi gures.

The Setting

Lawn Lake is a manmade enlarged natural lake, occupying a moraine-dammed
depression on the southeast side of the Mummy Range in Rocky Mountain National
Park (Colorado) at an elevation of about 11,000 ft (fig, 1). Local bedrock
cons i sts of Precambri an gnei sses and schi sts that are more than L 7 bi 11 ion
years old (Peterman and others, 1967), Lawn Lake is fed by the Roari ng Ri ver,
which originates upstream from Crystal Lake, a higher cirque lake about 1 mi
upstream, at an elevation of about 11,500 ft, Downstream from Crystal Lake,
the valley is steep and rugged. The Roaring River 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 percent along the Roaring River (fig, 2).
Mean annual preci pitat ion in the area vari es wi th e 1 evat ion, rangi ng from
20 in. at Estes Park to 40 in. or more on the Continental Divide above Lawn
Lake.

Downstream from Lawn Lake, the Roari ng Ri ver descends over a seri es of
steep bedrock falls and through gentle mountain meadows, The Roaring River
valley and the Fall River valley were repeatedly glaciated to the entrance of
Rocky Mountain Nat i ona 1 Park duri ng the Pl ei stocene 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 Roari ng Ri ver, vegetation cons i sts of spruce, fi r, 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),

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 hi 11 s surroundi ng Horseshoe Park are covered wi th
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 LO to LOS for
other steeper mountain streams in Rocky Mountain National Park.

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