FLOOD06904

• County: Larimer
• Basin: South Platte
• Title: Hydrology, Geomorphology, and Dam-Break Modeling of the July 15, 1982 Lawn Lake Dam and Cascade Lake Dam Failures, Larimer County
• Date: 1/1/1986
• Prepared For: Larimer County
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

GEOMORPHIC EFFECTS OF THE FLOOD

7. Long axes are perpendicular to the flow direction.
8. Some of the coarsest rocks are on the top of the
berms. ;1.
9. The tops of the berms may be above high-water
marks on valley sides. I
Step-pool long profiles are a common feature in high-
gradient mountain streams, where more shallow-gradient,
deep pools of water are separated by abrupt, steep sU;ps
or falls in the channel. Each step consists of a cluster of
very coarse boulders extending across the channel Water
flows over the steps or concentrations of large boulders
and into pools. where much of the fluid energy is dissi-
pated. The origin of step-pool features in mountain chan-
nels has been much debated; Whittaker and Jaeggi (1982)
reviewed the proposed origins. They concluded that s~p-
pool features form in high gradient mountain streams with
heterogeneous bed material sizes during high intensity,
low.frequency floods. These are extremely stable forms
during more frequent lower flows. i
Four well-developed steps ranging in height from 4 to
9 ft were formed within a distance of 110 ft in the Fall
River just upstream from Aspenglen Campground.
One step-pool bedform in boulder deposits in the Fall
River is shown in figure 44. Average sizes of the 10
largest boulders exposed in the front of each of the steps
were 3.4 ft, 3.5 ft, 4.0 ft, and 4.4 ft. The steps formed
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47

downstream from Cascade Lake dam probably were
analogous to the steppool forms so common in coarse-
boulder steep channels. and support the belief that they
originate as bedforms under catastrophic flood condi-
tions. probably collecting under hydraulic jumps formed
near the peak of the flood.

FALL RIVER AND BIG THOMPSON RIVER:
ESTES PARK POWERPLANT TO LAKE ESTES

Between the Estes Park powerplant just below the
Aspenglen Campground and the confluence of the Fall
River with the Big Thompson River in Estes Park. the
stream slope averages 2.3 percent. Water was 3.0 to 6.0 ft
deep in the overbank areas. Flood width varied with the
valley morphology, but typically was 200 ft wide (table 4).
Scour and channel widening were not as pronounced
as they were along other steeper parts of the flood path.
Estimates of channel scour from exposed channel banks
and tree roots were in the range of 0.5 to 2.0 ft, except
immediately downstream from the powerplant, where
scour approached 30 ft. Sediments were deposited in the
channel and on the flood plains. Channel deposits were
predominantly coarse, bouldery gravel bars and sand
bars. Flood-plain deposits consisted of sand splays in

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FIGURE' 44.-The front of one step-pool bedform in boulder deposits in the Fall River upstream from Aspenglen
Campground at river mile 7.0l.
I