FLOOD02750

• Document ID: 125
• County: Routt
• Stream Name: Elk River
• Basin: Yampa/White
• Water Division: 6
• Title: Streambank Stabilization Limited Watershed
• Date: 11/1/1999
• Prepared For: CWCB
• Prepared By: ICON Engineering, Inc.
• Watershed Pro - Doc Type: Educational/Technical/Reference Information

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4.3.3 Geomorphic Setting and Historical Analysis

The lower portion of the Elk River has been significantly modified in the past by human
activity. This portion of the river has provided a source of irrigation flow for adjacent
agricultural and nearby gravel mining uses. The channel is incised within a fairly broad
floodplain terrace area. Existing meanders are irregular and relatively mild with average
amplitude and wavelength of approximately 1000 ft and 2,500 ft, respectively. A fairly sharp
meander bend exists upstream of U.S. 40. Sinuosity (the ratio between thalweg length and
down valley distance) measures between 1.3 and 1.5 along the study reach. The lower reach of
the Elk River is characterized as a sinuous point bar channel.

A general overview of the planform character and field assessment of the study reach is shown
in Figure 4.1. The study reach was broken down into a series of individual reaches (numbered
I through 5) as shown on Figure 4.1. Reaches were determined based on a number of
observations including stream characteristics such as: the presence of man-made or artificial
controls, the level of channel incision, channel geometry, bank failure characteristics,
vegetation, apparent changes in bed or bank materials, observed flow, presence and types of
depositional features, and land use. A summary of the field assessment on a reach by reach
basis is presented in the following paragraphs.

Reach 1 (Downstream U.S. 40) - This reach extends from the confluence with the Yampa
River to U.S. 40. There is significant evidence of deposition along this reach with a majority of
deposition downstream of the Denver and Rio Grande Western Railroad bridge. Middle bar
deposits downstream of the railroad are well vegetated. Sediment has also been deposited
between the two bridges and under the eastern portion of the railroad bridge. This reach is
subject to backwater effects from the Yampa River, and the river would also provide some
natural grade control in this area.

Reach 2 (Upstream of U.S. 40) - This reach extends from the U.S. Highway 40 bridge to
approximately 1,400 ft upstream. A large point bar with significant vegetation dominates the
western portion of the channel forcing a skewed stream alignment with respect to the bridge.
Large boulders had been placed along the eastern channel bank to protect both the bank and the
eastern bridge abutment. Significant scour was noted around the bridge piers. The bar deposits
were a mixture of small cobble underlain by sandy gravel. The bridge appears to serve as a
man-made grade control for this reach. Some localized headcutting was observed in the
channel along this reach.

Reach 3 (Section 4 to Section 6) - This reach extends from approximately 1,400 feet upstream
of U.S. 40 through the sharp meander bend to approximately 3,000 feet upstream of U.S. 40.
The western channel bank is fairly steep with heights around 6 to 8 feet. The western bank
along this reach has been protected with a combination of broken concrete rubble, riprap, and
large boulders. A small irrigation diversion culvert is protected by a concrete structure in the
west channel bank. The eastern portion of the channel is dominated by a large point bar

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