9671

• UCREFRP Catalog Number: 9671
• Author: Rees, D. E., W. J. Miller and J. A. Ptacek.
• Title: Volume 1 Draft Final Report - Ecological and Physical Processes During Spring Peak Flow and Summer Base Flows in the Colorado River.
• USFW Year: 2003.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

community on which they depend. To understand the interactions between flows, sediment
delivery and biological productivity in the l5MR of the Colorado River, a series of physical and
biological investigations were made over a three year period at the Clifton study site. Those
investigations are continuing.

Physical Process Investigations

The primary goals of the physical process portion of this investigation were to address specific
aspects of the sediment dynamics of the l5-MR at meso- and micro-scale levels so that they
could be directly related to the quality of the habitat in the riffles and runs, and hence biological
productivity. These investigations focused on the role of peak flows and baseflows on bed
material mobilization and deposition, and removal of fine sediment from gravel and cobbles
within the various geomorphic subunits (riffles and runs) that are present in the 15-MR.

The specific areas of investigation and the findings of this study were as follows:

. Factors that control the supply of fine sediment to the 15-MR and the timing of the
sediment supply.

The fine sediment is derived from the lower elevations of the VCR basin that are
underlain by highly erodible sedimentary rocks. The fine sediment, that is composed
mainly of fine sands, silts and clays, is delivered to the Colorado River during baseflow
periods primarily by summer thunderstorms that do not increase the discharge of the
Colorado River greatly, but do increase the suspended sediment concentrations. Even
though suspended sediment concentrations have not changed appreciably in the last 50
years, they are lower currently than they were prior to the 1940' s.

· The hydrodynamic conditions within the reach that permit mud deposition and the
resulting spatial distribution of the mud within the reach.

Velocity and shear stress thresholds for fine sediment (mud) deposition and erosion were
identified from field measurements at the Clifton and Com Lake sites. At locations
where velocity and shear stress are higher than 2.5 fils and 0.03 lb/ft2, respectively, mud
is not deposited in appreciable quantities. The good correlation of the mapped
boundaries of the mud mapping units with the predicted boundaries from 2-D modeling
at Clifton enables the spatial distribution of the mud at the site to be predicted with
~easonable precision over a wide range of flows.

· The hydrodynamic conditions within the reach that permit general mobilization of the
gravel and cobble that constitute the bed sediment in the riffles and the runs.

Incipient motion calculations based on output from the 2-D model of the Clifton site
identified critical discharges for the riffles and the runs. The critical discharge in the
riffles is about 4,800 cfs, and in the run it is between 13,000 and 15,000 cfs. General
mobilization of the bed material throughout the site occurs at flows in excess of 20,000
cfs.

Draft Final Report - 15 Mile Reach Studies
Miller Ecological Consultants, Inc.

Page ES-3
October 24, 2003