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

. The range of flows required to deposit and remove fine sediment from the bed of the
channel.

Provided that fine sediment producing events have occurred in the upstream tributaries,
fine sediment (mud) is deposited at various locations within the Clifton site where the
velocity is less than 2.5 ft/s or shear stress is less than 0.03 lb/ft2. Mud is re-entrained
from these locations when the identified velocities of shear stress thresholds are
exceeded. The results of the 2-D modeling of the Clifton site indicate that at a flow of
800 cfs (equaled or exceeded 92 percent of the time), about 35 percent of the site is
relatively mud-free. At flows of 1,100 cfs and 1,400 cfs (equaled or exceeded about 80
percent of the time), about 47 to 54 percent of the site is relatively mud-free. At a flow of
2,000 cfs (equaled or exceeded 45 percent of the time), about 67 percent of the site is
mud-free. About 85 percent of the site is mud-free at a flow of 4,800 cfs (equaled or
exceeded 16 percent of the time).

· Whether it is necessary to mobilize the underlying coarse-grained bed material for
flushing of accumulated fine sediment.

Flushing of fine sediments from the gravels and cobbles that make up the bed of the river
in the riffles and runs at the Clifton site occurs at flows in excess of 4,800 cfs and 13-
15,000 cfs, respectively when the critical discharge is exceeded, and this is described by
the framework response of the process-response model. However, the results of the 2-D
modeling show that the surficial fine sediments can be re-mobilized and flushed byless
than the critical flows for the underlying bed material throughout the Clifton site. This is
described by the transient response portion of the process-response model.

Biological Investigations

The biological portion of this study was developed to provide specific and detailed information
describing the relationship between physical processes (including peak flows) and periphyton
and macro invertebrate communities. This approach provided an opportunity to assess the
influence of peak flows and other physical processes that may affect periphyton and
macro invertebrate communities during summer and fall seasons.

The objective for the biological investigations was to quantify the primary and secondary
production, and then determine if the present fish biomass appeared to be limited by the available
productivity. Specific objectives and conclusions from the biological portion of this study are as
follows:

· Are primary and secondary productivity and standing crop dependant on magnitude of
peak flow?

Magnitude of annual peak flow had little or no measurable effect on most of the variation
observed in periphytonand macroinvertebrate communities in the l5-MR during this
study.

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

Page ES-4
October 24, 2003