8267

• UCREFRP Catalog Number: 8267
• Author: Elliott, J. G. and L. A. Hammack.
• Title: Geomorphic and Sedimentologic Characteristics of Alluvial Reaches in the Black Canyon of the Gunnison National Monument, Colorado.
• USFW Year: 1999.
• USFW - Doc Type: Water-Resources Investigations Report 99-4082,
• Copyright Material: NO

Geomorphic and sedimentologic Characteristics of
Alluvial Reaches in the Black Canyon of the
Gunnison National Monument, Colorado
By John G. Elliott and Lauren A. Hammack1
ABSTRACT
The Black Canyon of the Gunnison River in
western Colorado includes several alluvial
reaches that are potentially sensitive to changes in
streamflow. Sediment derived from talus slopes,
rockfall, and tributary debris flows periodically
is reworked and redeposited on the streambed,
streambanks, and alternate bars in these alluvial
reaches, providing a growth medium for riparian
vegetation. Geomorphic and sedimentologic data
were collected and evaluated at two alluvial
reaches in the Black Canyon-Warner Point and
Red Rock Canyon. These data and the hydrologic
data from an upstream gaging station were used to
calculate the entrainment potential of a large
range of sediment sizes on a variety of fluvial
geomorphic surfaces typical of the Gunnison
River in the Black Canyon and other canyon
rivers.
The HEC-2 and HEC-RAS one-dimen-
sional water-surface profiles models were used to
estimate water-surface elevations, flow depths,
and hydraulic conditions for discharges from
2,000 to 20,000 cubic feet per second at surveyed
channel cross sections in the two study reaches. A
peak discharge of 9,470 cubic feet per second in
1995 and onsite observations and photographs in
1994 and 1995 confirmed sediment entrainment
or reworking on several geomorphic surfaces
inundated by the peak discharge. Physical
evidence of sediment entrainment, or absence of
sediment entrainment, on inundated sediment-
tNational Park Service, Fort Collins, Colorado.
measurement sites generally was consistent with
critical shear stresses estimated with a dimension-
less critical shear stress, ti*~, of 0.030 in the
Warner Point study reach. Sediment-entrainment
potential over a range of discharges was summa-
rized by the ratio of boundary shear stress to the
critical shear stress (io/~~), given local hydraulic
geometry and sediment-size characteristics.
Differing entrainment potential for similar
geomorphic surfaces indicates that estimation of
minimum streamflow requirements based on sedi-
ment mobility requires site-specific geomorphic
and sedimentologic data.
INTRODUCTION
The Black Canyon of the Gunnison River is
noted for its gorge-like characteristics, which include
expansive views, precipitous bedrock cliffs, and a
cascading river (Hansen, 1965; Warner and Walker,
1972). However, several reaches of the river through
the Black Canyon of the Gunnison National Monu-
ment (BLCA) exhibit characteristics of an alluvial
river: fluvially deposited banks and bars, riffle-pool
channel geometry, and a riparian ecosystem. Although
the large-scale features of canyon rivers, such as the
Gunnison River in the Black Canyon, are influenced
by regional geology, the channel and riparian zone of
many canyon river reaches are dominated by fluvial
processes. Geomorphic and botanical conditions in the
alluvial reaches of canyon rivers are controlled both by
the characteristics of fluvially deposited sediment and
the hydraulics of flood discharges. Consequently,
changes in streamflow can produce adverse effects in
canyon rivers similar to those in alluvial rivers (Elliott
and Hammack, in press).
Abstract