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

to the river. Debris flows from ephemeral tributaries
occur intermittently in both talus/rockfall and alluvial
reaches of BLCA.
Talus/rockfall reaches have steep river gradients
and very narrow canyon floors bordered by steep talus
or colluvial slopes and sheer bedrock cliffs. The
streambed and banks in these reaches are composed of
a high proportion of large, angular rock fragments that
have fallen directly into the channel and generally
remain there. These reaches have limited, if any,
deposits of finer grain, fluvially deposited sediments
where riparian plant communities can develop.
Talus/rockfall is the predominant reach category in
BLCA; photographic interpretation indicates that the
riparian zone in these reaches has changed very little
since 1939.
In other river reaches in BLCA, substantial
portions of the channel bed and banks are composed of
fluvially transported and deposited sand, gravel,
cobbles, and boulders. These are alluvial reaches.
Streambanks composed of well-rounded gravel and
cobbles are more prominent in these reaches than in
talus/rockfall reaches, and fluvially deposited bars can
be as much as several tens of feet wide (fig. 2). Allu-
vial deposits of notable size are present in the upper
and lower reaches of BLCA but generally are absent
between river mile (RM) 4.6 upstream from The
Narrows and RM 7.1 upstream from The Painted Wall
where the river gradient is very steep (fig. 3). River
miles in this report are expressed as the downstream
distance measured from the Gunnison Tunnel diver-
sion in the eastern part of BLCA (fig. 1).
The Gunnison River profile (fig. 3) over a
13-mile segment from the Gunnison Tunnel Diversion
to the western boundary of BLCA (fig. 1) was derived
from USGS 1:24,000 topographic maps with 20- and
40-foot contour intervals. Superimposed on the river
profile in figure 3 is a graph showing the percentage of
the river reach between adjacent contour crossings
bounded by alluvial deposits. Aerial-photograph inter-
pretation indicates that approximately 35 percent of
the Gunnison River in BLCA is bordered on one or
both banks by alluvial sedimentary deposits.
The greater abundance of fluvial sediments
deposited in alluvial reaches provides a growth
medium for riparian vegetation and the gravel- and
cobble-size bed material that is critical for some
aquatic biota. Alluvial reaches, in contrast to
talus/rockfall reaches, are areas dominated by stream-
flow-dependent processes, including intermittent sedi-
ment transport, storage, and remobilization.
Consequently, the fluvial geomorphology, riparian
vegetation, and bed-material composition in alluvial
reaches are more sensiti~re to streamflow-regime modi-
fication caused by climate change or anthropogenic
activity. Some alluvial n:aches have exhibited notice-
able geomorphic and vegetation changes since the first
aerial photographs were taken of BLCA in 1939.
Two alluvial reaches of BLCA were studied.
The Warner Point study reach (fig. 4) extended from
RM 8.1 to RM 8.7, and the Red Rock Canyon study
reach (fig. 5) extended from RM 10.9 to 11.4 (fig. 3).
Talus/rockfall reaches were not included in this study
because of their relative insensitivity to streamflow-
regime modification.
Hydrology
The Gunnison River Basin has a drainage area
of almost 8,000 mil at its confluence with the Colo-
rado River at Grand Junction, Colorado, and contrib-
utes 42 percent of the Colorado River annual
streamflow at the Colorado/Utah State line. Most of
the main-stem runoff is generated by spring snowmelt;
however, convective and monsoonal summer storms
generate runoff in ephemeral tributaries in the lower
parts of the watershed (Elliott and Parker, 1992).
Four major reservoirs and one transbasin diver-
Sion affect streamflow entering the Black Canyon.
Blue Mesa Reservoir, completed in October 1965,
Morrow Point Reservoir (1968), and Crystal Reservoir
(1977) comprise the Wayne N. Aspinall Unit, a group
of three large reservoirs in the central part of the
Gunnison River Basin just upstream from BLCA
(fig. 1). The three reservoirs of the Aspinall Unit have
a combined storage capacity of about 52 percent of the
average annual streamflow of the Gunnison River.
Diversion and impoundment before water year 1966
had relatively little effect on the flood hydrology of the
Black Canyon. Impoundment by Blue Mesa Reservoir
has had a substantial effect on the downstream flow
regime; therefore, the postregulation period for the
purpose of this study is defined as 1966 to the present
(Elliott and Parker, 1997).
The USGS streamflow-gaging station
09128000, Gunnison River below Gunnison Tunnel,
has recorded peak discharge since 1906 and contin-
uous discharge since 1911. The gage is located
approximately 2.3 miles downstream from Crystal
4 Geomorphic and Sedimentologic Characteristics of Alluvial Reaches in the Black Canyon of the Gunnison National Monument,
Colorado