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

Dam (the downstream-most of the Aspinall Unit
dams), about 0.4 mile downstream from the Gunnison
Tunnel (fig. 1), and 8.2 miles upstream from the begin-
ning of the Warner Point study reach. The Gunnison
Tunnel diverts as much as approximately 1,000 ft3/s
for irrigation from April through October. The
nonconsumed portion of the diverted water eventually
is returned to the Gunnison River by way of the
Uncompahgre River and does not pass through the
Black Canyon. The drainage area upstream from the
gage is 3,965 mil. Discharge data from 1911 through
1994 were used to determine magnitude-frequency
relations and flow-duration characteristics for periods
before (preregulation) and after (postregulation) reser-
voir construction.
Upstream reservoirs have altered the timing and
magnitude of Gunnison River flood peaks since
1966; however, the mean annual discharge in the
Black Canyon (954,000 acre-feet) has not changed
since about 1930. Monthly mean discharge during the
snowmelt season (April through July) has decreased
63 percent since reservoir regulation, whereas monthly
mean discharge for the remainder of the year (August
through March) has increased 170 percent (Elliott and
Parker, 1992). Flood magnitudes of given recurrence
intervals (RI) have decreased substantially since 1966.
Recurrence intervals, the reciprocal of the probability
that a specific discharge will be equaled or exceeded
in any given year, were determined according to
methods prescribed by the U.S. Interagency Advisory
Committee on Water Data (1982). The 10-year flood
decreased from 14,900 to 7,000 ft3/s, the 5-year flood
from 12,700 to 5,460 ft3/s, and the mean annual flood
from 9,290 to 4,040 ft3/s. While reservoir operation
has attenuated flood peaks in the Black Canyon, it
also has augmented moderate streamflows. The dura-
tion of daily mean streamflows between 1,140 and
3,000 ft3/s has increased from an average of 12 percent
of the time, or an average of about 44 days per year
from 1911 to 1965, to 38 percent of the time, or an
average of about 139 days per year from 1966 to 1994
(Elliott and Parker, 1997). Recurrence intervals and
streamflow durations of selected discharges in the
preregulation and postregulation periods are summa-
rized in table 1.
GEOMORPHIC AND SEDIMENTOLOGIC
CHARACTERISTICS
Geomorphic and sedimentologic data were
collected and evaluated at two alluvial reaches in the
BLCA. These data and the hydrologic data from the
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.
Aerial Photography
Aerial and oblique photography have been used
to document changes in geomorphology and riparian
conditions by several investigators (Webb, 1996;
Webb and others, 1996; Elliott and Hammack, in
press). Aerial photography was used to identify allu-
vial and talus/rockfall reaches of BLCA and to eval-
uate changes in alluvial deposits, debris-flow deposits,
and riparian vegetation through time. Black and white,
natural color, and color/infrared images covering part
or all of BLCA have been made intermittently since
1939. Most of these photographs were made in the late
summer or early autumn and facilitate comparison of
fluvial features at relatively low discharges. Years of
photography used in this study are 1939, 1950, 1954,
1966, 1976, 1982, 1989, and 1992. An index of most
of these aerial photographs is available from the USGS
Earth Science Information Center in Denver, Colo-
rado.
Monumented Oblique Photography
Oblique photographs were taken of the study
reaches, monumented cross sections, alluvial bars, and
sediment-sampling sites and can be used to document
future changes in alluvial features if they occur. Photo-
graphs of the BLCA study reaches were made from
monumented vantage points at river level to at least
100 feet above the river level to show different
perspectives of the Gunnison River. Several changes in
riparian vegetation and sediment deposits in the two
study reaches were observed and photographed
between 1994 and 1995 and confirm sediment entrain-
ment at several locations (Elliott and Hammack, in
press). Monumented photography locations and
numbers are shown in figures 4 and 5 and are listed in
GEOMORPHIC AND SEDIMENTOLOGIC CHARACTERISTICS g