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

Purpose and Scope
This report summarizes geomorphic, hydraulic,
and sedimentologic data collected in two alluvial
reaches of BLCA in 1990, 1994, and 1995 and hydro-
logic data recorded since the early 20th century.
The geomorphic and sedimentologic data were
collected and evaluated by the U.S. Geological Survey
(USGS) in cooperation with the National Park Service
(NPS). The objective of this study was to determine
the hydraulic conditions and minimum streamflow
necessary to entrain, or initiate the movement of, the
sediment median-particle size (dgp) of the alluvial
streambanks and bars in selected areas of BLCA
vulnerable to encroachment by riparian vegetation.
The data and these findings will be used by the NPS to
assess the sensitivity of the riparian corridor in BLCA
to potential changes in the Gunnison River streamflow
regime.
The hydrologic data are from streamflow-gaging
station 09128000 Gunnison River below Gunnison
Tunnel, located at the eastern end of BLCA (fig. 1).
This gage, operated by the USGS, has recorded peak
discharge since 1906 and continuous streamflow since
1911. Geomorphic data include channel geometry
characteristics determined from onsite surveys made
with atotal-station laser theodolite and sediment-size
distributions. Hydraulic data include flow depths and
boundary shear stresses at several channel cross
sections reconstructed from one-dimensional, water-
surface profiles modeling. Sedimentologic data
include particle-size analyses of streambanks, alluvial
bars, and a tributary debris-flow deposit. Estimates of
sediment-entrainment potential were based on the
sediment critical shear stress and modeled hydraulic
conditions at several locations on the cross sections.
The authors are especially grateful to Myron
Chase (NPS) and George Ingersoll (USGS) for their
dedication, invaluable logistical support, and assis-
tance in the field. Greg Auble, Jonathan Friedman, and
Mike Scott (USGS) contributed critical insight and
discussion concerning the sensitivity of riparian vege-
tation to hydrologic variability. Additional field assis-
tance was provided by Michelle DeLong, Chris Gable,
and Erica Van Wie (NPS) and by Randy Parker and
Erik Eggleston (USGS). Technical reviews of the
report were provided by David F. Meyer and Mark E.
Smith (USGS).
STUDY AREA
The area generally known as the Black Canyon
of the Gunnison River includes the Black Canyon of
the Gunnison National Monument (BLCA), the
Gunnison Gorge, and reaches now inundated by Blue
Mesa, Morrow Point, and Crystal Reservoirs (fig. 1).
This study was limited to the BLCA portion of the
Black Canyon.
Geologic Setting
The geology of the Black Canyon area exerts a
strong influence on the Gunnison River canyon and
fluvial geomorphology in BLCA. Incision of the
Gunnison River into folded and faulted areas of the
Gunnison uplift has exposed predominantly Precam-
brian igneous and metamorphic rocks at river level
(Hansen, 1965), but variations in the structure and
lithology give rise to a variety of canyon and fluvial
morphologies. Sediment transported by the river is
supplied by local talus slopes and rockfall and by
debris flows from ephemeral tributaries (Elliott and
Parker, 1997).
Acknowledgments
The study was made possible with the support
of the National Park Service and the cooperation of the
Bureau of Reclamation. Mark Wondzell (NPS) estab-
lished and surveyed several cross sections in 1990 that
were subsequently relocated and resurveyed by the
authors. In addition, Mark Wondzell organized logis-
tical support, assisted in all field-data collection
efforts, and provided critical oversight of methodology
and reviews of the early drafts of this report.
Riparian Zone Characteristics
The Black Canyon riparian zone can be classi-
fied into general reach categories determined by the
dominant physical processes affecting river
morphology and sediment characteristics. The two
most common reach categories are talus/rockfall and
alluvial. A third reach category, debris flow, is less
common although significant geomorphically because
of the large volume of mixed-size sediment delivered
2 Geomorphic and Sedimentologic Characteristics of Alluvial Reaches in the Black Canyon of the Gunnison National Monument,
Colorado