8260

• UCREFRP Catalog Number: 8260
• Author: Elliott, J. G., D. M. Murphy and K. S. Tucker
• Title: Resource Management Considerations in a Changing Physical Environment
• USFW Year: n.d.
• USFW - Doc Type: The Gunnison Gorge, Colorado
• Copyright Material: NO

channel geometry surveys and a step-backwater analysis (Shearman,1991). Bed-material
particle size at the threshold of entrainment, also known as the critical-particle size, was
estimated for several discharges using channel-geometry characteristics and the Shields
(1936) shear-stress equation. The estimated critical-particle size for a given discharge was
compared to the size distribution of existing bed material to assess the potential for bed-
material entrainment at the study reach. The frequency of probable bed-material
entrainment was estimated as the recurrence interval of the entraining discharge. Two
examples of discharge illustrate the effect of flow regulation on bed-material entrainment
and maintenance of channel characteristics.
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Figure 2. Annual Peak Discharges for the Gunnison River
in the Black Canyon, 1911-1993.
1) Bankfull discharge. The post-regulation bankfull discharge is a relatively common
event in the Gunnison Gorge (recurrence interval 1.3 years); however, at 62 m3/s, bed
material equal to or coarser than the d5o (cobble-size and larger) can be entrained at only
three of nine riffle/pool cross sections in the study area (Elliott and Parker, 1992). At the
other sites, the material that can be removed from the bed by the bankfull discharge is
considerably finer than the d50. Although sand and fine gravel probably can be flushed from
all riffle/pool and rapid reaches by the bankfull discharge, coarse material generally cannot,
and the bed remains immobile in most Gunnison Gorge reaches. Larger discharges are
needed to rework bed material and to remove coarse material introduced by tributaries.
2) Juniper-stage discharge. Floods greater than the bankfull discharge produce shear
stresses capable of moving larger bed material than that entrained by the bankfull discharge.
Critical-particle sizes were estimated for a flood that fills the channel to a prominent
geomorphic and vegetation feature common throughout the Gunnison Gorge. A distinctive
near-stream break in occurrence of old-growth Utah junipers (juniperus osteosperma) often
coincides with a terrace 1.5 to 2 m above the current bankfull stage. This delineation may
represent a relict, pre-regulation, flood trimline. Preliminary streamflow reconstruction
1920 1930 1940 1950 1960 1970 1960 1990
WATER YEAR