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

introduced during low flow,-tends to accumulate in pools and low-velocity areas. Large
racks are exposed in rapids at tributary debris-flow fans and below cliff failures, making raft
passage difficult for boats larger than about 3.5 m. The number of raft parties declines for
logistical reasons, but fishing is excellent and low water levels favor fishermen who wade to
preferred locations. Many instream and riparian areas normally used only by boat
fishermen are accessible by hiking and wading. The heaviest fishing use in the low-flow
period is from late July through early November. However, turbidity can adversely affect
fishing and can be high from warm-water algal growth during late summer. Management
issues during low-flow periods center on litter cleanup, trail and campsite maintenance, and
fishing and hunting regulation enforcement.
Moderate streamflows, from 30 to 85 m3/s, are common and can occur from March
through September. Annual flood peaks in this range have a recurrence interval not loner
than once every year or two. At streamflows below to just above bankfull discharge, 62 m /
s,, fine sediments are flushed from most cobble riffles, although they may remain in
slackwater areas. Bed shear stresses are not sufficient to disrupt trout feeding or redd areas,
and water temperatures tend to be colder than summer temperatures in a low-flow year.
Fishing is good if steady flow conditions are maintained, although wading is limited to
shallow, near-bank areas. Most campsites are accessible, but streamside trails are flooded
when discharge is above 62 m3/s. Many instream obstacles encountered at low flow are
inundated at moderate streamflows facilitating use of larger rafts, and moderate stream
velocities and abundant eddies for landings produce excellent whitewater rafting
conditions. In the summer, multiday float trips starting at Chukar Wash are popular, and
there are large numbers of fishermen and campers who use hike-in access from Duncan
Cramp, Ute Park, and Cedar Flats. These are the optimal conditions and months that attract
the greatest number of users whose activities overlap. This high-use period poses the
greatest threat to wilderness values of the Gunnison Gorge, both in terms of physical
impacts to the natural environment (damage and vandalism to campsites, streambanks,
vegetation, trails) and social impacts to visitors (excessive human encounters). The BLM
spends a large amount of time and resources mitigating resource damage and addressing
complaints and conflicts over campsites, launch times, and fishing spots.
High streamflows, peak discharges from 85 to 140 m3/s, have a recurrence interval of
1.7 to 4 years. Annual peak discharges were in this range 11 times since 1966, including the
period of USGS research during 1991, 1992, and 1993. Bed shear stresses accompanying
these discharges may be adequate to entrain some gravel- and cobble-size material from the
tied surface in some riffle/pool reaches. After the 1993 peak discharge of 145 m3/s, most of
the sand was flushed from pools above four observed rapids. Clay to gravel-size material
and some large cobbles and boulders were removed from the inundated debris-fan margin
al Long Gulch in 1993; however their subsequent depositional location is unknown. The
most heavily used river-side trails and many river-access points on banks and low terraces
were inundated. No bank or flood-plain erosion was observed, but increased soil moisture
or prolonged inundation may have affected encroaching exotic riparian plants. Sandy
overbank sedimentation and organic deposition was observed in isolated locations in 1993.
Conditions remained excellent for experienced whitewater users in this flow range in 1993,
although the number of commercial rafting trips declined due to safety concerns,
misinformation, and waning interest in fishing at high velocities and above-bankfull
discharges.
Very high streamflows, 140 to 250 m3/s, have become much less common since
construction of the upstream reservoirs (recurrence interval 4 to 20 years), and annual peaks