BOARD02108

• Board Meeting Date: 5/19/2003
• Description: ISF Section - Gunnison River New Appropriation Recommendation - Water Division 4 - Montrose & Delta Counties
• Board Meetings - Doc Type: Memo

: 05/09V2003 16: 09 FAX [phone]

AIS FISH/WI~DLlFE/PARK

~006

STREAMFJ,OWS ,lS'ECESSARY TO PROTECT T~ NATURAL ENVIRONl\g:NT

The stream flows described below are necessary to prot~ct, maintain, and preserve the unique
riverine environment, biological habitats, and the naturai processes and ecosystem components
of the Gunnison River within the Black Canyon and G1$rlson Gorge. This includes both the
physical (e.g., stream flow, sediment transport) and biological (e,g., riparian vegetation, aquatic
invertebrates) components of the aquatic and associatediterrestrial ecosystems.

Sediment: The Gunnison River carved the Black Canyqn of the Gunnison. Frequent high flows
and the constant grinding of water and sediment scoured the riverbed at a much faster rate than
the surrounding canyon walls could be worn away by a1;mospheric weathering, Although the
upstream dams have reduced the sediment load entering the main stem river, abundant sediment
continues to be supplied via rock fall and unregulated tp.butary canyons, Periodic redistribution
and transport of this material is critical to protecting biological habitats and rnaintaining channel
characteristics and canyon forming processes.

Elliott and Hammack (2000) computed sediment entraimnent potential and critical shear stress
necessary to mobilize sediment on a variety of fluvial geornorphic surfaces within a typical
alluvial reach of the GUnnison River. They found that the finer gravels and cobbles of the low-
flow channel (the channel that contains a flow of apprQximately 300 cfs) and the immediately
adjacent low-flow stream banks (those surfaces normally covered by flows of700-900 cfs) ar,e
rnobiIized by flows greater than 2,500 cfs. Flows betWeen 2,500-7,500 cfs mobilize ,gravels and
finer cobbles on approximately 30-60% of the low-flow banks sampled; however, these surfaces
constitute only a srnall fraction of the entire bottomland. The larger, remaining fraction of the
bottomland (i.e., dominant lateral gravel bars) requires flows in excess of 10,000 cfs to mobilize
the rnedian particle size. .

In summarizing past sediment-related research within! the canyon, W ohl (unpublished) reports
that flows of at least 3,500 to 6,000 cfs are needed to ):legin to move coarse sediments within
debris flow constrictions and rapids, and that flows greater than 12,000 cfs are needed to
substantially rework these constrictions, W ohI predicts that as sediment continues to enter the
canyon frorn canyon walls and tributaries it will progtessively accumulate along the river if it is
not flushed downstream by these high flows. Similatily, fine-grained sediments will accumulate
in vegetated banks along the river, which will gradually restrict the active channel. Ultimately,
accumulated sedirnent will constrict the river, creatiJ\g steeper rapids and a narrower channel
with higher, vegetated banks; portions of the river could develop into long pools separated by
steep drops over rapids and srnall falls.

To protect critical biological habitats and the spect~cular gorge, sediment transport capacities
and main stem channel proportions must be maintai~ed. Peak flows are necessary to scour the
riverbed and rework debris flows and other coarse-grained deposits within the main stem
channel. Peaks between 2,500 cfs and 10,000 cft effectively mobilize gravels and cobbles within
the low flow channel, adjacent low flow banks, and werj10w channels. However,flows in excess
of 10,000 cft are /'leeded in some years to mobilize materials on lateral gravel bars and debris
, flow deposits.

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