BOARD00438

• Board Meeting Date: 5/19/2003
• Description: EDO Director's Report
• Board Meetings - Doc Type: Memo

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'05/09V2003 16:09 FAX [phone]
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A/S FISH/WILDLIFE/PARK

14J006

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STREAMFLOWS NECESSARY TO PROTECT THE NATURAL ENVIRONMENT

The stream flows described below are necessary to protect, maintain, and preserve the unique
riverine environment, biological habitats, and the natural processes and ecosystem components
of the Gunnison River within the Black Canyon and Gunnison 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 associated terrestrial ecosystems.

Sediment: The Gunnison River carved the Black Canyon 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 atmospheric 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 tributary canyons. Periodic redistribution
and transport of this material is critical to protecting biological habitats and maintaining channel
characteristics aI1d canyon fonning processes.

Elliott and Hanunack (2000) computed sediment entrainment potential and critical shear stress
necessary to mobilize sediment on a variety of fluvial geomorphic 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 approximately 300 cfs) and the immediately
adjacent low-flow stream banks (those surfaces normally covered by flows of 700-900 cfs) ar,e
mobilized 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 small fraction of the entire bottomland. The larger, remaining fraction of the
bottomland (Le., dominant lateral gravel bars) requires flows in excess of 10,000 cfs to mobilize
the median particle size.

In summarizing past sediment-related research within the canyon, Wohl (unpublished) reports
that flows of at least 3,500 to 6,000 cfs are needed to begin 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 ohl predicts that as sediment continues to enter the
canyon from canyon walis and tributaries it will progressively accumulate along the river if it is
not flushed downstream by these high flows. Similarly, fine-grained sediments will accumulate
in vegetated banks along the river, which will gradually restrict the active channel. Ultimately,
accumulated sediment will constrict the river, creating 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 small falls.

To protect critical biological habitats and the spectacular gorge, sediment transport capacities
and main stem channel proportions must be maintained. Peak flows are necessary to scour the
riverbed and rework debris flows and other coarse-grained deposits within the main stem
channel. Pea!r.f between 2.500 cft and 10,000 eft effectively mobilize gravels and cobbles within
the lowflow channel, afijacent low flow banks, and overflow channels. However,flows in excess
. of 10,000 eft are peeded in some years to mobilize materials on lateral gravel bars and debris
flow deposits.

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