FLOOD02454

• County: Gunnison
• Community: Uncompahgre Valley
• Basin: Gunnison
• Title: Uncompahgre Valley Reclamation Project - Hydropower - Part 2 - AB Lateral Unit Water Supply Study
• Date: 12/1/1988
• Prepared For: Montrose Partners and Ucompahgre Valley Water Users Association
• Floodplain - Doc Type: Project

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2.3 Ooerational constraints
2.3.1 Gunnison River instream flows

After passing the East Portal of the Tunnel, the Gunnison River flows
through the Black Canyon of the Gunnison National Monument and the Gunnison
Gorge. Throughout this reach, the river is considered by the State of
Colorado to be a Gold Medal trout stream. In recent years, the popularity
of the Gunnison Gorge as a rafting opportunity has grown rapidly. The UVWUA
has historically coordinated its diversions to allow a minimum flow to enter
this reach during dry periods. After construction of the Aspinall Unit,
this flow was recognized as 200 cfs. However, recent studies indicate that
300 cfs could be more beneficial for the fishery and habitat [Reclamation,
1988]. As a result, 300 cfs has been recognized as the minimum flow in the
Gunnison for the past several years, and is used in this report to represent
existing conditions. The UVWUA has agreed to this amount for purposes of
hydropower generation, but has also retained its right to withdraw flows for
irrigation from the river to values less than 300 cfs under its 1901 water
right [Emanco, 1987 and Reclamation, 1988].

2.3.2

Gunnison Tunnel capacity

Water rights granted to the UVWUA by the State of Colorado decree a
maximum diversion of 1,300 cfs to be diverted through the Tunnel. However,
the UVWUA has not been able to perfect this entire amount, because of limited
Tunnel capacity resulting from hydraulic inefficiencies in various sections
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2.3.3

Uncompahgre River flooding

The third constraint affecting existing and proposed system operations
are the periodic flood conditions which occur on the Uncompahgre River.
These conditions have historically disrupted highway and rail traffic and
damaged or destroyed irrigation diversion structures and other riverine
improvements. Inundation of agricultural areas has eroded the land, damaged
or destroyed crops, damaged irrigation systems and other farm improvements,
and deposited sand, silt and debris on cropland. Major floods have required
the evacuation of homes and resulted in damaged or destroyed public utility
facilities, roads and bridges, and railroad track and bridges. The cost of
recovery from major floods have been substantial. '

Floods in the study area generally result from rapid melting of the
mountain snowpack from about the middle of May through early July, and from
general rainstorms that normally occur from July through October. On the
long-term average, rainfall flood events occur less frequently than snowmelt
events and do not constitute a serious flood threat. Snowmelt runoff
attenuates along the lower Uncompahgre River due to channel storage and the
numerous irrigation diversions.

Snowmelt flooding is characterized by moderate peak flows, large volume
and long duration, and marked diurnal fluctuation of flow.' Snowmelt runoff
may occasionally be augmented by rain. Rainfall flooding is characterized
by high peak flows of moderate volume and duration. Flooding is more severe

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