WSP09221

• File Number: 7630.425
• Description: Wild and Scenic - Piedra River
• State: CO
• Basin: Western Slope
• Water Division: 7
• Date: 1/1/1991
• Author: Julie Stromberg Dunc
• Title: Instream Flow Requirements for Cottonwoods at Bishop Creek - Inyo County - California
• Water Supply Pro - Doc Type: Report/Study

eastern Sierra Nevada stream that has been
partially diverted for hydroelectric power
production for more than 80 years, stream-
flow is considered to be more "important"
(Le" is the exclusive source of water) to
riparian vegetation at lower, effluent
reaches than at higher elevation. influent
reaches (Walsh et al. 1987). The objective
of this study was to determine instream
flow requirements for maintenance of ri.
parian cottonwood (Populus spp.) trees on
Bishop Creek. To this end. relations of an-
nual streamflow to tree ring width, and
tree ring width to canopy vigor and pop-
ulation mortality were explored. These re-
lations were compared for diverted stream
reaches with differing valley geomorphol-
ogy and hydrology (i.e., effluent versus in-
fluent) and for a reach receiving the di-
verted flows.
Riparian vegetation maintenance mod-

003076

els are necessary for streams such as Bishop
Creel<, !,v.en though the young age of many
of the cottonwood trees indicates that di-
version has not precl uded cotton wood es-
tablishment. The mere presence of cotton-
wood trees (albeit at low density in many
reaches) cannot be used as an indicator of
sufficient flows, because instream flows for
recruitment and those for maintenance are
two different concerns. Episodic recruit-
ment of cottonwoods at Bishop Creek has
been facilitated by the periodic presence
of sufficient water during appropriate pe-
riods in spring. The water becomes avail-
able because the storage capacity of the hy-
droplant reservoir system is exceeded in
wet years. However, subsequent years of
low or no flow can result in stress and mor-
tality. necessitating the development of in-
stream maintenance models for riparian
vegetation.

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STUDY AREA AND SITES

Bishop Creek drains a 180-km' water-
shed in the rain shadow of the eastern Si-
erra Nevada Mountains, In its upper reach-
es, Bishop Creek flows through steep alpine
and coniferous landscapes in a glacially
carved canyon. over bedrock and glacial
till. The stream in the lower reaches is sur-
rounded by Great Basin shrub desert and
flows through an alluvial fan before en-
tering the Owens River Valley. Precipita-
tion ranges from ca. 100 em near the Sierra
Nevada Crest (ca. 4.500 m elevation) to ca.
IS em at the confluence of Bishop Creek
with the Owens River (ca. 1,350 m eleva-
tion). Most precipitation falls in winter.
Natural flows in Bishop Creek are esti-
mated at 93 hm3/year.
The main fork of Bishop Creek has five
hydroelectric power plants (Figure I). Dur-
ing dry and normal hydrologic years, most
of the water is diverted from the channel
into a series of pipelines, penstocks. and
reservoirs, and it is used to generate power.
At each power plant the water carried in
the penstock. plus any water flowing in
the stream, is collected in a small reservoir
and delivered to the downstream power
plant through another penstock. Flows
from nearby McGee Creek and Birch Creek
(combined watershed size of 65 km') are
also diverted into Bishop Creek penstocks.

Flows are ultimately released into the
stream channel below the lowermost pow-
er plant. In dry and normal years, these
release flows have been 99% greater than
flows in the diverted reaches between
power plants, and in wet years they have
been 40% greater (Smith and Nachlinger
1987). Annual flow in the diverted reaches
has ranged from < 1.25 hm' in dry years
to nearly 50 hm' in wet years. Flows in
1982 were exceptionally high, with daily
flows exceeding the 100-year flood limit
for the town of Bishop (>40 m'/s).
The study sites were located in four
reaches along Bishop Creek. as described
below:
Reach 2 is between power plants 2 and
3 at an elevation of ca. 2.020 m. It is an
influent reach in a V-shaped valley with
Quaternary till and has a stream gradient
of ca. 5% (Simons, Li and Associates 1990).
Annual flow volume from 1969 to 1988 for
this diverted reach has been ca. 9 hm3,
of which 96% has been in the growing sea-
son (May-October) (Figure 2). (Pre-1969
flow data are unavailable for this reach and
other diverted reaches.) Annual average
precipitation is 34 em. The woody riparian
community is fairly dense, consisting
mainly of black cottonwood (Populus tricho-
carpa T. & G.), Jeffrey pine (Pinus jeftreyi

FIGURE 1.

Grev. & BalL), qu
uloides Michx.).
talis Hook), and
cottonwood is
within the reach
Reach 4 is bet
5 at ca. 1,475 m.
an alluvial out
gradient of ca. 4
eraged ca. 13.6
been in the gro
etation is sparse I
only scattered pa
of cottonwood, bi
mont cottonwoo<
in the reach. '
Reach 5 is be~
6 at ca. 1.390 mini
The upper porti
effluent, but in 1
flow is augment'
(Hess and Smith

I~I 2

Rivers. Volume 2. Number I

January 1991

J. C. Stromberl