- Summary
- Acknowledgments
- Introduction
- Scope of Present Status Review
- Key Questions in ESA Evaluations
- The "Species" Question
- Hatchery Fish and Natural Fish
- Thresholds for Threatened or Endangered
Status
- Summary of Information Relating to the Species
Question
- Environmental Features
- Ecoregions and Zoogeography
- Klamath Mountains Geological
Province
- California Current System
- In-Stream Water Temperature
- Life History
- Anadromy-Nonanadromy
- Steelhead
Run-Types
- Age
Structure
- Half-Pounders
- Oceanic
Migration Patterns
- Straying
- History of Hatchery
Stocks and Outplantings
- Steelhead Hatcheries
- Oregon Hatchery
Stocks
- California
Hatchery Stocks
- Population Genetic
Structure
- Previous
Studies
- New Data
- Discussion and
Conclusions on the Species Question
- Reproductive
Isolation
- Ecological/Genetic Diversity
- Conclusions
- Abundance and Threshold
Determination
- Population
Trends
- Coastwide
Overview of Steelhead Abundance
- Historical
Abundance in Southern Oregon and Northern California
- The Threshold
Question
- Reductions in
Available Steelhead Habitat
- Dams
- Other
Activities
- Discussion and
Conclusions on the Status of the ESU
- Threshold
Assessment
- Conclusion
- Citations
- Appendix A - Glossary
- Appendix B - Stock Abundance
and Trends
SUMMARY
The Endangered Species Act (ESA) allows listing of "distinct
population segments" of
vertebrates as well as named species and subspecies. The policy of
the National Marine
Fisheries Service (NMFS) on this issue for Pacific salmon and
steelhead is that a population
will be considered "distinct" for purposes of the ESA if it
represents an Evolutionarily
Significant Unit (ESU) of the species as a whole. To be considered
an ESU, a population or
group of populations must 1) be substantially reproductively isolated
from other populations,
and 2) contribute substantially to ecological/genetic diversity of
the biological species. Once
an ESU is identified, a variety of factors related to population
abundance are considered in
determining whether a listing is warranted. NMFS received a petition
in May 1992 asking that
winter steelhead of Oregon's Illinois River be listed as a threatened
or endangered species
under the ESA. In May 1993, NMFS published a Federal Register notice
concluding that
Illinois River winter steelhead did not by themselves constitute a
species as defined by the
Endangered Species Act (ESA). At the same time, NMFS indicated that
it would undertake a
broader status review to determine the boundaries of the
Evolutionarily Significant Unit (ESU)
that contains Illinois River winter steelhead and determine whether
this broader group was
threatened or endangered. This report summarizes biological and
environmental information
gathered in that status review
Based on genetic, life history, zoogeographic, geologic, and
environmental
information, we conclude that the ESU that contains Illinois River
winter steelhead extends
from the vicinity of Cape Blanco in southern Oregon to the Klamath
River Basin (inclusive) in
northern California. These are essentially the boundaries of a
prominent geologic feature
known as the Klamath Mountains Province. Both winter- and summer-run
steelhead are
included in this ESU, as well as populations sometimes referred to as
"fall-run" in California.
Within this geographic area, most steelhead populations show a
declining trend in abundance,
and 10 stocks have been identified in independent stock assessment
reports as being at
moderate or high risk of extinction. Furthermore, the declines are
even more dramatic when
only natural fish (progeny of naturally spawning fish) are
considered. We conclude that
steelhead within this ESU are likely to become endangered in the
foreseeable future.
ACKNOWLEDGMENTS
This review was conducted, in effect, as a continuation of the
status review for Illinois
River winter steelhead (Busby et al. 1993). Although the legal
requirements for responding to
the Illinois River winter steelhead petition were fulfilled by the
initial status review, which
concluded that the petitioned population was not a "species" under
the Endangered Species
Act, the National Marine Fisheries Service Directorate initiated this
extended status review,
and the authors of this Technical Memorandum would like to
acknowledge their support.
As with all ESA status reviews, information submitted by
agencies, organizations, and
individuals was invaluable. The authors acknowledge the efforts of
all who contributed to the
Administrative Record.
The biological review team for this status review included:
Peggy Busby, Dr. David
Damkaer, Robert Emmett, Dr. Stephen Grabowski, Dr. Jeffrey Hard, Dr.
Orlay Johnson,
Dr. Conrad Mahnken, Gene Matthews, Dr. Michael Schiewe, Dr. Thomas
Wainwright,
William Waknitz, Dr. Robin Waples, Laurie Weitkamp, Dr. John
Williams, and Dr. Gary
Winans.
INTRODUCTION
In May 1992, the National Marine Fisheries Service (NMFS)
received a petition
(ONRC et al. 1992) to list southwest Oregon's Illinois River winter
steelhead (Oncorhynchus
mykiss) as a threatened or endangered "species" under the U.S.
Endangered Species Act (ESA)
of 1973 as amended (U.S.C. 1531 et seq.). A status review completed
in May 1993 (Busby
et al. 1993, NMFS 1993) concluded that winter steelhead of the
Illinois River, a tributary to
the Rogue River (Fig. 1), did not by itself constitute a species
under the ESA. However, it
was also concluded that steelhead from the Illinois River are part of
a larger Evolutionarily
Significant Unit (ESU) whose boundaries remained to be determined.
Whether the larger ESU
that contains Illinois River winter steelhead would merit protection
under the ESA could not be
determined until the nature and extent of the ESU were identified and
additional information
about patterns of abundance in coastal steelhead was compiled.
On 20 May 1993, NMFS published a Federal Register notice (NMFS
1993) initiating a
status review of coastal steelhead populations in California, Oregon,
and Washington for the
purpose of identifying ESU(s) in these areas and determining whether
listing under the ESA
was warranted for any identified ESU. The present report summarizes
information considered
in this status review relative to the ESU that includes Illinois
River winter steelhead.
The biological species Oncorhynchus mykiss includes
several forms of nonanadromous
and anadromous trout, including rainbow trout, redband trout, and
steelhead (the name applied
to the anadromous form of the species). Taxonomy of O.
mykiss, and the relationship between
the various forms, has been well studied, yet remains challenging
(e.g., Kendall 1920, Snyder
1940, Behnke 1992). Behnke (1992) proposed that there are seven or
eight subspecies within
O. mykiss (Table 1).
Figure 1. Map showing the Illinois River and other key geographic
locations discussed in this
status review. See Figure 2 for detail.
Table 1. Proposed taxonomy of various forms (subspecies) of
Oncorhynchus mykiss (Behnke
1992).
| Scientific name |
Common name and comments |
|
Rainbow trout of coastal basins |
|
O. mykiss irideus |
Coastal rainbow trout from Alaska to
California (anadromous form is called
steelhead) |
O. mykiss mykiss |
Kamchatka rainbow trout or mikizha
(anadromous form is called steelhead) |
| Redband trout of northern inland basins |
|
O. mykiss gairdneri |
Columbia redband trout of the Columbia and
Fraser River Basins east of the Cascades,
including Kamloops trout (anadromous form is
called steelhead) |
| Redband trout of eastern Oregon basins |
|
O. mykiss newberrii |
Upper Klamath redband trout (including Upper
Klamath Lake) |
(no name given) |
Oregon desert basin redband trout (other than
Upper Klamath Lake) |
| Redband trout of the Sacramento Basin |
|
O. mykiss aguabonita |
California golden trout |
O. mykiss gilberti |
Kern and Little Kern River golden trout |
O. mykiss stonei |
Sacramento redband trout (McCloud River
subspecies) |
Two major genetic groups of O. mykiss are presently
recognized: the inland and
coastal groups, separated by the Cascade crest (Huzyk and Tsuyuki
1974, Allendorf 1975,
Utter and Allendorf 1977, Okazaki 1984, Parkinson 1984, Schreck et
al. 1986, Reisenbichler
et al. 1992). Both inland and coastal steelhead occur in British
Columbia, Washington, and
Oregon; Idaho has only inland steelhead; California has only coastal
steelhead. Based on
Schreck et al. (1986), the demarcation between coastal and inland
steelhead in the Columbia
River Basin occurs between the Hood River and Fifteenmile Creek in
Oregon and between the
Klickitat River and Rock Creek in Washington (i.e., in the vicinity
of The Dalles Dam).
These genetic groups apply to both anadromous and nonanadromous forms
of O. mykiss; that
is, rainbow (redband) trout east of the Cascades are genetically more
similar to steelhead from
east of the Cascades than they are to rainbow trout west of the
Cascades. Behnke's (1992)
terminology for subspecies of O. mykiss reflects this genetic
difference. Coastal rainbow trout
of North America, as well as coastal steelhead, are placed in the
subspecies O. m. irideus
(Behnke 1992). The inland Columbia redband trout of the Columbia and
Fraser River Basins,
as well as inland steelhead, are placed in the subspecies O. m.
gairdneri (Behnke 1992).
Illinois River winter steelhead fall within the coastal group,
subspecies O. m. irideus.
Present distribution of coastal steelhead in Washington, Oregon,
and California extends
from the U.S.-Canada border south to Malibu Creek, California.
Within this distribution are
two major life-history types: summer-run (summer steelhead) and
winter-run (winter
steelhead). These run-types are primarily differentiated by time and
duration of spawning
migration and state of sexual maturity at the time of river entry.
Summer steelhead enter fresh
water between May and October, in a sexually immature condition.
After several months in
fresh water, summer steelhead mature and spawn. Winter steelhead
enter fresh water between
November and April with well-developed gonads and spawn shortly
thereafter. Both summer
and winter steelhead are found in some drainages, including the Rogue
River Basin of
southwest Oregon. The Illinois River is generally considered to have
only winter-run
steelhead.
Scope of Present Status Review
The environmental and biological information developed in
the Illinois River winter
steelhead status review (Busby et al. 1993) indicated that the ESU
that contains that population
might extend somewhat north of the Rogue River Basin and south into
northern California.
Therefore, the present status review concentrates on environmental
and biological information
for that geographic area.
KEY QUESTIONS IN ESA EVALUATIONS
Two key questions must be addressed in determining whether a
listing under the ESA is
warranted:
1) Is the entity in question a "species" as defined
by the ESA?
2) If so, is the "species" threatened or endangered?
The "Species" Question
As amended in 1978, the ESA allows listing of "distinct
population segments" of
vertebrates as well as named species and subspecies. However, the
ESA provided no specific
guidance for determining what constitutes a distinct population, and
the resulting ambiguity led
to the use of a variety of criteria in listing decisions over the
past decade. To clarify the issue
for Pacific salmon, NMFS published a policy describing how the agency
will apply the
definition of "species" in the ESA to anadromous salmonid species,
including sea-run cutthroat
trout and steelhead (NMFS 1991). A more detailed description of this
topic appeared in the
NMFS "Definition of Species" paper (Waples 1991). The NMFS policy
stipulates that a
salmon population (or group of populations) will be considered
"distinct" for purposes of the
ESA if it represents an evolutionarily significant unit (ESU) of the
biological species. An ESU
is defined as a population that 1) is substantially reproductively
isolated from conspecific
populations and 2) represents an important component in the
evolutionary legacy of the
species. Information that can be useful in determining the degree of
reproductive isolation
includes incidence of straying, rates of recolonization, degree of
genetic differentiation, and
the existence of barriers to migration. Insight into evolutionary
significance can be provided
by data on genetic and life-history characteristics, habitat
differences, and the effects of stock
transfers or supplementation efforts.
Hatchery Fish and Natural Fish
Because artificial propagation of Pacific salmonids has been
widespread for many
years, the influence of hatchery fish needs to be considered in most
ESA status reviews.
NMFS policy stipulates that in determining whether a population is
distinct for purposes of the
ESA, attention should focus on "natural" fish, which are defined as
the progeny of naturally
spawning fish (Waples 1991). This approach directs attention to fish
that spend their entire
life cycle in natural habitat and is consistent with the mandate of
the ESA to conserve
threatened and endangered species in their native ecosystems.
Implicit in this approach is the
recognition that fish hatcheries are not a substitute for natural
ecosystems.
The decision to focus on natural fish is based entirely on
ecosystem considerations; the
question of the relative merits of hatchery vs. natural fish is a
separate issue. Fish are not
excluded from ESA consideration simply because some of their direct
ancestors may have
spent time in a fish hatchery, nor does identifying a group of fish
as "natural" as defined here
automatically mean that they are part of a listed ESU. For a
discussion of artificial
propagation of Pacific salmon under the ESA, see Hard et al. (1992).
Thresholds for Threatened or Endangered Status
The ESA (sec. 3) defines the term "endangered species" as
"any species which is in
danger of extinction throughout all or a significant portion of its
range." The term "threatened
species" is defined as "any species which is likely to become an
endangered species within the
foreseeable future throughout all or a significant portion of its
range." Neither NMFS nor the
U.S. Fish and Wildlife Service (USFWS), which share authority for
administering the ESA,
has an official policy interpreting these definitions in terms of
thresholds for considering ESA
"species" as threatened or endangered. An information document on
this topic (Thompson
1991) published by NMFS suggests that conventional rules of thumb,
analytical approaches,
and simulations may all be useful in making this determination.
There is considerable interest
in incorporating the concepts of population viability analysis (PVA)
into ESA threshold
considerations for Pacific salmon. However, available PVA models
generally require
substantial life-history information that is not available for most
Pacific salmon populations, so
quantitative PVA is not practical at this time.
Therefore, NMFS considers a variety of information in evaluating
the level of risk
faced by an ESU. Important factors include 1) absolute numbers of
fish and their spatial and
temporal distribution; 2) current abundance in relation to historical
abundance and carrying
capacity of the habitat; 3) trends in abundance, based on indices
such as dam or redd counts or
on estimates of spawner-recruit ratios; 4) natural and
human-influenced factors that cause
variability in survival and abundance; 5) possible threats to genetic
integrity (e.g., selective
fisheries and interactions between hatchery and natural fish); and 6)
recent events (e.g., a
drought or a change in management) that have predictable short-term
consequences for
abundance of the ESU.
In evaluating these factors, the role of artificial propagation
is an important issue.
Because of the ESA's emphasis on conserving species in their native
ecosystems, threshold
determinations must focus on the status of natural fish.
Artificial production may have direct or indirect impacts on the
status of a population
through direct supplementation of numbers, by altering the genetic
composition of the
population, or through ecological interactions (competition,
predation, disease transmission,
etc.) between artificially-produced and natural fish. A mixture of
artificially-produced and
natural fish in a population also makes assessment of the natural
fish difficult: abundance and
viability of the natural stock is difficult to estimate unless
artificially-produced fish are clearly
marked, and trends in the natural stock can be obscured by the
infusion of artificially-produced
fish and their progeny into the natural population. An important
question for many natural
populations is the following: Is natural production sufficient to
maintain the population
without the constant infusion of artificially-produced fish?
According to the ESA, the determination whether a species is
threatened or endangered
should be made on the basis of the best scientific information
available regarding its current
status, after taking into consideration conservation measures that
are proposed or are in place.
In this review, we did not evaluate likely or possible effects of
conservation measures.
Therefore, we do not make recommendations as to whether identified
ESUs should be listed as
threatened or endangered species, because that determination requires
evaluation of factors not
considered by us. Rather, we have drawn scientific conclusions about
the risk of extinction
faced by identified ESUs under the assumption that present conditions
will continue
(recognizing, of course, that natural demographic and environmental
variability is an inherent
feature of "present conditions").
SUMMARY OF INFORMATION RELATING TO THE SPECIES
QUESTION
In this section, we summarize biological and environmental
information that is relevant
to determining the nature and extent of the ESU that includes
Illinois River winter steelhead.
Information presented in this section forms the basis for conclusions
regarding the species and
threshold questions, which are addressed in the following section.
Environmental Features
Ecoregions and Zoogeography
Many efforts have been made to describe and classify the
distributions of the Earth's
plant and animal species and their habitats. These have resulted in
the development of several
classification schemes for continental and marine environments.
Bailey (1976, 1980), Omernik and Gallant (1986), and Omernik
(1987) used geologic,
edaphic, climatic, and vegetational patterns to delineate ecoregions
for the United States.
Franklin and Dyrness (1973) used similar information to delineate
physiographic provinces for
Oregon and Washington. Most recently, the Forest Ecosystem
Management Assessment Team
(FEMAT) delineated aquatic and terrestrial physiographic provinces
for the northern spotted
owl region (USFS and BLM 1994).
Zoogeographic regions and provinces in the marine environment
have been described
using climate and faunal distribution. Allen and Smith (1988)
discussed the marine
zoogeographic literature and presented zoogeographic regions and
provinces and marine life
zones for the North Pacific Ocean and Bering Sea. Within the
Oregonian Province, Allen and
Smith (1988, p. 144) found that Cape Mendocino, California "is an
important southern limit
(at least in abundance) of many northern species" of marine fish.
Ichthyogeography--Snyder (1907) may have been the first
to attempt to categorize the
freshwater ichthyofauna of Oregon and California into geographic
groups. Based on
phenotypic characteristics and assemblages of fluvial fish species,
Snyder (1907) identified
three ichthyofaunal groups for the coastal area between the Columbia
and Sacramento Rivers:
Columbia River Fauna, Klamath River Fauna, and Sacramento River
Fauna. The Columbia
River faunal group included basins north of the Rogue River. The
Klamath River faunal group
included the Rogue and Klamath River Basins. The Sacramento River
faunal group included
rivers south of the Klamath Basin.
Moyle (1976) discussed the zoogeography of inland fish of
California. He noted that
the Klamath smallscale sucker (Catostomus rimiculus) is common to
both the Rogue and
Klamath Rivers, yet the Rogue River "lacks other Klamath fishes, such
as speckled dace
[Rhinichthys osculus] and marbled sculpin [Cottus klamathensis], and
contains reticulate
sculpin [C. perplexus], a species abundant in coastal streams further
north" (Moyle 1976,
p. 15). Moyle considered coastal streams of northern California
(south of the Klamath River)
to be part of the Sacramento River inland fish fauna, as did Snyder
(1907). Moyle specifically
noted that "the Mad, Eel, Bear, Navarro, Gualala, and Russian rivers,
as well as three
tributaries to Tomales Bay (Walker, Papermill, and Olema creeks) all
contain freshwater fishes
derived from the Sacramento-San Joaquin River system" (p. 17).
Hughes et al. (1987) described ichthyogeographic regions in
Oregon based on the
distribution of 68 native fish species. They found a similarity
between these
ichthyogeographic regions and the physiographic provinces of Franklin
and Dyrness (1973)
and the aquatic ecoregions of Omernik (1987). Hughes et al.
concluded that seven
ichthyogeographic regions could be delineated in Oregon. Coastal
steelhead are known from
all of these regions except the Blue Mountains region, which contains
inland steelhead, and the
Endorheic Lakes region of eastern Oregon, which contains redband
trout.
Physiographic and zoogeographic classification of southwest
Oregon--With respect
to the above, the Illinois River (Rogue River Basin) of southwest
Oregon falls within the Coast
Range and Sierra Nevada ecoregions of Omernik (1987) and the Klamath
Mountains
physiographic region of Franklin and Dyrness (1973) and FEMAT (USFS
and BLM 1994).
The Rogue River Basin is within the Rogue River/Sierra Nevada
ichthyogeographic region of
Hughes et al. (1987) and the Klamath/Siskiyou and Franciscan aquatic
ecosystems
physiographic provinces of FEMAT (USFS and BLM 1994). The Rogue
River Basin shares
some inland ichthyofaunal commonalities with the Klamath River to the
south, as well as with
some streams to the north (Snyder 1907, Moyle 1976, Behnke 1992).
Each of the studies
cited above has consistently delineated an ecoregion, that includes
the Illinois River, that
largely corresponds with a geological feature known as the Klamath
Mountains Geological
Province.
Klamath Mountains Geological Province
The Klamath Mountains Geological Province includes a complex
of mountain ranges in
southwest Oregon and northwest California (Fig.
2). Collectively,
these are called the
Klamath Mountains; they include the Trinity Alps, Salmon Mountains,
Marble Mountains, and
Siskiyou Mountains (Wallace 1983). Ecologically, the region is
classified in the Marine
Division of the Humid Temperate Domain (Bailey 1980); however, it
exhibits influence from
the warmer, drier Mediterranean Division (Atzet
- footnote 1). This region
includes diverse localized
climates including cool, wet coastal areas and hot, dry interior
valleys that receive less
precipitation than any other location in the Pacific Northwest west
of the Cascade Range
(Franklin and Dyrness 1973). For example, average annual
precipitation in the interior Rogue
River valley ranges between 30 and 94 cm (Oregon Water Resources
Committee 1955), while
at Cave Junction in the middle Illinois Valley it is 152 cm, and Gold
Beach at the mouth of the
Rogue River receives 229 cm (USFS 1989).
The Siskiyou Mountains include northern extensions of geological
formations typical of
those found in the California Coast Ranges and the Sierra Nevada
(Franklin and Dyrness
1973). The unusual geology and climate result in vegetation which
"combines elements of the
California, north coast, and eastern Oregon floras, with a large
number of species indigenous
only to the Klamath Mountains region" (Franklin and Dyrness 1973, p.
130).
Figure 2. Map of the southern Oregon-northern California area.
Shaded area is the Klamath
Mountains Geologic Province (modified from Irwin 1966 and Walker and
MacLeod 1991).
California Current System
The rivers and streams of California, Oregon, and Washington
drain into the California
Current system of the eastern North Pacific Ocean. This current
system includes the California Current, the California Undercurrent,
and the Davidson Current (Hickey 1989).
The prevailing California Current has a southerly flow. The
California Undercurrent (or
Countercurrent) flows northward on the continental slope. Both the
California Current and
California Undercurrent are strongest in summer (Hickey 1989). The
Davidson Current is a
north-flowing seasonal surface current, existing in winter and early
spring (Doyle 1992).
Upwelling--Upwelling occurs when warm, nutrient poor
surface waters are driven
offshore by wind stress and are replaced by cold, nutrient-rich
subsurface water. On the
Oregon coast, prevailing winds from September through March are from
the southwest. At
the end of March, wind direction shifts, typically coming from the
northwest, and Ekman
transport causes surface water to move offshore and deep water moves
up to replace it
(Ingmanson and Wallace 1973, Neshyba 1987). These wind patterns
result in summer
upwelling and winter downwelling.
The strength and consistency of upwelling south of Cape Blanco
yields highly
productive waters. This is demonstrated in satellite imagery,
described by Pearcy (1992), that
shows chlorophyll-rich water extending several hundred kilometers
offshore south of Cape
Blanco to approximately 33�N latitude (Thomas and Strub 1989).
In-Stream Water Temperature
Data from the U.S. Geological Survey for coastal streams in
Oregon and northern
California demonstrate that with decreasing latitude there is an
upward trend in water
temperature (Table 2). There appears to be a geographical
break-point in the average high
water temperature south of the Mad River, California.
Table 2. Average daily high water-temperatures (°C), by month,
recorded by U.S. Geological
Survey for selected coastal streams
in Oregon and California. Streams are listed from north to
south (Hydrosphere 1993).
|
Stream |
Recording
location |
Month
(January-June) |
Month
(July-December) |
Year* |
Years of
record |
Number of
years
recorded |
| Oregon |
| Siuslaw River |
Mapleton |
Jan: 6.7
Feb: 7.5
Mar: 8.9
Apr: 10.5
May:14.8
Jun: 18.8 |
Jul: 22.5
Aug: 22.6
Sep: 18.7
Oct: 13.5
Nov: 9.1
Dec: 7.5 |
13.4 |
1970-82 |
11 |
| Umpqua River |
Elkton |
Jan: 5.9
Feb: 6.9
Mar: 9.0
Apr: 11.6
May: 15.6
Jun: 19.9 |
Jul: 23.7
Aug: 23.4
Sep: 19.9
Oct: 14.4
Nov: 9.1
Dec: 6.3 |
13.8 |
1971-91 |
21 |
| Rogue River |
Agness |
Jan: 5.9
Feb: 7.2
Mar: 8.8
Apr: 11.2
May: 15.1
Jun: 19.4 |
Jul: 22.7
Aug: 22.3
Sep: 18.7
Oct: 13.5
Nov: 9.0
Dec: 6.7 |
13.4 |
1961-88 |
28 |
| California |
| Smith River |
Crescent City |
Jan: 7.5
Feb: 8.2
Mar: 9.0
Apr: 10.6
May: 13.7
Jun: 17.6 |
Jul: 20.4
Aug: 20.4
Sep: 17.9
Oct: 13.6
Nov: 10.2
Dec: 8.0 |
13.1 |
1966-81 |
16 |
| Klamath River |
Klamath |
Jan: 6.8
Feb: 7.7
Mar: 9.0
Apr: 11.5
May: 14.4
Jun: 18.6 |
Jul: 22.4
Aug: 22.2
Sep: 20.1
Oct: 16.2
Nov: 11.4
Dec: 7.5 |
14.0 |
1966-81 |
16 |
| Redwood Creek |
Orick |
Jan: 7.5
Feb: 8.8
Mar: 9.6
Apr: 11.6
May: 15.8
Jun: 20.5 |
Jul: 22.2
Aug: 21.4
Sep: 19.6
Oct: 14.9
Nov: 9.9
Dec: 8.1 |
14.2 |
1966-79 |
14 |
| Mad River |
Arcata |
Jan: 7.8
Feb: 9.3
Mar: 10.2
Apr: 12.6
May: 16.2
Jun: 18.8 |
Jul: 21.5
Aug: 21.2
Sep: 19.6
Oct: 16.0
Nov: 11.6
Dec: 8.7 |
14.5 |
1962-79 |
16 |
| Eel River |
Scotia |
Jan: 8.2
Feb: 9.0
Mar: 10.5
Apr: 12.8
May:16.8
Jun: 19.9 |
Jul: 21.7
Aug: 21.6
Sep: 20.5
Oct: 16.9
Nov: 12.5
Dec: 9.1 |
15.0 |
1962-82 |
20 |
| Mattole River |
Petrolia |
Jan: 8.7
Feb: 10.0
Mar: 11.7
Apr: 14.0
May: 17.8
Jun: 22.1 |
Jul: 23.4
Aug: 22.5
Sep: 21.1
Oct: 16.7
Nov: 12.3
Dec: 9.9 |
15.9 |
1966-79 |
13 |
| Noyo River |
Fort Bragg |
Jan: 8.5
Feb: 9.6
Mar: 10.8
Apr: 12.8
May: 16.4
Jun: 18.8 |
Jul: 20.3
Aug: 19.8
Sep: 18.3
Oct: 14.2
Nov: 11.2
Dec: 8.9 |
14.1 |
1966-79 |
14 |
| Navarro River |
Navarro |
Jan: 8.7
Feb: 10.1
Mar: 11.7
Apr: 13.8
May: 18.4
Jun: 21.5 |
Jul: 22.4
Aug: 21.7
Sep: 20.0
Oct: 16.2
Nov: 11.9
Dec: 8.9 |
15.4 |
1966-79 |
13 |
| Garcia River |
Point Arena |
Jan: 10.6
Feb: 11.4
Mar: 12.6
Apr: 14.5
May: 17.1
Jun: 18.7 |
Jul: 19.6
Aug: 19.6
Sep: 18.8
Oct: 16.7
Nov: 13.6
Dec: 11.2 |
15.4 |
1964-79 |
16 |
| Russian River |
Guerneville |
Jan: 9.9
Feb: 11.6
Mar: 13.3
Apr: 16.2
May: 20.3
Jun: 23.6 |
Jul: 25.6
Aug: 24.6
Sep: 22.3
Oct: 18.2
Nov: 13.6
Dec: 10.3 |
17.5 |
1964-82 |
19 |
*The average of the monthly values.
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