On 14 March 1994, the National Marine Fisheries Service (NMFS) was petitioned by the Professional Resources Organization-Salmon (PRO-Salmon) to list spring-run populations of chinook salmon (Oncorhynchus tshawytscha) in the North Fork and South Fork Nooksack River, the Dungeness River1, and the White River (Fig. 1) as threatened or endangered species under the Endangered Species Act (ESA) either singly, or in some combination (PRO-Salmon 1994). At about the same time, NMFS also received petitions to list additional populations of other Pacific salmon species in the Puget Sound area. In response to these petitions and the more general concerns for the status of Pacific salmon throughout the region, NMFS announced on 12 September 1994 that it would initiate ESA status reviews for all species of anadromous salmonids in Washington, Oregon, California, and Idaho (NMFS 1994d). This proactive approach was intended to facilitate more timely, consistent, and comprehensive evaluations of the ESA status of Pacific salmonids than would be possible through a long series of reviews of individual populations. Subsequent to this announcement, NMFS was petitioned on 1 February 1995 by the Oregon Natural Resources Council (ONRC) and Siskiyou Project Staff Ecologist Dr. Richard K. Nawa to list 197 stocks of chinook salmon either separately or in some combination.
This document reports results of the comprehensive ESA status review of chinook salmon from Washington, Oregon, California, and Idaho. To provide a context for evaluating these populations of chinook salmon, biological and ecological information for chinook salmon in British Columbia, Alaska, and Asia were also considered. This review thus encompasses, but is not restricted to, the populations identified in the PRO-Salmon and ONRC-Nawa petitions.
Because the ESA stipulates that listing determinations should be made on the basis of the best scientific information available, NMFS formed a team of scientists with diverse backgrounds in salmon biology to conduct this review. This Biological Review Team (BRT) for chinook salmon included: Peggy Busby, Dr. Stewart Grant, Dr. Robert Iwamoto, Dr. Robert Kope, Dr. Conrad Mahnken, Gene Matthews, Dr. James Myers, Philip Roni, Dr. Michael Schiewe, David Teel, Dr. Thomas Wainwright, F. William Waknitz, Dr. Robin Waples, and Dr. John Williams of NMFS Northwest Fisheries Science Center; Gregory Bryant and Craig Wingert of NMFS Southwest Region; Dr. Steve Lindley and Dr. Peter Adams from NMFS Southwest Region (Tiburon Laboratory); Alex Wertheimer of NMFS Alaska Fisheries Science Center (Auke Bay Laboratory); and Dr. Reg Reisenbichler from the USGS Biological Resource Division. NMFS received scientific and technical information from Pacific Salmon Biological and Technical Committees (PSBTCs) convened in Washington, Oregon, and California. Meetings of the PSBTC were not held in Idaho because all chinook salmon populations in Idaho are already listed under the ESA. The BRT discussed and evaluated scientific information gathered at the PSBTC meetings, and also reviewed information submitted to the ESA administrative record for chinook salmon, including specific comments by co-managing agencies on a draft version of this document (CDFG 1997b, HVTC 1997, IDFG 1997, LIBC 1997, NWIFC 1997a, ODFW 1997a, and WDFW 1997a, YTFP 1997a).
In determining whether a listing under the ESA is warranted, two key questions must be addressed:
These two questions are addressed in separate sections of this report. If it is determined that a listing(s) is warranted, then NMFS is required by law (1973 ESA Sec. 4(a)(1)) to identify one or more of the following factors responsible for the species' threatened or endangered status: 1) destruction or modification of habitat, 2) overutilization by humans, 3) disease or predation, 4) inadequacy of existing regulatory mechanisms, or 5) other natural or human factors. This status review does not formally address factors for decline; except insofar as they provide information about the degree of risk faced by the species in the future if current conditions continue. A separate document identifies factors for decline of chinook salmon from Washington, Oregon, California, and Idaho, and is presented subsequent to any proposed listing recommendation.
As amended in 1978, the ESA allows listing of "distinct population segments" of vertebrates as well as named species and subspecies. However, the ESA provides no specific guidance for determining what constitutes a distinct population, and the resulting ambiguity has 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 document 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 discussion of this topic appeared in the NMFS "Definition of Species" paper (Waples 1991b). 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 of the evolutionary legacy of the species.
The term "evolutionary legacy" is used in the sense of "inheritance," that is, something received from the past and carried forward into the future. Specifically, the evolutionary legacy of a species is the genetic variability that is a product of past evolutionary events and that represents the reservoir upon which future evolutionary potential depends. Conservation of these genetic resources should help to ensure that the dynamic process of evolution will not be unduly constrained in the future.
The NMFS policy identifies a number of types of evidence that should be considered in the species determination. For each of the criteria, the NMFS policy advocates a holistic approach that considers all types of available information as well as their strengths and limitations. Isolation does not have to be absolute, but it must be strong enough to permit evolutionarily important differences to accrue in different population units. Important types of information to consider include natural rates of straying and recolonization, evaluations of the efficacy of natural barriers, and measurements of genetic differences between populations. Data from protein electrophoresis or deoxyribonucleic acid (DNA) analyses can be particularly useful for this criterion because they reflect levels of gene flow that have occurred over evolutionary time scales.
The key question with respect to the second ESU criterion is, if the population became extinct, would this represent a significant loss to the ecological/genetic diversity of the species? Again, a variety of types of information should be considered. Phenotypic and life-history traits such as size, fecundity, migration patterns, and age and time of spawning may reflect local adaptations of evolutionary importance, but interpretation of these traits is complicated by their sensitivity to environmental conditions. Data from protein electrophoresis or DNA analyses provide valuable insight into the process of genetic differentiation among populations but little direct information regarding the extent of adaptive genetic differences. Habitat differences suggest the possibility for local adaptations but do not prove that such adaptations exist.
On 7 November 1985, NMFS received a petition from the American Fisheries Society (AFS) to list the winter-run chinook salmon in the Sacramento River as a threatened species under the federal ESA. NMFS initially announced its decision not to list this population as threatened or endangered on 27 February 1987 (NMFS 1987). Subsequently, the winter-run chinook salmon population experienced a further decline, and an emergency listing to list the population as threatened was made on 4 August 1989 (NMFS 1989); the listing was extended on 2 April 1990 (NMFS 1990a). A final rule to list the Sacramento River winter-run chinook salmon as threatened was made on 5 November 1990 (NMFS 1990b). The winter run continued to decline and was subsequently listed as endangered 4 January 1994 (NMFS 1994b).
On 7 June 1990, NMFS received a petition from Oregon Trout and five co-petitioners to list Snake River spring-run chinook salmon, Snake River summer-run chinook salmon, and Snake River fall-run chinook salmon under the ESA. A final rule was announced on 22 April 1992 (NMFS 1992), which determined that Snake River chinook salmon should be listed as threatened under the ESA. Furthermore, it was determined that the spring- and summer-run populations collectively constituted a separate ESU from the fall-run chinook salmon under the ESA. As a result of record low adult returns in 1994 and projected returns for 1995, an emergency interim rule was announced 18 August 1994 to reclassify the Snake River spring/summer run and Snake River fall run as endangered (NMFS 1994c); however, both Snake River chinook salmon ESUs were subsequently classified (17 April 1995) in a final ruling as being threatened (NMFS 1995a).
A petition for the listing of summer-run chinook salmon in the mid-Columbia River2 was submitted to NMFS on 3 June 1993, by the American Rivers and ten co-petitioners. On 23 September 1994, NMFS determined that the mid-Columbia River summer-run chinook salmon stocks petitioned did not constitute an ESU, but belonged to a larger fall- and summer-run chinook salmon ESU located along the mainstem Columbia River between the Chief Joseph and McNary Dams (NMFS 1994a). NMFS concluded that this ESU did not warrant a listing of endangered or threatened.
This section briefly summarizes information presented by the petitioners (Professional Resources Organization (PRO)-Salmon 1994, Oregon National Resources Council (ONRC) and Nawa 1995) to support their arguments that specific chinook salmon stocks in Washington, Oregon, Idaho, and California qualify as threatened or endangered species under the ESA. Previous ESA petitions for chinook salmon under the ESA have been evaluated and summarized in elsewhere (NMFS 1987, Matthews and Waples 1991, Waples et al. 1991b, Waknitz et al. 1995).
The PRO-Salmon (1994) petition requested that NMFS evaluate four stocks of chinook salmon in Washington state for listing as threatened or endangered under the ESA: the North Fork Nooksack River spring run, South Fork Nooksack River spring run, Dungeness River spring run, and White River spring run. The petitioners presented several alternative groupings of these stocks into one or more ESUs, which might also include stocks not specifically mentioned in their petition. The ONRC and Nawa (1995) petition listed 197 "stocks" in Washington, Oregon, California, and Idaho to be considered for listing as threatened or endangered, either separately or in one or more ESUs. The authors specifically included non-native stocks, such as Clearwater River spring-run chinook salmon, which contains components of other spring-run stocks from the Snake River spring- and summer-run ESU. They argued that if an ESU that contains the original components of a mixed stock is identified and listed as threatened or endangered, then the mixed stock should be included in the ESU.
ONRC and Nawa suggested several alternative scenarios for chinook salmon, specifically, to list:
Both the PRO-Salmon (1994) and ONRC and Nawa (1995) petitions cited extensive information to document the decline of specific chinook salmon stocks. PRO-Salmon (1994) cited the work of Nehlsen et al. (1991), who considered the four stocks of chinook salmon in the petition to be at a high or moderate risk of extinction, and WDF et al. (1993), who identified the status of the four stocks as "critical," based on "chronically low" escapement or redd counts. The spring run on the White River had declined from 5,432 in 1942 to a low of 66 in 1977, and return numbers have averaged less than 200 fish from 1978-91 (PRO-Salmon 1994). Escapement estimates for the North Fork Nooksack River spring run are less accurate because of unfavorable river conditions for sampling. Spawner/redd surveys nevertheless indicate a considerable decrease in stock size.
ONRC and Nawa (1995) surveyed and categorized 417 stocks of chinook salmon, of which they considered 67 (16.1%) to be extinct, 21 (5.0%) nearly extinct, 95 (22.8%) declining, 75 (18.0%) composite production [in which the hatchery contribution exceeds natural production], and a further 37 (8.9%) of unknown status. Using information from a number of sources, the petitioners presented overall and regional estimates of the decline of chinook salmon stocks. Nehlsen et al. (1991) listed 64 stocks of chinook salmon that they determined to be at a high or moderate risk of extinction or of special concern. WDF et al. (1993) determined the status of 40 of the 108 (37.0%) chinook salmon stocks in Washington State to be "critical" or "depressed." The Wilderness Society (1993) reported that 63% of spring- and summer-run chinook salmon stocks in Washington, Oregon, California, and Idaho were considered to be extinct, with a further 24% being endangered or threatened. Similarly, among fall chinook salmon stocks, 19% were extinct, and 25% endangered or threatened.
On a regional basis, the Central Valley of California had the highest percentage of extinct stocks (40%), with only one wild stock classified as not declining according to ONRC and Nawa (1995). Stocks within the coastal basins south of Cape Blanco, Oregon had also experienced a similar decrease in abundance, with 67% of the stocks classified as extinct, nearly extinct, or declining. Within the Columbia River Basin, chinook salmon stocks below McNary Dam (River Kilometer [RKm] 470) have been heavily influenced by artificial propagation, and only six wild stocks were identified that were not declining. According to ONRC and Nawa, the Columbia River chinook salmon stocks above McNary Dam have experienced the second highest level of extinction (28%), with 44% of the stocks being classified as declining. In the Snake River, the petitioners identified 13 stocks (28%) as being extinct and 22 stocks (47%) to be in decline. No wild stocks were found that were not declining. Among chinook salmon stocks in Puget Sound, 50% of the spring-run stocks were extinct. Only coastal stocks north of Cape Blanco, Oregon were not found to be seriously declining. ONRC and Nawa (1995) presented individual stock historical abundance information for many of the 417 stocks surveyed. This information further documented many of the regional declines noted above.
The petitioners identified several factors which they believe have either singly or in combination resulted in the chinook salmon stock declines in abundance described above. Because the petitions cover such a wide geographic area, encompassing several terrestrial and marine ecological regions, and because the populations surveyed have been impacted by varying anthropogenic factors, only a very generalized review of this topic will be given.
PRO-Salmon (1994) and ONRC and Nawa (1995) both cited references indicating that habitat degradation is the major cause for the decline in the petitioned chinook salmon stocks. The influence of dams3 was most commonly implicated by ONRC and Nawa (1995) as being responsible for the decline or extinction of chinook salmon stocks. Of the stock extinctions surveyed in the coastwide region, 76% were dam related. This was most noticeable in the Central Valley, California where 100% of the extinctions surveyed were dam related (Campbell and Moyle 1990). Furthermore, 48 of the spring- and summer-run stocks found to be in decline were affected by dams. Two of the four chinook salmon stocks petitioned by PRO-Salmon (1994) were impacted to some extent by dam operation, but logging4 and agricultural land use/water diversion (including diking) also figured as major factors in all four stocks. The Nooksack Technical Group (1987) indicated that sedimentation from logging activities had seriously impacted the quality of the spawning habitats in both the North and South Forks of the Nooksack River. PRO-Salmon (1994) considered water diversion for agricultural use to be a major contributor to the decline of the Dungeness River spring run. Overall, ONRC and Nawa (1995) estimated that logging was responsible, in part, for 60% of the declines and 6% of the extinctions among the stocks surveyed. Similarly, agriculture, water withdrawal, mining and urbanization factors were implicated in 58% of the declines and 9% of the extinctions among the 417 stocks surveyed. Both petitioners also presented evidence that the exploitation rates on the stocks were sufficiently high to have seriously depleted stocks or been partially responsible for the extinction of stocks (Dosewallips, Duckabush, and Mokelumne Rivers spring-run chinook salmon (ONRC and Nawa 1995)).
The other major concern of the petitioners was the impact of introduced and/or artificially propagated fish on indigenous stocks. Potentially deleterious impacts of artificial propagation presented by ONRC and Nawa (1995) include: interbreeding of fall and spring runs in California due to habitat alterations (Campbell and Moyle 1990), interspecies hybridization between chinook and coho salmon (Oncorhynchus kisutch Walbaum) (Bartley et al. 1990), competition between hatchery and native stocks, interbreeding between hatchery and native chinook salmon stocks, disease introductions by artificially propagated fish, and the unsustainability of hatchery stocks in general. Finally, ONRC and Nawa (1995) suggested the "inadequacy of existing regulatory mechanisms" was a general reason for the overall decline in abundance of chinook salmon.
In this section, we summarize biological and environmental information and consider the relevancy of each in determining the nature and extent of West Coast chinook salmon ESUs. ESU boundaries were determined by the BRT on the basis of the team's professional opinion of the degree to which environmental and biological attributes exhibited significant changes with respect to the reproductive isolation and ecological/genetic diversity of West Coast chinook salmon.
Chinook salmon, also commonly referred to as king, spring, quinnat, Sacramento, California, or tyee salmon, is the largest of the Pacific salmon (Netboy 1958). The species distribution historically ranged from the Ventura River in California to Point Hope, Alaska in North America, and in northeastern Asia from Hokkaido, Japan to the Anadyr River in Russia (Healey 1991). Additionally, chinook salmon have been reported in the Mackenzie River area of northern Canada (McPhail and Lindsey 1970). Of the Pacific salmon, chinook salmon exhibit arguably the most diverse and complex life history strategies Healey (1986) described 16 age categories for chinook salmon, 7 total ages with 3 possible freshwater ages. This level of complexity is roughly comparable to sockeye salmon (O. nerka), although sockeye salmon have a more extended freshwater residence period and utilize different freshwater habitats (Miller and Brannon 1982, Burgner 1991). Two generalized freshwater life-history types were initially described by Gilbert (1912): "stream-type" chinook salmon reside in freshwater for a year or more following emergence, whereas "ocean-type" chinook salmon migrate to the ocean within their first year. Healey (1983, 1991) has promoted the use of broader definitions for "ocean-type" and "stream-type" to describe two distinct races of chinook salmon. This racial approach incorporates life history traits, geographic distribution, and genetic differentiation and provides a valuable frame of reference for comparisons of chinook salmon populations. For this reason, the BRT has adopted the broader "racial" definitions of ocean- and stream-type for this review.
The generalized life history of Pacific salmon involves incubation, hatching, and emergence in freshwater, migration to the ocean, and subsequent initiation of maturation and return to freshwater for completion of maturation and spawning (Fig. 2). Juvenile rearing in freshwater can be minimal or extended. Additionally, some male chinook salmon mature in freshwater, thereby foregoing emigration to the ocean. The timing and duration of each of these stages is related to genetic and environmental determinants and their interactions to varying degrees. Salmon exhibit a high degree of variability in life-history traits; however, there is considerable debate as to what degree this variability is the result of local adaptation or the general plasticity of the salmonid genome (Ricker 1972, Healey 1991, Taylor 1991).
Several types of biological evidence were considered in evaluating the contribution of West Coast chinook salmon to ecological/genetic diversity of the biological species under the ESA. Life-history traits examined for naturally spawning chinook salmon populations included smolt size and outmigration timing, age and size at spawning, river-entry timing, spawn timing, fecundity, and ocean migration. These traits are believed to have both a genetic and environmental basis, and similarities among populations could indicate either a shared genetic heritage or similar responses to shared environmental conditions.
The analysis of life-history trait information is complicated by several factors. Data collected from different locations during different years are confounded by spatial and temporal environmental variability. This variability creates considerable "noise," which may be as large as differences between geographically distant populations, and may mask subtle regional patterns. High interannual variability also complicates the comparison of results from studies conducted during different time periods. For chinook salmon, for which a single broodyear may return from the ocean over a 5- or 6-year period, variations in ocean productivity due to events such as the 1983 El Niño (Johnson 1988b) may bias estimates of age distribution, age-size relationships, and/or age and size-related fecundity estimates. Furthermore, it may be difficult to distinguish between fish from different runs emigrating from, or returning to, the same river system. Direct comparisons of chinook salmon life-history traits between stocks under controlled conditions are limited in number, and the extent to which inference can be made to wild populations is uncertain.
A third confounding complication is that the expression of life-history traits may be altered by anthropogenic activities such as land-use practices (Hartman et al. 1984, Holtby 1987), harvest (Ricker 1981), or artificial propagation (Steward and Bjornn 1990, Flagg et al. 1995b). To help limit any bias introduced by artificial propagation, life-history trait comparisons in this status review have focused on naturally spawning populations. However, because of the widespread practice of off-station plants of hatchery-reared fry and smolts, many studies of naturally spawning populations may have inadvertently included first-generation hatchery fish or fish whose ancestors have been hatchery reared. Life-history trait information from hatchery populations was used only when insufficient information from naturally spawning populations was available, as in the case of ocean migration patterns. As with environmental variability, the effects of anthropogenic activities may confound the expression of life-history traits and are difficult to factor out.
Because of these potential sources of variability, we felt that statistical analyses of life-history trait variability would not be particularly informative. Instead, data were collected from as many sources as possible from each system to give some indication of the mean and range in character traits. Older data sets were especially sought to provide insight into chinook salmon population characteristics prior to the proliferation of hatchery programs, which have produced fish with relatively high juvenile survival and growth rates and modified saltwater entry dates.
The geologic events of the last 20,000 years have provided mechanisms for genetic isolation, colonization, and population interbreeding. In determining ESU boundaries it is useful to understand the factors that may have shaped present day chinook salmon population distributions. Much of the present distribution of aquatic and terrestrial species in western North America is a legacy of the volcanic, tectonic, and glacial forces that have shaped this region. Events such as headwater transfer or stream capture have altered the flow of major rivers and the aquatic species that inhabit them. The Cordilleran ice sheet was the last major glacial event to affect the distribution of chinook salmon. At its height some 10,000-15,000 years ago, vast areas of Southeast Alaska, British Columbia, Washington, and Idaho were covered with ice (McPhail and Lindsey 1970). This created a discontinuous distribution of chinook salmon stocks. Two major ice-free refugia existed: Beringia, composed of the Bering land bridge connecting Eastern Siberia and Western Alaska; and Cascadia, composed of the lands south of the mid-Columbia River drainage (McPhail and Lindsey 1970). An additional ice-free refuge existed on the coast of the Olympic Peninsula in the area of the Chehalis River. The drop in sea level during the glacial periods may have created minor refugia along the coast of Vancouver Island or the present-day Queen Charlotte Islands (McPhail and Lindsey 1986). As the ice sheet receded, the colonization of newly exposed freshwater habitat began from the two refugia.
Chinook salmon colonization during the postglacial period (approximately beginning 10,000 years ago) occurred through a number of possible pathways. Straying adults could invade coastal river systems, as could salmon that moved farther upriver to headwaters exposed by the receding glaciers. Ice dams and land expansion after the retreat of glacial ice sheets caused rivers to alter course and change watersheds. Watershed capture has resulted in the exchange of aquatic organisms between several major river systems. Parts of the present day Fraser River drainage flowed into the Columbia River via the Okanogan River and Shuswap Creek during the last deglaciation (McPhail and Lindsey 1986). Species that moved into the Upper Fraser River from the Columbia River also gained access to southeastern Alaskan coastal rivers. The Stikine, Skeena, and Nass Rivers at various times drained east into the Fraser River Basin relative to their current westerly flow to the Gulf of Alaska (McPhail and Lindsey 1986). Similarly, the Alsek River in Alaska, which also flows to the Gulf of Alaska, drained what is now part of the Yukon River headwaters (Lindsey and McPhail 1986). Presently, the headwaters of the Taku, Stikine, and Yukon Rivers lie within 50 miles of one another. Chinook salmon populations from Beringia also had access to the Mackenzie River in Canada during the deglaciation, which may explain recurring reports of chinook salmon in that river system (McPhail and Lindsey 1970).
The fidelity with which chinook salmon return to their natal stream implies a close association between a specific stock and its freshwater environment. The selective pressures of different freshwater environments may be responsible for differences in life-history strategies among stocks. Miller and Brannon (1982) hypothesized that local temperature regimes are the major factor influencing life-history traits. If the boundaries of distinct freshwater habitats coincide with differences in life histories it would suggest a certain degree of local adaptation. Therefore, identifying distinct freshwater, terrestrial, and climatic regions may be useful in identifying chinook salmon ESUs. The Environmental Protection Agency (EPA) has established a system of ecoregion designations based on soil content, topography, climate, potential vegetation, and land use (Omernik 1987). These ecoregions are similar to the physiographic provinces determined by the Pacific Northwest River Basins Commission (PNRBC 1969) for the Pacific Northwest. Historically, the distribution of chinook salmon in Washington, Oregon, California, and Idaho would have included 13 of the present day EPA ecoregions (Fig. 3). Similarly, there is a strong relationship between ecoregions and freshwater fish assemblages (Hughes et al. 1987). We have retained the ecoregion names and numbers used by Omernik (1987) and included physiographic information presented by PNRBC (1969), present day water use information (USGS 1993), river flow information (Hydrosphere Products, Inc. 1993), and climate data from the U.S. Department of Commerce (1968) into the appropriate ecoregion description (Omernik and Gallant 1986, Omernik 1987). Additional information for British Columbia (Environment Canada 1977, 1991) and Alaska (Hydrosphere Products, Inc. 1993) is included for comparative purposes. The following ecoregions are wholly or partially contained within the historical natural range of chinook salmon in Washington, Oregon, California, and Idaho.
Extending from the Olympic Peninsula through the Coast Range proper and down to the Klamath Mountains and the San Francisco Bay area, this region is influenced by medium to high rainfall levels due to the interaction between marine weather systems and the mountainous nature of the region. Topographically, the region averages about 500 m in elevation, with mountain tops under 1,200 m. These mountains are generally rugged with steep canyons. Between the ocean and the mountains lies a narrow coastal plain composed of sand, silt, and gravel. Tributary streams are short and have a steep gradient; therefore, surface runoff is rapid and water storage is relatively short term during periods of no recharge. These rivers are especially prone to low flows during times of drought. Regional rainfall averages 200-240 cm per year (Fig. 4), with generally lower levels along the southern Oregon coast. Average annual river flows for most rivers in this region are among the highest found on the West Coast when adjusted for watershed area (Fig. 5). River flows peak during winter rain storms common in December and January (Fig. 6). Snow melt adds to the surface runoff in the spring, providing a second flow peak, and there are long periods when the river flows are maintained at least 50% of peak flow (Fig. 7). During July or August there is usually no precipitation; this period may expand to 2 or 3 months every few years. River flows are correspondingly at their lowest (Fig. 8) and temperatures at their highest during August and September (Fig. 9). Oregon coastal rivers have the largest relative difference in minimum and maximum flows, where minimum flows are 2-5% of the maximum flows.
The region is heavily forested primarily with Sitka spruce, western hemlock, and western red cedar. Forest undergrowth is composed of numerous types of shrubs and herbaceous plants.
Situated between the Coast Range and Cascade Range Ecoregion, this region experiences reduced rainfalls (50-120 cm) from the rainshadow effect of the Coast Mountains. The area is generally flat with high hills (600 m) at the southern margin of the ecoregion. Soils are composed of alluvial and lacustrine deposits. These deposits are glacial in origin north of Centralia, Washington. This area tends to have large groundwater resources, with groundwater from the bordering mountain ranges helping sustain river flows during drought periods. Peak river flow varies from December to June depending on the contribution of snowpack to surface runoff for each river system. Rivers tend to have sustained flows (5 to 8 months of flows at 50% of the peak or more), and low flows are generally 10-20% or more of the peak flows.
Douglas fir represent the primary subclimax forest species, with other coniferous species (lodgepole, western white, and ponderosa pines) locally abundant. Prairie, swamp, and oak, birch, or alder woodlands are also common. The land is heavily forested, and wood-cutting activities (including road building, etc.) contribute to soil erosion, river siltation, and river flow and temperature alteration.
The region is heavily urbanized, and domestic and industrial wastes impact local water systems. Urban run-off and sewage treatment influence water quality west of the Cascade Mountains, with the exception of the Olympic Peninsula coastal and northern Puget Sound rivers. Glacial sediment also influences water quality, especially in the Skagit, North Fork Nooksack, Nisqually, and Puyallup/White River Basins.
Adjoining the southern border of the Puget Sound Lowland Ecoregion at the Lewis River, this region was not glacially influenced. A rainshadow effect, similar to the one influencing the Puget Sound Lowlands, limits rainfall to about 120 cm per year. River flows peak in December and January and are sustained for 6 or 7 months of the year. Low flows occur in August and September, although the volume is generally 20% of the peak flow.
Much of the land has been converted to agricultural use, with Douglas fir and Oregon white oak stands present in less-developed areas. Irrigation is commonly employed, and stream flows, especially in the southern portion of this region, can be significantly affected. Agricultural and livestock practices contribute to soil erosion and fertilizer/manure deposition into stream systems.
Water quality is impacted by agricultural and urban activities. Many water quality problems are exacerbated by low water flows and high temperatures during the summer. Pulp and paper mill discharges of dioxin into the Columbia and Willamette Rivers were cited as another water quality concern, although this situation has been much more serious in the past (USGS 1993).
This region is composed of the Cascade Range in Washington and Oregon and the Olympic Mountains in Washington state. Peaks above 3,000 m are distributed throughout the region. The crest of the Cascade Range (averaging 1,500 m) captures much of the ocean moisture moving eastward in addition to providing a biological barrier. Rainfalls can average 280 cm per year (up to 380 cm in the Olympic Mountains), much of which is in the form of heavy snowpack. Intensive rainstorms, those depositing more than 2.5 cm per hour, are rare. Rainfall is generally spread over the year with the majority occurring between October and March. Except where porous rock substrate exists, there is little capacity for long-term groundwater storage. In these porous rock areas, streams receive 75-95% of their average discharge as groundwater, and are able to maintain their flows during dry periods. Surface water flow originating in the Cascades and Olympic Mountains influences river flows throughout this region.
Currently the area is primarily forested with Douglas fir, noble fir, and Pacific silver fir (all subclimax species), whereas western hemlock and red cedar are common climax species. At higher elevations, these trees are replaced by Englemann spruce, whitebark pine, and mountain hemlock. Forest undergrowth tends to be dense on the western slopes of this region and rather sparse on the eastern slopes. Heavy rainfall, combined with woodcutting activities, has resulted in increased soil erosion.
To the south of the Cascades Ecoregion lies a similar mountainous ecoregion, comprised of portions of the Klamath, Sierra, Trinity, and Siskiyou Mountains. Annual rainfall varies considerably, from 40 cm to over 150 cm, depending on elevation and the degree of rainshadowing. Most of the rain comes in the winter months, with summers being hot and dry. Topographically, the region rises to over 2,000 m with an average elevation of 1,000 m. This region contains the headwaters for the Rogue, Klamath, and Sacramento Rivers. Peak flows occur in February, with low flows in August, September, or October. As a result of water diversion and impoundment activities, flows are now more evenly apportioned throughout the year. This has occurred primarily through irrigation/flood mitigation-related reductions in peak flows and less so through increased spillage during the historical time of minimum flows.
Douglas fir is the predominant tree species, but mixed coniferous-oak stands are common. Soils tend to be unstable, and timber harvest or livestock grazing can result in severe erosion. Hydraulic placer mining has had a considerable impact on stream quality and hillslope stability.
To the east and in the rainshadow of the Coastal Mountain range, the tablelands and hills of this region have generally low levels of annual rainfall (40-100 cm). Tributary rivers to the Sacramento and San Joaquin Rivers flow through this region. Vegetation is composed of California oaks and manzanita chaparral with extensive needlegrass steppe. Livestock grazing in the open woodlands is the predominant land use.
The Sacramento and San Joaquin Rivers are the key features of the Central California Valley Ecoregion. The broad flat lands that border the river naturally support needlegrass and marshgrasses, although much of the region has been extensively converted to agricultural use. The annual rainfall for the region is 40-80 cm. The Sacramento and San Joaquin Rivers peak in February with a 6-month period of high flows (>50% of peak flow). Low flows occur in September and October. Changes in the hydrology of tributaries and irrigation withdrawals from the mainstem rivers have drastically altered the flow characteristics of these rivers over the course of the last 100 years. An estimated 90% of the surface water withdrawals were used for irrigation in 1990 (USGS 1990). The maintenance of livestock and cultivation, irrigation, and chemical treatment of crop land has resulted in increases in fecal coliform, dissolved nitrate, nitrite, phosphorus, and sulfate concentration levels (USGS 1993). Industrial and mining runoff from sites, such as the copper mines near Spring Creek in the Sacramento River Basin, also impact water quality in the immediate area.
This ecoregion marks the transition between the high rainfall areas of the Cascades Ecoregion and the drier basin ecoregions to the east. The area receives 30 cm to 60 cm of rainfall per year. Streamflow is intermittent, especially during the summer dry season. Surface and groundwater contributes to flows in the Yakima, Deschutes, Klickitat, and White Salmon Rivers.
Ponderosa and lodgepole pine are common throughout the region, with little forest undergrowth. Soils tend to be volcanic, young, and highly erodible. Primary land uses are timber harvest and mixed grazing/timber areas. Agriculture is limited to valleys and irrigation is commonly employed.
This ecoregion is typified by irregular plains, tablelands, and high hills/low mountains. The plateau spans from the Cascade Mountains to the Blue Mountains in the south and southeast. Much of the basin is covered with glacial and alluvial deposits. The loose surface substrate is prone to erosion. There is little rainfall and the majority of the water discharge comes from the mountains that border the basin. Because tributaries to the mid- and upper Columbia River receive much of their water from snowmelt, peak river flows are in May and June, except for the Deschutes, John Day, and Umatilla Rivers, which peak in April. Peak flows are not as sustained as on the coast, generally lasting 2-3 months. Annual rainfalls of 20-60 cm support sagebrush and wheatlands. Most smaller streams are ephemeral, partially due to irrigation withdrawals (Omernik and Gallant 1986). The Columbia Plateau experiences a prolonged drought of 1 to 3 months every year, with longer events occurring frequently. Low river flows occur during the late summer and early fall, August-October, when irrigation demand is heavy. Nitrates, sulfites, and pesticides commonly associated with crop irrigation are found in most of the rivers in the Columbia River Basin. Heavy metal contamination from Canadian mining operations has been detected at several downstream sites on the Columbia River (USGS 1993).
Sagebrush and wheatgrass constitute the primary natural vegetation for this region. Much of the land has been converted to dryland wheat agriculture, with smaller irrigated areas supporting the cultivation of peas and potatoes. Irrigation and agriculture have changed the flow and course of smaller rivers and streams (Omernik and Gallant 1986).
The Blue, Wallowa, Ochoco, Strawberry, and Aldrich Mountains are contained in this ecoregion. The mountains are a mix of older sedimentary and younger volcanic peaks. Mountainous regions contain ponderosa pine, grand fir and Douglas fir, and Englemann spruce stands. Rainfall varies from 25-50 cm in the lowlands, and as much as 100 cm in the mountains, most of which falls as snow. The aquifers that develop in these mountains feed into numerous river systems: the John Day, Umatilla, and Walla Walla Rivers, which flow into the Columbia River, and the Tucannon, Grande Ronde and Imnaha Rivers, which flow into the Snake River. Peak flows occur from April to June, but only last 2 to 4 months; however, flood events historically have occurred from December through February as rain on snow events (WDFW 1997a). Minimum flows occur predominantly in August or September, except in the mountains where flows are at a minimum in January and February.
Lowlands contain sagebrush, wheatgrass, and bluegrass. Land-use activities correspond to vegetation, with timber harvest more prevalent in the mountains and grazing prevalent in the lowlands. Both of these activities have led to considerable localized stream-side erosion.
This region spans southeastern Oregon, southern Idaho, northeastern California, and northern Nevada. Passage of chinook salmon into most of the region has been blocked by dams, but the region still exerts a considerable influence on downstream habitat. This area is geologically very new and contains extensive areas of lava and other volcanic material. The rock substrate is very permeable, streams tend to lose much of their flow through percolation and evaporation, and only the larger rivers that lie below the water table contain substantial flows year round. Rainfalls are generally less than 30 cm annually, but may be as high as 60 cm on the borders of the ecoregion. Extended dry intervals are very common in the Snake River Plateau.
Sagebrush and wheatgrass are prevalent with much of the area utilized as rangeland. Agriculture (potatoes, corn, grains) is sustained where water resources are available. Rivers in the southern half of Idaho are affected by agricultural and urban development. Irrigation return flows, livestock grazing, and urban activities were associated with high nutrient concentrations in the Boise and Snake Rivers (USGS 1993).
Forming the northeast boundary of the Columbia Basin Ecoregion, this region is a mosaic of mountain crestlines (up to 2,500 m) and valleys. Rainfall varies accordingly from 50 to 150 cm or more per year, some of which falls in intense local storms. Winter snowpack is the major contributor to the streamflows; river flows peak with the spring melt in May or June lasting only 2-3 months. One- and 2-month drought periods are fairly common; however, longer periods are quite rare, especially in the higher mountains, where drought periods of even 1 month are rare (once in 5 years). Low flows correspond with low periods of precipitation in August and September except in the higher elevations, where winter temperatures limit flow. In many areas, soil and subsoil development have created important areas for water storage. Seepage is an important water source for major rivers in this area. The Salmon and Clearwater Rivers drain the southern portion of this region and are the only major tributaries to which chinook salmon still have access. The Spokane, Kootenai, and Pend Oreille Rivers drain into the Columbia River from the eastern and northern portions of this ecoregion; however, runs that historically existed on these rivers have been eliminated by impassable dams (Fulton 1968).
Forests are dominated by conifers: western white pine, lodgepole pine, western red cedar, western hemlock, western larch, Englemann spruce, subalpine fir, and Douglas fir. Prairie and mixed forest/grassland are also common. Forestry is the primary land-use activity, although mining and grazing activities are commonplace. Water systems in the northern half of Idaho, the Coeur d'Alene and Clearwater Rivers, are impacted by mining and logging operations; however, containment ponds appear to limit metal concentrations downstream (USGS 1993).
The marine habitat can be subdivided into three general regions--estuary, coastal, and ocean. Chinook salmon with different life-history strategies use these regions to different extents; therefore, changes in the conditions in one region may selectively affect some populations more than others.
Ocean-type chinook salmon reside in estuaries for longer periods as fry and fingerlings than do with yearling, stream-type, chinook salmon smolts (Reimers 1973, Kjelson et al. 1982, Healey 1991). The diet of outmigrating ocean-type chinook salmon varies geographically and seasonally, and feeding appears to be opportunistic (Healey 1991). Aquatic insect larvae and adults, Daphnia, amphipods (Eogammarus and Corophium spp.), and Neomysis have been identified as important food items (Kjelson et al. 1982, Healey 1991). Rivers with well developed estuaries are able to sustain larger ocean-type populations than those without (Levy and Northcote 1982). Juvenile chinook salmon growth in estuaries is often superior to river-based growth (Rich 1920a, Reimers 1971, Schluchter and Lichatowich 1977). Stream-type chinook salmon move quickly through the estuary, into coastal waters, and ultimately to the open ocean (Healey 1983, Healey 1991). Very limited data are available concerning the ocean migration of stream-type chinook salmon; they apparently move quickly offshore and into the central North Pacific, where they make up a disproportionately high percentage of the commercial catch relative to ocean-type fish (Healey 1983, Myers et al. 1987). The Stikine, King Salmon, and Chilkat Rivers are notable exceptions to this general stream-type migration pattern. Apparently, a portion of fish from these stocks remain in the coastal waters of southeast Alaska throughout their lives (ADFG 1997). In contrast, throughout their ocean residence ocean-type chinook salmon inhabit coastal waters, where coded-wire tag (CWT)-marked fish are recovered in substantial numbers (Healey and Groot 1987).
The utilization of estuaries by ocean-type chinook salmon makes them more susceptible to changes in the productivity of that environment than stream-type chinook salmon. Estuaries may be "overgrazed" when large numbers of ocean-type juveniles enter the estuary en masse (Reimers 1973, Healey 1991). The potential also exists for large-scale hatchery releases of fry and fingerling ocean-type chinook salmon to overwhelm the production capacity of estuaries (Lichatowich and McIntyre 1987). The loss of coastal wetlands to urban or agricultural development may more directly impact ocean-type populations. Dahl (1990) reported that California has lost 94% of its wetlands. Furthermore, an estimated 80-90% of the undiked tidal marshlands in the Sacramento River Delta area, the major nursery area for Central Valley chinook salmon stocks, has been lost (Nichols et al 1986, Lewis 1992). A similar reduction has been reported in Washington and Oregon wetlands: a 70% loss in the Puget Sound, 50% in Willapa Bay, and 85% in Coos Bay (Refalt 1985).
The ocean migrations of chinook salmon extend well into the North Pacific Ocean. The productivity of various ocean regions has been correlated with the degree of wind-driven upwelling (Bakun 1973, 1975). Under normal conditions this upwelling decreases along the coast from California to Washington and British Columbia (Bakun 1973). Changes in wind directions related to sea level pressure (SLP) systems, most notably the Aleutian low pressure (ALP) or Central North Pacific (CNP) pressure indices, can greatly alter upwelling patterns (Ware and Thompson 1991, Beamish and Bouillon 1993). Upwelling brings cold, nutrient-rich waters to the surface, resulting in an increase in plankton and ultimately salmon production (Beamish and Bouillon 1993). Strong ALP measurements (high pressure readings) tend to result in minimal upwelling in the North Pacific. Similarly, atmospheric pressure systems in the Central Pacific can alter trade wind patterns to bring warmer water up along the California coast; this occurrence is better known as an El Niño. El Niño events suppress coastal upwelling off the Washington, Oregon, and California coasts and tend to bring warmer water and warm-water species northward (McLain 1984). One difference between El Niño events and ALP events is that the northerly flow of warm waters associated with El Niño events may stimulate ocean productivity off Alaska (McLain 1984). Ocean migratory pattern differences between and within ocean- and stream-type chinook salmon stocks may be responsible for fluctuations in abundance. Moreover, the evolution of life-history strategies has, in part, been a response to long-term geographic and seasonal differences in marine productivity and estuary availability.