The South Fork (Kern River) Wildlife Area
Will the Commitment Be Forgotten?[1]
Carolyn Fleshman and Darrell S. Kaufman[2]
Abstract.—This study reports the ecological effects of reservoir inundation on a portion of the floodplain riparian system of the South Fork, Kern River, Kern County, California. The South Fork Wildlife Area, lying within gross pool of Isabella Reservoir and administered by the US Army Corps of Engineers, constitutes the prime portion of one of the largest and healthiest riparian forests remaining in California. In spring, 1980, the entire sanctuary was inundated by Isabella Reservoir, drastically reducing its unusually high bird populations and contributing to a reduction in the overall habitat values of the forest.
Introduction
The South Fork Wildlife Area (SFWA) is an ecological reserve and wildlife management area established by the Sacramento District, US Army Corps of Engineers (CE), within the gross pool (the area subject to flooding when the reservoir fills to its highest level) of Isabella Reservoir, Kern County. While the reservoir dams both the North and South Forks of the Kern River (see fig. 1), the most luxuriant riparian forest (and the SFWA) occur on the floodplain of the South Fork.
The SFWA serves as a unique example of preservation and enhancement of an area of extremely rich biological diversity and productivity by the CE, an agency often criticized for its disregard for natural values. The SFWA project has won the CE, Sacramento District, several awards for excellence as a conservation initiative. However, a recent change in water management and a proposed water development project threaten to reverse all improvements the CE has made, and, if implemented, destroy the resource that it has striven to protect.
Figure 1.
Map of the Kern River watershed showing the
South Fork Valley and the SFWA location.
The first part of this paper presents background information on the ecology and management of the SFWA, and a description of Isabella Reservoir management policies affecting it. The second part details the results of bird censuses and vegetative transects designed to assess the impacts of reservoir inundation on the SFWA.
[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].
[2] Carolyn Fleshman and Darrell S. Kaufman, Environmental Field Program, University of California, Santa Cruz. Co-authors in alphabetical order.
South Fork Valley
The South Fork Kern River (fig. 1) supports one of the largest contiguous riparian forests in the State, encompassing over 1,000 ha.[3] The forest is one of the best remaining examples of a native deciduous riparian system in California (USDI Fish and Wildlife Service 1980).
Hydrology and Geology
The South Fork, unlike most Sierran streams, flows through a broad, gently sloping floodplain. Draining a watershed of 1,370 km2 , the South Fork begins in the Golden Trout Wilderness and flows south, then east through the South Fork Valley. Originally it joined the North Fork, but now it empties into Lake Isabella, a reservoir created in 1954 by damming the North and South Forks of the Kern River at their confluence.
The South Fork once cut through a deeply incised canyon, into the underlying rocks of the Sierra Nevada basement complex at the time of the Sierra Nevada uplift. Changes in topography, probably the result of block faulting at or near the present dam site, raised the elevation of the Kern River drainageway. This in turn produced a sediment trap, causing the canyon to be partially filled with unconsolidated alluvial sediments, largely decomposed granite, carried down from the surrounding mountains. This accumulated material now forms the South Fork floodplain and is the substrate of the South Fork riparian system.[4]
The low hydraulic energy of the South Fork, and the gradual topography and porous soils of its floodplain provide aquifer storage and a high water table for the Valley throughout the year. These characteristics create optimum growing conditions for a luxuriant native riparian woodland (fig. 2).
Figure 2.
View of the lush cottonwood/willow forest
of the South Fork Valley, looking east.
Vegetation
Dominated almost entirely by a mature growth of various willows (Salixgooddingii , S . lasiandra , and S . laevigata ) and Fremont cottonwood (Populusfremontii ), some of them estimated to be up to 300 years old,[5] the South Fork forest forms a meandering vegetated corridor through nearly the entire length of the Valley. Its total length is about 16 km., its average width 0.75 km. At the eastern end, the cottonwoods and willows intergrade with a relict stand of Joshua trees (Yuccabrevifolia ). An extensive network of marshes weaves in and out of the forest for much of this length. The valley floodplain offers a distinct visual contrast to the sparse, xerophytic vegetation of the adjacent slopes.
Subjected to various land use practices, the structure of the forest reflects the activities of humans since the early settlers arrived following the Walker expedition of 1834. Portions of the forest were cleared for agricultural fields or cut for fuel (Powers 1971). Grazing, which continues today, has inhibited regeneration of the forest, resulting in areas of homogenous old growth with a dense understory of mulefat (Baccharisviminea ) and stinging nettle (Urtica sp.). Areas where grazing has recently been eliminated are showing a remarkable recovery rate, with proliferous new growth of cottonwood and willow seedlings.
Wildlife
This unusually verdant valley supports an extraordinary abundance and diversity of wildlife. Singularly rich in avifauna, the South Fork riparian forest owes its diversity of birdlife to its location at the confluence of five distinct biogeographic regions, a situation which occurs nowhere else in North America north of Mexico. In the South Fork Valley, the Sierran Forest, Intermountain Sagebrush, California Grassland, American Desert, and California Chapparral provinces converge (Bailey 1978). Over 200 species, including songbirds, woodpeckers, waterfowl, raptors, and others, fill the South Fork forest with a jungle-like symphony of song in the breeding season. Riparian-dependent
[3] Gaines, D.A. 1977. Riparian cottonwood-willow forests in California, an inventory of potential preserves. Report prepared for The Nature Conservancy, Western Regional Office.
[4] Warner, R.E. 1980. Riparian vegetation/wildlife relationships in the South Fork Kern watershed. Unpublished field study prepared for the Kern County Wildlife Resources Commission. 26 p.
[5] Shevock, James. 1981. Personal conversation. Plant ecologist, USDA Forest Service, Sequoia National Forest, Calif.
species such as Wood Duck,[6] Red-shouldered Hawk, Yellow-breasted Chat, Blue Grosbeak, and Yellow Warbler, once common throughout California's riparian forests, but now absent from many of them,[3] still inhabit the South Fork.
The cottonwood/willow forest is a haven for flycatchers, warblers, swallows, and other migrants on their flights over the mountains and deserts. Unusual species such as Indigo Bunting and Rose-breasted Grosbeak are also drawn to the forest.
The South Fork Valley supports an exceptionally high breeding bird population of 98 species, more than one third of the regular nesting species in California identified by Hehnke and Stone (1978). The largest population of the State Rare Yellow-billed Cuckoo outside of the Colorado River region, an estimated 24 pairs, breeds in the South Fork forest.[7] A pair of Osprey also nest in the Valley, one of only two breeding pairs between Mexico and Sacramento.[7] In addition, there are two Great Blue Heron rookeries with approximately 100 active nests.
The South Fork represents one of the last refuges for many species of birds threatened by habitat loss. Thirty-eight species found here are included on the 1981 Audubon Blue List for California, and the California Natural Diversity Data Base List of Special Animals.
Isabella Reservoir
Authorization
The CE Isabella Project was authorized by the Flood Control Act of 1944 to protect the city of Bakersfield, built on the floodplain of the Kern River, from floods. Project authorization included provisions for irrigation and recreation in addition to flood control. The Federal Government paid for approximately 70% of the project costs, with irrigation and hydroelectric power interests contributing 20% and 8% respectively.[8] The project, involving construction of a main and auxiliary dam, began in 1948 and was completed in 1954.
Normally, all water released from the main dam is used for irrigation of agricultural fields in Kern County. When flows are in excess of demand they can be diverted into the California Aqueduct via the Kern River/California Aqueduct Intertie, or to the Tulare and Buena Vista Lakebeds. Releases of floodwaters into the Lakebeds are usually damaging to crops, however.
South Fork Wildlife Area
Riparian System .—The creation of Isabella Reservoir resulted in the immediate loss of approximately 1,300 ha. of South Fork riparian forest.[9] An additional 1,417 ha. of riparian system lie within gross pool elevation (2,605.5 ft. msl). Of this additional area, only approximately 125 ha. are presently heavily forested, the remainder having been deforested by agriculture, woodcutting, grazing, and fire. The area is periodically inundated by the reservoir during years of especially high runoff. This portion of the forest, where the South Fork broadens into a braided delta, is the greatest in width and most dense (fig. 3).
In 1966, in a planning aid letter to the CE, the USDI Fish and Wildlife Service (FWS) recognized the superior value of the remaining cottonwood/willow forest as wildlife habitat. They recommended that grazing be eliminated in the forest because of its adverse impacts, and that the CE manage the area for the enhancement of wildlife values (US Army Corps of Engineers 1979a).
Figure 3.
Morning view of the South Fork
Wildlife Area, looking southeast.
Management Plan .—In 1977, a wildlife management plan for what was to become the SFWA was developed by local CE rangers with professional training in wildlife management. The first phase of the plan called for restoration and wildlife improvement activities in 505 ha. of floodplain riparian zone, from the eastern boundary of the reservoir's gross pool line west to elevation 2,580 ft. msl. The major objective of the plan, to convert agricultural land within the riparian zone back to riparian forest by allowing agricultural leases to expire, has been partially achieved (ibid .) Leases have not been termi-
[6] Scientific names of bird species mentioned in the text are included in Appendix A.
[7] Barnes, Robert. 1981. Personal conversation. President, South Fork Watershed Association, Calif.
[8] Schontzler, G. 1981. Politicians, farmers wrangle over who gets Kern River water. The Bakersfield Californian, Feb. 16, 1981. Sec. B2, p. 1.
[9] Estimated from an aerial photograph taken in 1946, prior to construction of Isabella Dam.
nated, but have instead been placed on inactive status, allowing grazing or agriculture on an interim, short-term basis, as prescribed by the District Engineer.[10]
Other improvements under the SFWA plan have included construction of nesting structures for Mourning Dove, Wood Duck, and Osprey; and construction of a trail system for which an interpretive guide is being prepared. In addition, a riparian reforestation and habitat manipulation program is being implemented.
Because the SFWA lies within gross pool elevation of Isabella Reservoir, the management plan was designed to function under normal operations of the reservoir. Prior to 1978, the reservoir rarely inundated the riparian forest except in years of exceptionally high runoff, and then only for short durations (US Army Corps of Engineers 1978).
SFWA Awards .—In recognition of an outstanding demonstration of its knowledge and appreciation of the most important environmental values, the CE, Sacramento District, received a National Honor Award in the US Army Chief of Engineers Design and Environmental Awards competition for 1979. It also won a CE Divisional Award during the same year. In addition to numerous other commendations, the SFWA management plan earned the CE, Sacramento District, a National Audubon Society award, which was presented in a special on-site ceremony.
Reservoir Water Management .—The flood control function of the reservoir and, secondarily, irrigation demand dictate reservoir operation. According to the Lake Isabella Regulations Manual (ibid .), reservoir level should be drawn down to 170,000 acre-feet (AF) by 1 November each year and kept at this level until 1 February when snowmelt runoff predictions become available. This seasonal drawdown prepares the reservoir for both rain and snowmelt floodwaters. Any water remaining in the reservoir above this 1 November level, that is, within Flood Control Space, is called "winter carryover," and is dependent upon "prevailing conditions" (ibid .). Under no circumstances, however, is the storage of winter carryover permitted above 245,000 AF between 1 November and 1 March, as this minimum reservation is required to control rainfloods (ibid .).
To accomodate irrigation interests, since 1978 winter carryover has been substantially higher than the prescribed flood control capacity, as described in the Regulations Manual. In fact, since 1978 water has encroached on the rainflood reservation space of 325,000 AF (reservoir capacity above 245,000 AF) 40% of the time.[11] A comparison of historical reservoir operations shows that reservoir storage for the 1978–1981 period was substantially higher than ever before (fig. 4). First-of-the-month reservoir storage for the 1978–1981 period was an average of 40,000 AF higher than that of the 1967–1970 period when the total reservoir inflow was actually 13% higher.
The representative for the Kern County water districts maintains that the high winter carryover is the result of a CE reevaluation of the Kern River Basin hydrology.[12] However, the most recent hydrological study shows that the Probable Maximum Flood is actually higher than previously predicted (US Army Corps of Engineers 1979b).
The CE, Sacramento District, Operations Division claims that the completion of the California Water Project, making more water available to Kern County users and allowing more conservation of Kern River water, is responsible for the difference.[13]
Although both parties deny any connection, the Kern River/California Aqueduct Intertie was completed in 1977, reducing the flood hazard to Tulare Lake Basin—the ultimate destination for Kern River floodwaters—by absorbing floodwaters from the Kern River and transporting them south to the Los Angeles area (US Army Corps of Engineers 1978). The Intertie thus reduces the hazard of permitting encroachment into the Flood Control Space of Isabella Reservoir. It also results in the further diversion of water from the basin, which in the past had partially recharged the badly depleted groundwater aquifer.
Regardless of the reason for the change in reservoir storage, it has not been officially acknowledged, and thus no environmental assessment of its effects has been considered. Because of the high winter carryover, the reservoir fills earlier and to a higher elevation during the annual spring snowmelt runoff season, thus changing the timing and increasing the frequency, duration, and depth of reservoir inundation of the SFWA. With a reservoir storage increase of 40,000 AF (the minimum average increase since 1978 as compared to the 1967–1970 period) the reservoir is expected to inundate the lower boundary of the South Fork riparian forest once every four and one-half years, as compared to the 1977 calculation (ibid .) of once every six years. The entire sanctuary will be flooded every 20 years, as compared to once in 100 years.
Prior to 1980, the reservoir reached gross pool capacity only once in its history, in 1969. In 1980, as the result of high winter carryover
[10] Slattery, Thomas. 1981. Personal correspondence. US Army Corps of Engineers, Sacramento District, Real Estate Division.
[11] Calculated from Isabella Project Office daily records.
[12] Williams, Chuck H. 1981. Personal conversation. Kern River Watermaster, Bakersfield, Calif
[13] Verke, Mark. 1981. Personal conversation. US Army Corps of Engineers, Sacramento District, Operations Division, Reservoir Control Section.
Figure 4.
Comparison of Isabella Reservoir storage for three time periods.
(265,000 AF average from 1 November to 1 March) and high runoff (nearly 200% of normal), Isabella filled to gross pool again and rose an additional 0.46 m. (1.5 ft.) over the dam spillway, an unprecedented event. Encroachment of the riparian forest of the SFWA began in early April, peaked in early July when the entire sanctuary was inundated, and receded in September.
Effects of Reservoir Inundation
Identified by the FWS as the fourth preservation priority in the State, the importance and unique value of the South Fork forest has been widely recognized (see Gaines 1977, and Warner 1980, cited in footnotes 3 and 4), yet no systematic, baseline studies have been conducted in the area. Consequently, when Isabella Reservoir filled to gross pool capacity in 1980, the effects of reservoir inundation on the SFWA could not be predicted. The purpose of the present study was to monitor these effects on the avian community and to evaluate the overall effects of periodic inundation on the habitat values of the SFWA. Necessary to achieve this end, and important in itself, was the establishment of baseline data on bird population diversity and density in 1981.
Methods
Bird Censusing
A variable circular plot technique was used in estimating bird populations.[14] This method has been recommended for assessing the effects of habitat manipulation (Verner 1980).
In order to reduce variability, as suggested by Dawson (1980) and Verner (1980), the number of individuals of a species detected through time, rather than absolute densities, was used in measuring the responses of avifauna to flooding in 1980. Bird densities and diversities were calculated from 1981 data, when the high degree of disturbance from inundation was not a factor.
Fourteen observation stations, representative of five subhabitat-types, were established within the 160 ha. study area:
4 along a forest/field ecotone where inundation is frequent;
3 along a relatively undisturbed ecotone;
[14] Reynolds, R.T., J.M. Scott, and R.A. Nussbaum. 1980. A variable circular-plot method for estimating bird numbers. Unpublished report.
5 in closed-canopy forest;
1 in a dense grove of young trees; and
1 in an open stand of mature trees.
The bird censuses consisted of five "counts", each comprised of six "surveys", completed during April and May 1980 and 1981. A total of 360 person-hours was spent conducting bird surveys. An intensive search for active nests was conducted on the mornings of 22 and 23 May, 1981 at four stations within the closed-canopy forest.
Vegetative Transects
To characterize the difference in vegetative composition, structure, and condition at various frequencies of reservoir inundation, four 100-point transects, modified from Davis (1977), were conducted. The transects ran north to south along elevations corresponding to the 5-, 10-, and 25-year reservoir stage recurrence intervals taken from Chart 20 of the Reservoir Regulations Manual (US Army Corps of Engineers 1978). A control transect was run at the 300-year reservoir stage recurrence interval. Although not subjected to reservoir inundation, the control transect area was actively grazed.
Results and Discussion
Displacement of Birds
When the study period began on 7 May, 1980, Station 1, the lowest in elevation, was under 7.5 cm. (0.25 ft.) of reservoir water. The reservoir steadily rose, inundating five ecotonal stations by 30 May, 1980. In response to reservoir inundation, bird populations and species richness showed significant declines. Figures 5 and 6 depict the decline in these parameters in relation to water depth for the five stations subjected to inundation during the study period.
Figure 5.
Bird populations and Isabella Reservoir
inundation levels, 1980, at five stations.
Figure 6.
Bird species richness and Isabella Reservoir
inundation levels, 1980, at five stations.
Because bird species have different habitat requirements, utilizing the riparian environment differently, their responses to inundation varied. Bird species were grouped into the following four guilds based on their preferred nesting sites: 1) species which nest on the ground; 2) species which nest in shrubs; 3) species which nest among the foliage or branchwork of trees; and 4) species which nest in tree cavities.
The declines in bird populations and species richness are depicted for each guild in figure 7. The figure illustrates proportionately greater decline in ground and shrub nesters. The percent decline represents the average for the five stations once the reservoir level reached each station and assumes a linear decline. Data show, however, the decline in bird species abundance was roughly logarithmic, showing the greatest de-
Figure 7.
Average decline of bird populations and
species richness for four nesting guilds. The
percent decline per foot of inundation once
Isabella Reservoir water level reached each
station was averaged for five ecotone stations.
cline during the first several feet of inundation, then leveling off.
As the water rose, birds utilizing the ground and shrubs for nesting and food sources were displaced to higher ground. Nests were destroyed and unfledged young were drowned. In general, tree nesting species stayed in flooded territory for a short time, but were eventually forced to leave because of reduction in available food and the threat to nests as the reservoir filled. The emigrating forest birds were temporarily replaced by shorebirds foraging along the shoreline of the advancing reservoir. Once inundation was complete, however, this habitat niche was also eliminated.
The influx of emigrating birds to unflooded regions, illustrated by increases in population densities and species richness just prior to inundation of a station, upset the balance of the entire avian community. There was a marked increase in agonistic activity, especially among the wrens and orioles, as competition for scarce unclaimed territories was heightened. The more aggressive species such as Starlings and blackbirds were observed to dominate perch sites previously shared by many species. Interspecific conflicts, indicative of lack of suitable nesting sites, were frequently observed. All of these factors cut into the parents' time and energy budgets. Smaller, more crowded territories, with a less abundant food supply, reduced nesting success and eliminated the opportunity for second and third broods.
Bird Census, 1981
Average aggregate bird densities, species diversity, and species richness for each count in each subhabitat-type are shown in table 1. Densities and diversities include only breeding populations. The highest average aggregate density was 3959 birds per 40 ha. in the closedcanopy forest. The low species diversity in the new-growth subhabitat-type is attributed to the predominance of Redwinged Blackbirds, which comprised over 60% of the total density; when this species is excluded, the new-growth subhabitattype had the lowest density as well.
The exceptionally high bird densities of the South Fork forest well exceed the high density limits established by Peterson (1941), and are comparable even to worldwide maxima (Johnson et al . 1977). The intensive search for and location of 66 nests at four of the survey stations supports the high estimates obtained. Twenty-two active nests were found within a radius of 30 m. from Station 8 alone; the resulting breeding bird density would be over 6,000 birds per 40 ha. Bird densities for individual species within the SFWA are given in Appendix A.
When the rate of decline of bird populations observed in 1980 is applied to the high density estimates of 1981, the effects of reservoir inundation are clearly shown to be devastating.
Vegetation
Caution must be used in drawing conclusions about the effects of reservoir inundation on the condition of vegetation in the SFWA, as it has been affected by land use practices such as grazing, farming, and woodcutting as well (fig. 8). However, it is clear that prolonged inundation has significant immediate and long-term effects on vegetation. While some effects are readily observable after one season, others are not until some years later, when trees may die abruptly after a period of continued growth (Ewel 1978).
The immediate result of flooding, described by Teskey and Hinckley (1977), is the creation of an anaerobic environment around root systems, producing a variety of physiological plant stresses. The duration, depth, and rate of movement of floodwater are important factors controlling oxygen availability to the roots and hence the degree of stress produced. The time of year is also important; plants are much more susceptible to stress during the growing season (Walters etal . 1980).
Table 2 summarizes the data from four vegetative transects along the 5-, 10-, 25-, and 300-year reservoir stage recurrence intervals. Data show that total tree density, tree species diversity, average percent canopy cover, and average tree height all decrease with greater frequencies of reservoir inundation. The number of trees that died during the 1980 growing season (dead trees in 1981, with the previous year's leaves remaining), number of fallen trees, and degree of mistletoe (Phoradendron flavescens ) infestation decreases with increasing elevation.
The decline in total tree density from 550 trees per ha. at transect 4 to 1 tree per ha. at transect 1 is dramatic. Although transect 2 appears to be more dense than transect 3, the filling in of a former open field with young black willow (Salix Gooddingii ), of which 51% are more than 50% decadent, accounts for the high density figure.
Tree condition related to three age-classes for each tree species is shown in figure 9. The histogram illustrates a greater degree of decadence at lower elevations where inundation is most frequent.
Ewel (1978) concluded that the dependence of species composition on hydroperiod and depth of flooding is clearly the most sensitive characteristic of riparian ecosystems. Long periods of flooding eliminate the more sensitive species and generally favor the early successional stage species (Walters etal . 1980). The degree of decadence and mistletoe infestation were especially high among the cottonwoods along the lower elevation transects. In contrast, along the same transects, there was a high proportion of healthy black willow. These results indicate that Fremont cottonwood is least tolerant and black willow most tolerant of prolonged inundation in
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the SFWA. Teskey and Hinckley (1977) classified Fremont cottonwood as "intermediately tolerant," that is, able to withstand one to three months of partial inundation during the growing season, and black willow as "tolerant," or able to withstand flooding for most of one growing season.
Figure 8.
Scattered willows and cottonwoods of the
SFWA where the forest enters Isabella Reservoir.
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Figure 9.
Tree species composition, degree of decadence, and age-class for four vegetative transects within the SFWA, 1981.
The combined effects of grazing and inundation have retarded the regeneration of new trees, especially at lower elevations where large open fields have shown high potential for riparian reforestation. On transect 1, there were no seedling, sapling, or young cottonwoods. All seedling willows were concentrated in one narrow slough and had sprouted in 1981. Transect 2 had a high percentage of young willows concentrated in the interior forest, where aerial photographs show a former open field. This area was probably not easily accessible to cattle. Transect 4, an actively grazed area above gross pool, also had few seedlings or saplings of any species and was the only transect which showed a high percentage of mulefat—a shrub which is less palatable to cattle. The majority of the saplings, preferred food for cattle, were able to survive by growing through the protective center of a mulefat shrub.
The increase in percentage of fallen trees from transect 4 to transect 1 indicates that increased inundation also reduces soil stability, resulting in less support for root systems.
Another effect within the study area was the change in herbaceous vegetation in open fields. Prior to inundation, dominant plants were alfalfa (Medicagosativa ), mustards (Rorrippa and Descurainia spp.), and various grasses (Graminae). When the water receded, the fields were recolonized by cocklebur (Xanthium ) of little wildlife value and a nuisance to visitors.
The plant species composition also changed in a cattail (Typha ) marsh of 4 ha. The cattails did not regenerate after inundation and were replaced with curly dock (Rumex spp.) and monkey flower (Mimulusguttatus ).
Vegetation-Bird Relationships
The longer term effects of reservoir inundation on bird populations will be the result of an overall decrease of habitat value of the forest. Bird densities along ecotone stations are correlated with tree densities, both increasing with elevation. This correlation is also supported by the findings of Carothers and Johnson (1975) in a homogenous cottonwood forest. They found a linear relationship between bird density and tree density with a more rapid decline in bird populations if tree density fell below 25 trees/ha. Johnson (1970) also found that breeding bird populations in a cottonwood forest were reduced when tree density declined.
Tree species composition and age-class diversity also increased with elevation, showing an association between less diverse forest and lower bird densities at lower elevations.
The reduction in density of mature cottonwoods, in particular, has negative implications for bird populations. Nest site locations in the SFWA show that although mature Fremont cottonwood composed only 12% of the forest tree composition in the proximity of stations 8, 10, and 13 (transect 3), they supported 89% of the 45 nests found at these stations. Willow species, constituting 83% of the trees, supported only 9% of the nests. Three percent of the nests were found in herbaceous understory. In a study of nesting birds of a cottonwood/willow community in California, Ingles (1950) also found that hole-nesting birds preferred cottonwood over other tree species.
Although new growth is essential to the continued viability of the forest and contributes to structural diversity, study results show that uniform stands of young trees are less favored habitat for birds than mature groves. Anderson etal . (1978) concluded that along the lower Colorado River, as trees mature they approach their full value to wildlife.
The elimination of the 4 ha. cattail marsh displaced a blackbird colony of several thousand breeding birds in 1980. Because the cattails failed to regenerate, the colony was reduced to less than 50 pairs in 1981.
As Best etal . (1978) has pointed out: "Birds with restricted habitat usage patterns are more vulnerable to changes in land use practices than those occupying a wider variety of environments." The high dependency of birds on riparian vegetation makes them especially susceptible to the effects of habitat degradation.
Additional Effects
Inundation also had important secondary impacts on wildlife and habitat improvements of the SFWA. Fishing activities, popular once the forest was under water, caused considerable disturbance to the Great Blue Heron rookery and to nesting Osprey. Because of the interference, no Osprey were hatched in 1980. Perch poles, necessary for successful utilization of nesting platforms, were washed away.
Reductions in mammals and other vertebrates were not quantified, but animals floating on debris or drowned were encountered daily during the study period in 1980. While mature birds were able to fly ahead of the advancing water level in the reservoir, many other terestrial animals were trapped. A prominent local naturalist has noted the absence of numbers and species of animals throughout the winter after inundation.[15] The recolonization rates for all species are unknown.
Inundation caused substantial damage to CE-constructed improvements in the SFWA. Artificial nesting boxes were inundated, many with occupants. All three footbridges were washed out and required reconstruction. Water control structures, including Waterman gates, levees, and ponds were destroyed, and cattle exclusion fences severely damaged. Repeated inundation will result in the destruction of the water system rebuilt in 1981, as well as other repairs and improvements; and damage or death to newly planted cottonwoods and willows.
Inundation also precluded visitor use of the SFWA for at least three spring and summer months during the season when visitation at Isabella Reservoir is highest.
Conclusions
Reservoir inundation has many adverse impacts on vegetation and wildlife in the SFWA. Flooding drastically reduces the unusually high bird populations of the SFWA by drowning and displacement. Prolonged flooding decreases tree density and diversity, which reduces the overall habitat value of the forest. The timing of inundation, during the spring nesting season of birds, and growing season of trees, amplifies these effects. Inundation is also destructive to CE structural and other improvements and significantly reduces the opportunity for visitor use of the area.
While high winter carryover is an immediate concern, enlargement of Isabella Reservoir, which is currently under consideration,[16] poses a more long-term threat of permanent inundation of part of the SFWA and more frequent inundation of the remainder.
Although the CE has made commendable habitat improvements, it maintains that it is helpless to prevent seasonal inundation of the SFWA. It has often been stated that because the SFWA is not one of the authorized purposes of Isabella Reservoir, it cannot be included in the determination of winter carryover. Failure to consider the management of the SFWA as a part of the management of the reservoir is a result of the lack of official recognition of the CE's responsibility to protect the values of the SFWA, and a lack of recognition of a change in winter carryover policy as a significant action requiring environmental impact assessment and mitigation.
If the CE's commitment to enhancement of natural values is to be honored, the South Fork Wildlife Area must be recognized as a legitimate concern and incorporated into reservoir operations and management.
Acknowledgements
This research was supported by a grant from the Environmental Field Program, University of California, Santa Cruz. We wish to thank Dr.
[15] Wilson, Lee. 1981. Personal conversation. Kernville, Calif.
[16] US Army Corps of Engineers, Sacramento District. 1977. Kern River Basin investigation.
Richard Warner for invaluable assistance throughout the study. Thanks also to the US Army Corps of Engineers, Sacramento District, and Isabella Project Office for providing information, logistic support, and other helpful assistance throughout the study.
Literature Cited
Anderson, B.W., R.D. Ohmart, and J. Disano. 1978. Revegetating the riparian floodplain for wildlife. p. 318–333. In : R.R. Johnson and J.F. McCormick (tech. coord.). Strategies for protection and management of floodplain wetlands and other riparian ecosystems: Proceedings of the symposium. [Callaway Gardens, Ga., Dec. 11–13, 1978]. USDA Forest Service GTR-WO-12. 410 p.
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