THE ROLE OF LARGE WOODY DEBRIS IN ROCKY REACH RESERVOIR
First-Draft
ROCKY REACH HYDROELECTRIC PROJECT FERC Project No. 2145
November 9, 2000
Prepared by: BioAnalysts, Inc. Boise, Idaho
Prepared for: Public Utility District No. 1 of Chelan County Wenatchee, Washington
Large Woody Debris
TABLE OF CONTENTS
SECTION 1: INTRODUCTION ...... 1
SECTION 2: SOURCE OF LARGE WOODY DEBRIS...... 3
SECTION 3: FUNCTION OF LARGE WOODY DEBRIS...... 5 3.1 Submerged Large Woody Debris ...... 5 3.2 Floating Large Woody Debris ...... 6 3.3 Large Woody Debris on the Floodplain ...... 7
SECTION 4: SUMMARY...... 9
SECTION 5: REFERENCES ...... 11
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Large Woody Debris
SECTION 1: INTRODUCTION
Large woody debris (LWD) is an important component of aquatic ecosystems (Harmon et al. 1986; Maser and Sedell 1994; Gurnell et al. 1995). LWD provides physical structure that aquatic organisms use as habitat, and it alters water movements and hydrological processes (Triska and Cromack 1980; Franklin et al. 1981; Harmon et al. 1986; Maser and Sedell 1994). LWD affects the flow of organic matter from terrestrial ecosystems into surface waters (Franklin et al. 1981), and the transport of organic materials within aquatic ecosystems (Beschta 1979; Bilby and Ward 1991). The balance among inputs, decomposition rates, and removal processes determines the biomass of LWD inputs in freshwaters. Climate, soils, stream flow rates, stream gradient, topography (geomorphology), forest age, forest density, and community composition collectively determine the distribution and abundance of LWD in aquatic ecosystems (Harmon et al. 1986; Harmon and Chen 1991; Tyrrell and Crow 1994).
Although LWD performs several essential functions in streams and rivers, the relative importance of each function varies with stream size. In small, steep headwater streams (first and second-order streams), LWD tends to dominate hydraulic processes by increasing the frequency and volume of pools, decreasing the effective streambed gradient, and increasing the retention of organic material and nutrients within the system (Bisson et al. 1987). In mid-order streams, LWD functions primarily to increase channel complexity and flow heterogeneity by anchoring the position of pools along the thalweg, creating backwater along the stream margin, causing lateral migration of the channel, and increasing depth variability (Maser et al. 1988). Another important function of LWD in mid-order streams includes the retention of carcasses and organic detritus, which provide nutrients to the biota within the stream and in the adjacent riparian area (Bilby et al. 1996; Cederholm et al. 1999). The function of LWD in high-order streams is less understood; however, historical records indicate that large debris jams once played a major role in floodplain and channel development on large rivers (Sedell and Luchessa 1981). In these high-order streams, LWD increased channel complexity by creating side channels, backwaters, and ponds, as well as refugia for aquatic organisms during winter storm events. LWD also trapped sediments on floodplains and in riparian zones during high flows.
Schuett-Hames et al. (1994) describe three types of LWD: logs, rootwads, and large debris jams. To qualify as a log, a piece of wood must have a root system that is wholly or partially detached and is no longer capable of supporting the log’s weight. The wood must have a diameter of at least 10 cm for at least 2 m of its length. A root wad is defined as a piece of wood less than 2-m long (except for old-growth stumps), with a root system attached. It must be at least 20-cm diameter at the base of the stem where it meets the roots. The latter must be detached from their original position. A large debris jam is defined as an accumulation of 10 or more logs or rootwads in contact with one another. At Rocky Reach Dam, one could define LWD as any piece of wood that does not pass through the trash rack. Here, I follow the definition presented in Schuett-Hames et al. (1994).
This paper investigates the source, function, and fate of LWD in the Rocky Reach Reservoir, emphasizing the function of LWD in the reservoir. Because there is virtually no information on LWD in Rocky Reach Reservoir, I draw on information from other systems, and rely mostly on
Draft Study Report Rocky Reach Project No. 2145 November 9, 2000 Page 1 SS/2486 Large Woody Debris studies that examined the function of LWD in lakes. I found no studies that described the function of LWD in reservoirs of “run-of-the-river” hydroelectric projects. The Rocky Reach Natural Sciences Working Group will use this report to either formulate management decisions and plans or to recommend additional studies.
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SECTION 2: SOURCE OF LARGE WOODY DEBRIS
Large wood enters streams and rivers through two different pathways: (1) the steady toppling of trees as they die or are undercut by streamflow, and (2) catastrophic inputs associated with windstorms, mass failures, and debris torrents (Bisson et al. 1987; Cummins et al. 1994). Beavers (Castor canadensis) also contribute wood to streams (Maser and Sedell 1994). In addition, land management activities, such as logging and clearing, can influence the amount of wood entering streams. Once in the channel, LWD can either move downstream or stay in the channel or on the floodplain. Stream flows, size of the wood, and channel morphology determine whether the wood is transported or retained. In general, the larger the size of the wood, the greater its stability in the channel, since higher flows are needed to displace larger pieces (Bilby and Ward 1989).
Wood enters the Rocky Reach project area from the riparian vegetation along the reservoir and from upstream locations. I found no estimates of the amount of LWD recruited annually to the project area. Whatever the total amount is, it would appear that riparian vegetation along the reservoir provides only a small fraction. Riparian vegetation occurs only intermittently along the margins of the reservoir and consists of grasses, forbs, shrubs, and deciduous trees (CPUD 1991). Generally, only trees provide a source of LWD. CPUD (1991) indicated that riparian vegetation types represent only about 40% of the shoreline. Presently, riparian vegetation types represent more than 40% of the shoreline, although the total amount is unknown (S. Hays, CPUD, personal communication). Therefore, the majority of the LWD recruited to the project area likely enters from upstream sources, such as the Entiat River and wood that passes Wells Dam. Until recently, large rafts of driftwood passed Wells Dam into Rocky Reach Reservoir. Currently, however, Douglas PUD mechanically removes a large fraction of the driftwood at their project (R. Klinge, DPUD, personal communication). Thus, the amount of LWD entering the project area from the Columbia River upstream from Wells Dam is relatively small.
Once in the reservoir, LWD can become submerged in littoral1 areas or at the bottom of the reservoir, float at or near the water surface, strand on the floodplain, or pass through the dam.
1 The littoral zone extends from the shore just above the influence of waves and spray to a depth where light is insufficient for rooted aquatic vegetation (Goldman and Horne 1983). The pelagic zone is the deep-water area that extends beyond the littoral zone.
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SECTION 3: FUNCTION OF LARGE WOODY DEBRIS
LWD in aquatic systems can affect nutrient cycling and production, and provide refuges from predation (Wege and Anderson 1979; Lynch and Johnson 1989; Christensen et al. 1996). In lakes and reservoirs, it appears that both small- and large-scale processes affect the biological and physical functions of LWD (Maser and Sedell 1994). Small-scale processes control how LWD decomposes. In water, wood decomposes more slowly than on land because water-logging prevents oxygen from penetrating deeply into the wood (Harmon et al. 1986). Large-scale processes affect how LWD influence flows and channel structure (generally not important in reservoirs), and habitat for fish and invertebrates. Therefore, the function of LWD depends on its location within the reservoir.
3.1 Submerged Large Woody Debris Submerged LWD increases habitat structure, which provides cover and perhaps increases foraging opportunities for fish (Moring et al. 1989). Moring et al. (1986) studied the importance of submerged logs in a Maine reservoir and found that suckers (Catostomus spp.) and shiners (Notemigonus spp.) were attracted to LWD. Yellow perch (Perca flavescens), on the other hand, did not demonstrate a clear preference for LWD. In Rocky Reach Reservoir, suckers, shiners (Richardsonius spp.), bass (Micropterus spp.), pumpkinseed (Lepomis gibbosus), bluegill (Lepomis macrochirus), crappie (Pomoxis spp.), northern pikeminnow (Ptychocheilus oregonensis), possibly subyearling summer/fall chinook (Oncorhynchus tshawytscha)2, and perhaps other fish species would use submerged LWD in littoral areas. In deeper water, species such as sculpins, suckers, bullheads (Ameiurus spp.), tench (Tinca tinca), and minnows would use wood at the bottom of the reservoir. Bennett (1991) cautioned that the presence of brush and trees in littoral areas could increase the survival of northern pikeminnow, which may reduce the survival of anadromous fish through predation.
Invertebrates also use submerged LWD in lakes and reservoir. These organisms use LWD as attachment sites and some graze on the fungi, algae, and bacteria that grow on the wood. Moring et al. (1986, 1989) and Negus (1987) found that the total biomass of macroinvertebrates was greater in the sediments than on logs in reservoirs. This contrasts with increases in the abundance and diversity of invertebrates living on submerged logs in streams (Maser and Sedell 1994). France (1997) concluded that macroinvertebrates opportunistically colonize wood in boreal lakes, probably for its use as either a biofilm substrate or a predation refuge. Based on these studies, I would expect invertebrates in Rocky Reach Reservoir to be more abundant in sediments than on wood; although submerged LWD likely provides a foraging substrate and predation refuge for some invertebrates.
Little is known about decay rates of LWD in lakes or reservoirs. The few studies available suggest that decay of LWD is a function of the size of the wood, oxygen availability, and the type of wood (Maser and Sedell 1994; Christensen et al. 1996; Bilby et al. 1999). Large boles decay at much slower rates than smaller boles (Harmon et al. 1986; Murphy and Koski 1989). As I indicated
2 Hillman et al. (1989) found large numbers of subyearling chinook salmon closely associated with woody debris jams in large pools in the Wenatchee River. Large pools without woody debris supported fewer chinook. It is quite likely, therefore, that subyearling summer/fall chinook would use woody debris in the Rocky Reach Reservoir.
Draft Study Report Rocky Reach Project No. 2145 November 9, 2000 Page 5 SS/2486 Large Woody Debris earlier, oxygen within the wood affects rates of decomposition. Water-logged parts of LWD decompose in thin layers, starting at the outer surface. As the decomposed surface is removed, oxygen penetrates deeper into the wood, which then becomes food for the decomposers (bacteria and fungi). In backwater areas with high BOD or anoxic conditions, wood decomposition nearly stops. This may occur in areas with large amounts of aquatic macrophytes (e.g., Eurasian milfoil (Myriophyllum spicatum)).
The type or species of wood also affects the rate of decomposition. Bilby et al. (1999) studied the effects of water immersion on the decay of wood from five species of trees. They found that diameter loss for the species ranged from 10.6 mm (western hemlock (Tsuga heterophylla)) to 21.8 mm (bigleaf maple (Acer macrophyllum)) after 5 years. Decreases in the density of surface wood for the five species ranged from 23% (red alder (Alnus rubra)) to 31% (western hemlock). The two hardwood species (bigleaf maple and red alder) generally had higher levels of microbial activity than conifers (Douglas fir (Pseudotsuga menzesii), western hemlock, western redcedar (Thuja plicata)). In general, conifers decay more slowly than deciduous trees (Harmon et al. 1986).
There is virtually no information on the decay of LWD in pelagic areas of lakes or reservoirs. Typically, in deep lakes or reservoirs that stratify, LWD that settles to the bottom decays very slowly because of a lack of oxygen and cold temperatures near the bottom. Rocky Reach is essentially a run-of-the-river project and hence its reservoir does not stratify thermally (CPUD 1991). Therefore, wood that sinks to the bottom of the reservoir in the pelagic area would decay but probably at a slower rate than wood in the littoral zone (W. Minshall, ISU, personal communication). Macroinvertebrates that aid in the decomposition of wood in the littoral areas may be less abundant in the deeper pelagic areas.
3.2 Floating Large Woody Debris LWD that does not sink to the reservoir bottom remains at or near the water surface. It ultimately ends up in backwater areas, on floodplains, in the forebay of Rocky Reach dam, or passes the dam. Although it is unknown what fraction of the driftwood that enters the project area ends up in each location, it appears that most collects in the forebay3, where the PUD mechanically removes it and reduces it to wood chips (until recently the PUD burned all wood removed from the forebay). Because the floodplain is relatively small (CPUD 1991), probably a small fraction of the wood entering the project area ends up there.
LWD that floats into littoral areas increases habitat structure and provides cover for fish. Helfman (1979) studied the effects of floating wood on fish behavior in littoral zones of a small temperate lake. He found that fish densities were significantly higher under and around wood structures than in control areas (i.e., areas with no wood structures). Densities ranged from 12-83 fish/m2 and correlated directly with the size of the wood structure. In Helfman’s (1979) study, bluegill, pumpkinseed, crappie, and shiners were attracted to the structures. These species also live in the Rocky Reach project area (see Hillman 2000).
3 For the past 5 years, the PUD estimates that on average about 108 yrd3/yr (83 m3/yr) of debris has been removed from the trashracks at Rocky Reach Dam (T. Treat, CPUD, personal communication). Debris consists mostly of wood, but tires, bottles, and car parts are also included in the estimate.
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Climatic conditions affect the behavior of fish near floating structures. Helfman (1979) observed that fish more frequently used the structures during clear, calm periods than during overcast and rough-surface conditions. In addition, fish frequented the structures more during the day than at night. Helfman (1979) believed that prey fishes used the structures during clear, calm conditions to avoid predators. Floating wood produces shade, which reduces conspicuousness of prey, while enhancing the ability of the prey to view predators approaching from sunlit surroundings. Conversely, floating wood also provides a shaded region for lurking predators such as bass, trout Oncorhynchus, Salvelinus, and Salmo spp., and northern pikeminnow. In addition, fish-eating birds use floating LWD to increase foraging opportunities.
Floating LWD also provides a substrate for bacteria, fungi, algae, and some macroinvertebrates. These organisms colonize the water-logged side of the LWD and begin to slowly decompose it. During SCUBA observations, Levy et al. (1990) found that bacteria grew on the underside of logs and formed a gelatinous slime-mat 1-3 cm thick. They also observed stands of bacterial slime hanging beneath log bundles. Macroinvertebrates use the wood for attachment and hiding, grazing on fungal-algal communities, and for preying on other organisms (Maser and Sedell 1994).
3.3 Large Woody Debris on the Floodplain Some unknown fraction of the driftwood that enters the project area becomes stranded on the floodplain. Because the Rocky Reach Project operates with only a four-foot drawdown4 (703-707 ft) and most banks along the reservoir are relatively steep (CPUD 1991; Bennett 1991), the floodplain is relatively small. Nevertheless, where it exists, LWD can accumulate there. The role of LWD on the floodplain in the project area is unknown. The fluctuations in water-surface elevation within the usual operating level should not allow most LWD to remain on the floodplain for long periods of time. However, those larger pieces that do remain on the floodplain may play an important role in riparian structure and function.
Maser and Sedell (1994) reported that LWD anchored on the floodplain allows riparian vegetation to establish. Large, well-anchored wood reduces the force of water, causing it to drop part of its suspended sediments on the downstream side of the wood. As sediments accumulate, seedlings become established and grow. And as the vegetation grows, it helps to stabilize banks and reduce erosion. A large diversity of organisms use LWD on floodplains, including insects, small mammals, and birds (Maser and Sedell 1994). These organisms all benefit from LWD on the floodplain.
4 The pool elevation usually fluctuates ≤2 ft on average (W. Fields, Chelan PUD, personal communication). For flood control purposes, the Army Corp of Engineers can order a pool elevation raise in the Rocky Reach Reservoir to 710 ft.
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Large Woody Debris
SECTION 4: SUMMARY
Presently, there is no information on the amount of LWD that enters the project area, nor is there much information on the function and fate of LWD in the reservoir. It appears that most wood enters the project area from upstream locations, such as the Entiat River and from wood that passes Wells Dam. Riparian areas along the reservoir probably contribute little LWD to the project area. Wood that enters the reservoir can submerge in littoral areas or at the bottom of the reservoir, float at or near the water surface, strand on the floodplain, or pass Rocky Reach Dam. Wood that becomes anchored on the floodplain can trap sediments and aid in establishing riparian vegetation. Wood recruited to the reservoir from riparian areas along the pool may stay in the project area for extended periods of time if the wood remains partially attached to the shore.
Both submerged and floating LWD increase habitat structure and provide habitat for fish and macroinvertebrates. Several species of fish use submerged and floating wood for cover. Prey fishes use wood to make themselves less conspicuous to predators, while lurking predators use wood to conceal themselves from potential prey. Predators such as northern pikeminnow, bass, and trout would likely use wood in littoral areas in Rocky Reach Reservoir. Fish may also use wood to increase foraging opportunities (i.e., prey on invertebrates on the wood). Macroinvertebrates use wood for attachment, hiding, grazing, and for preying on other organisms. However, macroinvertebrates appear to be more abundant in sediments than on wood.
The fate of LWD in the project area depends on the location of the wood in the reservoir. Wood that collects in the forebay is mechanically removed and chipped. Wood that ends up on the floodplain decomposes there or water transports it to a new location. Provided they are not resuspended, submerged logs decay in place over long periods of time. The rate of decay depends on wood size, species of wood, and oxygen concentrations in the wood. Conifers decay slower than hardwoods and both resist decay under anoxic conditions. Submerged wood in littoral areas probably decays faster than wood in deeper water. Driftwood may eventually submerge and decay in place, it may pass downstream, or it may deposit on the floodplain.
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SECTION 5: REFERENCES
Bennett, D. 1991. Potential for predator increase associated with a three foot pool rise in Rocky Reach Reservoir, Columbia River, Washington. University of Idaho. Report to Chelan County Public Utility District, Wenatchee, WA.
Beschta, R. 1979. Debris removal and its effects on sedimentation in an Oregon coast range stream. Northwest Science 53:71-77.
Bilby, R. and J. Ward. 1989. Changes in characteristics and function of woody debris with increasing size of streams in western Washington. Transactions of the American Fisheries Society 118:368-378.
Bilby, R. and J. Ward. 1991. Characteristics and function of large woody debris in streams draining old-growth, clear-cut, and second-growth forests in southwestern Washington. Canadian Journal of Fisheries and Aquatic Science 48:2499-2508.
Bilby, R., B. Fransen, and P. Bisson. 1996. Incorporation of nitrogen and carbon from spawning coho salmon into the trophic system of small streams: evidence from stable isotopes. Canadian Journal of Fisheries and Aquatic Sciences 50:164-173.
Bilby, R., J. Heffner, B. Fransen, J. Ward, and P. Bisson. 1999. Effects of immersion in water on deterioration of wood from five species of trees used for habitat enhancement projects. North American Journal of Fisheries Management 19:687-695.
Bisson, P., R. Bilby, M. Bryant, C. Dolloff, G. Grette, R. House, M. Murphy, K. Koski, and J. Sedell. 1987. Large woody debris in forested streams in the Pacific Northwest: past, present, and future. Pages 143-190 in: E. Salo and T. Cundy, editors. Streamside management: forestry and fishery interactions. Contribution No. 57, Institute of Forest Resources, University of Washington, Seattle, WA.
Cederholm, J., M. Kunze, T. Murota, and A. Sibatani. 1999. Pacific salmon carcasses: essential contributions of nutrients and energy for aquatic and terrestrial ecosystems. Fisheries 24:6- 15.
Chelan County Public Utility District (CPUD). 1991. Application for raising the pool elevation from 707’ to 710’. Rocky Reach Hydroelectric Project No. 2145, Wenatchee, WA.
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Christensen, D., B. Herwid, D. Schindler, and S. Carpenter. 1996. Impacts of lakeshore residential development on coarse woody debris in north temperate lakes. Ecological Applications 6:1143-1149.
Cummins, K., D. Botkin, H. Regier, M. Sobel, and L. Talbot. 1994. Status and future of salmon of western Oregon and northern California: management of the riparian zone for the conservation and production of salmon. Draft Research Report, The Center for the Study of the Environment, Santa Barbara, CA.
Franklin, J., K. Cromack, Jr., W. Denison, A. McKee, C. Maser, J. Sedell, F. Swanson, and G. Juday. 1981. Ecological characteristics of old-growth douglas fir forests. USDA Forest Service General Technical Report PNW-188.
France, R. 1997. Macroinvertebrate colonization of woody debris in Canadian shield lakes following riparian clearcutting. Conservation Biology 11:513-521.
Goldman, C. and A. Horne. 1983. Limnology. McGraw-Hill Book Company, New York, NY.
Gurnell, A., K. Gregory, and G. Petts. 1995. The role of coarse woody debris in forest aquatic habitats: implications for management. Aquatic Conservation: Marine and Freshwater Ecosystems 5:143-166.
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Harmon, J., J. Franklin, F. Swanson, P. Sollins, S. Gregory, J. Lattin, N. Anderson, S. Cline, N. Aumen, J. Sedell, G. Lienkaemper, K. Cromack Jr., and K. Cummins. 1986. Ecology of coarse woody debris in temperate ecosystems. Advances in Ecological Research 15:133-302.
Helfman, G. 1979. Fish attraction to floating objects in lakes. Pages 49-57 in: D. Johnson and R. Stein, editors. Response of fish to habitat structure in standing water. North Central Division American Fisheries Society Special Publication 6.
Hillman, T. 2000. Fish community structure and the effects of resident predators on anadromous fish in the Rocky Reach project area. BioAnalysts, Inc. Report to Chelan County Public Utility District, Wenatchee, WA.
Hillman, T., D. Chapman, and J. Griffith. 1989. Seasonal habitat use and behavioral interaction of juvenile chinook salmon and steelhead. I: Daytime habitat selection. Pages 42-82 in: Don Chapman Consultants, Inc. Summer and winter ecology of juvenile chinook salmon and
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steelhead trout in the Wenatchee River, Washington. Report to Chelan County Public Utility District, Wenatchee, WA.
Levy, D., I. Yesaki, and B. Christensen. 1990. Impacts of log storage upon epilimnetic dissolved oxyen and juvenile sockeye salmon in Babine Lake, British Columbia. Water Resources 24:337-343.
Lynch, W., Jr., and D. Johnson. 1989. Influences of interstice size, shade, and predators on the use of artificial structures by bluegills. North American Journal of Fisheries Management 9:219- 225.
Maser, C. and J. Sedell. 1994. From the forest to the sea: the ecology of wood in streams, rivers, estuaries, and oceans. St. Lucie Press, Delray Beach, FL.
Maser, C., R. Tarrant, J. Trappe, and J. Franklin. 1988. From the forest to the sea: a story of fallen trees. USDA Forest Service General Technical Report PNW-GTR-229, Portland, OR.
Moring, J., P. Eiler, M. Negus, and K. Gibbs. 1986. Ecological importance of submerged pulpwood logs in a Maine reservoir. Transactions of the American Fisheries Society 115:335-342.
Moring, J., M. Negus, R. McCullough, and S. Herke. 1989. Large concentrations of submerged pulpwood logs as fish attraction structures in a reservoir. Bulletin of Marine Science 44:609- 615.
Murphy, M. and K. Koski. 1989. Input of woody debris in Alaska streams and implications for streamside management. North American Journal of Fisheries Management 9:427-436.
Negus, M. 1987. The influence of submerged pulpwood on feeding and condition of fishes in a reservoir. Hydrobiologia 148:63-72.
Schuett-Hames, D., A. Pleus, L. Bullchild, and S. Hall. 1994. Timber-fish-wildlife ambient monitoring program manual. Timber, Fish and Wildlife TFW-AM9-94-001, Northwest Indian Fisheries Commission, Olympia, WA.
Sedell, J. and K. Luchess. 1981. Using the historical record as an aid to salmonid habitat enhancement. Pages 210-223 in: N. Armantrout, editor. Acquisition and utilization of aquatic habitat inventory information. Western Division American Fisheries Society, Portland, OR.
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Triska, F. and K. Cromack, Jr. 1980. The role of wood debris in forests and streams. Pages 171- 190 in: R. Waring, editor. Forests: fresh perspectives from ecosystem analysis. Oregon State Press, Corvallis, OR.
Tyrrell, L. and T. Crow. 1994. Structural characteristics of old-growth hemlock-hardwood forests in relation to age. Ecology 75:370-386.
Wege, G. and R. Anderson. 1979. Influence of artificial structures on largemouth bass and bluegills in small ponds. Pages 59-69 in: D. Johnson and R. Stein, editors. Response of fish to habitat structure in standing water. North Central Division of the American Fisheries Society Special Publication 6.
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