The Formation of Channel and Floodplain Habitats - Essay Example

The Formation of Channel and Floodplain Habitats Large Wood (LW), also commonly called large woody debris, are trees or wood pieces captured mostly from surrounding riparian forest, carried off and deposited by streams, and which exert influence on stream channel evolution and important ecological function (Naiman et al., 2002). They are logs with diameter measuring more than 10 cm along 2m of their length, and may be whole trees with intact rootward and limbs or pieces of cut logs at least 1m in length (Schuett-Hames, et al. 1999; Center for Streamside Studies. 1999). Furthermore, they differ from smaller logs in that the latter are more transient and decay rapidly. The abundance of LW in stream channels is an indication of the intimate association of river ecosystems with the surrounding terrestrial environment (Naiman and Bilby 1998, Naiman and Décamps 1997). Understanding this relationship is crucial in river basin management. Large wood abundance in the watershed depends in channel size, channel type, and the surrounding riparian forest. LW is more abundant in small channels on a per unit area basis because LW are easily transported in larger channels; it follows that whilst abundance in large channels is lower, the average LW sizes are bigger (Bilby and Ward 1989). LW is also more abundant where the stream channel is unconstrained and has fine substrate compared to constrained channels with boulder substrate (Bilby and Wasserman 1989). Moreover, LW is more abundant in forests where conifers are dominant compared to forests where hardwoods are dominant because conifers are larger making them less transient (Harmon et al., 1986). Abundance of LW is also more pronounced where the forest is mature than in forest dominated by young stands of small hardwoods (Bilby and Ward 1991). Woodland river ecosystems in their natural and pristine conditions are surrounded by riparian forest and have multiple channels. Over the years, anthropogenic disturbance has greatly reduced riparian forest cover, the topography graded for agricultural purposes, and rivers were constrained by levees into a single channel (Caroll & Robinson, 2007; Florsheim & Mount, 2002). Other than forest denudation, this also has deleterious effects on the morphological development of stream and the stream biota. This is because large wood is an important component of river ecosystems, having a profound influence on the physical structure and development of the stream, the processes of the stream system, the dynamics of the riparian forest, and health of the biological community (Bilby & Ward, 1989). The complex river systems are further simplified by the clearing of large woody debris from stream channels (Hampson et al., 2005). It is only in the last few decades that the significance of LW in the physical and biological structures of streams was recognized (Bilby & Bisson, 1998); even more recently was the identification of the importance of LW in large river channels (Collins and Montgomery, 2002). Influence on river channel geomorphic development and structural stability The effects of large wood on the geomorphic structures of stream and rivers, specifically structural stability and retention, are determined by channel size, the size of LW and its position in the channel (Lienkaemper & Swanson, 1987). Logjams that span across the river channel affect the route of the stream and sediment, the formation of channel and floodplains, and the nutrient storage and supply within the riverine ecosystem (Collins et al, 2002). LW at the site scale is also essential in the creation and maintenance of pool habitats and storage of sediments. Large whole trees in large streams can accumulate smaller wood pieces. Over time and as accumulation builds up, the logjam increases laterally or may be channel spanning which can cause the backwatering of an entire stream reach or increase the number of meanders; these actions increase channel width and complexity of the river (Swanson and Lienkaemper 1978). LW in large streams also creates scour pools and the pools associated with LW are usually up to three times deeper than free-formed pools (Collins and Montgomery, 2002). Pool formation around LW also exhibits increased pool frequency compared to pools in unobstructed alluvial channels (Gurnell & Sweet, 1998). LW in large rivers can also affect the creation and maintenance of vegetated bars and islands (Abbe & Montgomery, 1996). Log jams could either be along channel margins or form at mid-channel. The bar formed in the channel margin limits erosion on the outside meander of the stream and the bar formed is integrated with the floodplain. Meanwhile, the bar formed in mid-channel forms an elliptical bar which diminishes channel migration causing sediments to accumulate in the area; sediment height will eventually exceed the water level and an island is formed. In smaller channels, LW traps substrate to form log steps (Marston, 1982); the log steps facilitate the dissipation of the energy of the river flow and control the hydrological interface between surface water and groundwater (Wondzell & Swanson, 1999). LW in small rivers also increase flow complexity and water retention (Ehrman & Lamberti, 1992) which helps to ease flood peaks while increasing flood peak travel times (Gregory et al, 1985). Logjams in a series were shown to increase the roughness of a reach with resulting reduction in average velocity and increase in water surface elevation. The reduced velocity may increase sediment deposition as well as the frequency of overbank flooding which, in turn, enhance hyporheic flow for better temperature regulation, nutrient processing, and improved habitat (Edwards, 1998). Large wood obstructions are also associated with mineral, nutrient, and sediment storage and retention within the stream channel and along channel margins (Bilby & Ward, 1989). The retention of sediments maintains sediment transport equilibrium especially in steep channels; Beschta (1979) observed that removal of the logjam resulted to increased reach bed scour and channel instability. Wood dams thus increase channel stability; LW provides bank protection especially when it is oriented parallel to the bank (Hampson et al., 2005). Also, wood accumulation provide sites for avulsion and where secondary channels can be initiated (Harwood & Brown, 1993) to develop a stable hierarchy of channels particularly on smaller streams (Gurnell & Linstead, 1999). Influence on floodplain habitat and biodiversity Influence on habitat formation As mentioned earlier, large wood has the tendency to accumulate wood debris which may result to the formation of log jams; log jams may form bars and eventually islands. The deposition of organic matter, accumulation of sediments, moisture retention, and nutrients from decomposing wood provides a “resource node” (Pettit and Naiman 2005) which supports vegetation growth and may lead to the initiation of forest succession (Abbe and Montgomery 1996). Vegetation colonization was observed to stabilize the bars and further increase deposition; the island thus expands to provide more sites for riparian vegetation establishment (Naiman et al., 2000). Influence on riparian floral biodiversity The vegetation diversity of the riparian forest is enhanced by the presence of large wood. LW assists in the development of a variety of landforms and improves the diverse microclimates by promoting structural heterogeneity to the surrounding landscape (Bilby and Bisson 1998). LW also increases the age class variety through the stabilization of the the bank and channel allowing for old-growth vegetation to persist among more dynamic stands (Naiman et al., 2002). Moisture-laden sites are also ideal for germination and early growth of riparian forest species (Naiman et al., 2002) while decomposing logs provide a substrate for conifer regeneration (Thomas et al., 1993), and hemlock and spruce seedling establishment (McKee et al., 1982). LD also provides a microsite where tree seedling can establish without competition from other forest floor plants (Harmon & Franklin 1989), protection from flooding where sites are elevated, and provides moisture during droughts (Naiman et al., 2002). Impact on riparian fauna Approximately two-thirds of riparian wildlife has a need for river corridors at some point in their life cycles. Large wood also impacts riparian fauna – especially small mammals and birds – by providing habitats, shelter from predators or environmental extremes, and sources of food (Steel et al., 1999). Also, riparian wildlife composition is influenced by successional stages of forest development which, as we have seen, may be facilitated by LW (Kelsey and West 1998). Benefits to riverine ecosystem productivity The presence of LW increases stream productivity by increasing food availability and habitat quality (Naiman et al., 2002). Streams usually do not produce their own nutrients and must depend on the influx of nutrients from terrestrial sources such as leaves, needles, and twigs, and dissolved nutrients leached from the adjacent land (Naiman et al., 2002). Large wood traps and retain organic material which invertebrates utilize as food; also, plant and animal detritus settle in pools created by LW to enrich the food resources of benthic invertebrates (Bilby & Bisson 1998). The increased food and prey availability due to increased organic supply and increased invertebrate biomass also enhance fish production (Bilby and Bisson, 1987). Large wood also contributes to improved habitat quality by providing a stable substrate for invertebrates to feed and breed (Harmon et al., 1986). LD also assists in the creation and maintenance of lateral habitats that add “structural and hydraulic diversity near stream margins” (Naiman et al, 2002) which benefits a host of aquatic organisms. The deep pools formed by LD as described earlier are also the preferred habitats of some juveniles (e.g., coho salmon) as it provides them with refuge and cover from extreme flows (Connelly 1997) and protection from terrestrial and aquatic predators (Harvey and Stewart 1991). The deep pools also provide refuge from extreme temperature fluctuation during summer and winter (Tschlapinski and Hartman 1983). The habitat complexity afforded by the presence of LW also contributes to increased fish production by encouraging habitat partitioning which decrease competition in sympatric species (Reeves et al, 1997). Implications on river basin management In the last century, there has been a dramatic increase in natural resource extraction and agricultural production that saw to the large-scale denudation of forest habitats and the oversimplification of ecosystems. This has led to ecological destruction the complexity, severity, and magnitude of which we are have barely come to understand. The interrelationship of adjacent ecosystems is never more so underscored than the interaction of river ecosystem and riparian forest which we are only beginning to see (Collins et al. 2002); specifically, it was only recently that the importance of large wood in maintaining fluvial process and the health of the associated biota (Hampson et al, 2005). These realizations are ushering in efforts to restore stream and aquatic habitats but such efforts must take into account the reestablishment of dynamism between forests, wood recruitment, and wood jams (Collins & Montgomery, 2002). The importance of the role of large wood in fluvial process requires that sustainable river restoration must also consider the recovery of in-channel wood and how the quality of the riparian forest translate into the quality and function of wood jams. Collins and Montgomery (2002) suggest that self-sustaining river systems are dependent on the recruitment of trees large for log jams, access to a large number of recruitable trees, dynamic flow regime, and banks that permit channel migration. “River restoration can be accomplished in a series of stages, from engineered jams initially (1–10 years), to jams initiated by fast-growing largely deciduous pieces (50–100 years), followed in the longer term (more than 100 years) by slower-growing but more durable pieces” (Collins & Montgomery, 2002: p. 246). The strategy couples river and forest restoration and relies on “restoration succession” which aims to restore key processes as a long term solution. River restoration and management that focus only on wood placement approaches and which do not consider riparian forest restoration is but an interim solution. River management schemes will only be sustainable if it include riparian reforestation which will supply a sustainable source of LW to stream channels; this philosophy also accepts the occurrence of natural bank erosion and avulsion as processes that supply LW to the rivers. REFERENCE LIST Abbe, T. B., and Montgomery, D. R. (1996). Large woody debris jams, channel hydraulics and habitat formation in large rivers. Regulated Rivers Research and Management 12:201–221. Beschta R.L. (1979) Debris removal and its effects on sedimentation in an Oregon coast range stream. Northwest Science 53: 71–77. Bilby, R. E. and Bisson, P. A. (1987). Emigration and production of hatchery coho salmon (Oncorynchus kisutch) stocked in streams draining an old-growth and a clear-cut watershed. Canadian Journal of Fisheries and Aquatic Sciences 44: 1397-1407. Bilby, R. E., and Bisson, P. A. (1998). Function and distribution of large woody debris. Pages 324–346 in R. J. Naiman and R. E. Bilby, editors. River ecology and management: lessons from the Pacific Coastal Ecoregion. New York: Springer. Bilby R.E. & Ward J.W. (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. E., and Ward, J. W. (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 Sciences 48: 2499-2508. Bilby, R. E. and Wasserman, L. J. (1989). Forest practices and riparian management in Washington State: data based regulation development. In Gresswell; R. E.; Barton, B. A.; Kershner, J. L., editors. Practical approaches to riparian resource management. Billings, MT: U.S. Bureau of Land Management; 87-94. Carroll, S. and Robinson, E.G. (2007). The effects of large wood on stream channel morphology on three low-gradient stream reaches in the coastal redwood region. USDA Forest Service Gen Tech Rep. PSW-GTR-194. Center for Streamside Studies. (1999). Large woody debris: How much is enough? Fact Sheet, January 2002. University of Washington. Collins, B.D., and Montgomery, D.R. (2002). Floodplain Forests, Wood Jams, and River Restoration in the Puget Lowlands, Washington. Restoration Ecology Vol. 10 No. 2, pp. 237–247. Collins, B. D., Montgomery, D. R. and Haas, A. D. (2002). Historical changes in the distribution and functions of large wood in Puget Lowland rivers. Canadian Journal of Fisheries and Aquatic Sciences 59:66–76. Connolly, P. J. (1997). Influence of stream characteristics and age-class interactions on populations of coastal cutthroat trout. In: Bisson, P. A.; Gresswell, R. E., editors. Searun cutthroat trout: biology, management, and future conservation. Corvallis, OR: Oregon Chapter, American Fisheries Society; 173-174. Ehrman T.P. & Lamberti G.A. (1992) Hydraulic and particulate matter retention in a 3rd-order Indiana stream. Journal of the North American Benthological Society, 11: 341–349. Florsheim, J.L. and Mount, J.F. (2002). Restoration of floodplain topography by sand-splay complex formation in response to intentional levee breaches, Lower Cosumnes River, California. Geomorphology 44: 67–94. Gregory K.J., Gurnell A.M. & Hill C.T. (1985) The permanence of debris dams related to river channel processes. Hydrological Sciences Journal, 30: 371–381. Gurnell A.M. & Linstead C. (1999) Interactions between large woody debris accumulations, hydrological processes and channel morphology in British headwater rivers. In: Hydrology in a Changing Environment, Vol. 1 (Eds H. Wheater & C. Kirby), pp. 381–395. Gurnell, A.M, Piegay, H., Swanson, F.J., and Gregory, S.V. (2002). Large wood and fluvial processes. Freshwater Biology 47: 601–619. Gurnell A.M. & Sweet R. (1998) The distribution of large woody debris accumulations and pools in relation to woodland stream management in a small low-gradient stream. Earth Surface Processes and Landforms, 23: 1101– 1121. Gurnell, A., Tockner, K., Edwards, P., and Petts, G. (2005). Effects of deposited wood on biocomplexity of river corridors. Front Ecol Environ 3(7): 377–382 Harmon, M.E., Franklin, J.F., Swanson, F.J., Sollins, P., Gregory, S.V., Lattin, J.D., Anderson, N.H., Cline, S.P., Aumen, N.G., Sedell, J.R., Lienkaemper, G.W., Cromack, K., and Cummins, K.W. (1986). Ecology of coarse woody debris in temperate ecosystems. Advances in Ecological Research 15: 133-302. Harmon, M. E. and Franklin, J. F. (1989). Tree seedlings on logs in Picea-Tsuga forests of Oregon and Washington. Ecology 70: 48-59. Harwood K. & Brown A.G. (1993) Fluvial processes in a forested anastomosing river: flood partitioning and changing flow patterns. Earth Surface Processes and Landforms, 18: 741–748. Kelsey, K. A. and West, S. D. (1998). Riparian wildlife. In: Naiman, R. J.; Bilby, R. E., editors. River ecology and management: lessons from the Pacific Coastal Ecoregion. New York: Springer-Verlag; 235-258. Lienkaemper G.W. & Swanson F.J. (1987) Dynamics of largewoody debris in streams in old-growth Douglas-fir forests. Canadian Journal of Forest Research, 17: 150–156 Naiman, R.J., Balian, E.V., Bartz, K.K., Bilby, R. and Latterell, J.J. (2002). Dead Wood Dynamics in Stream Ecosystems. USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. Naiman, R. J. and Bilby, R. E., editors. (1998). River ecology and management: lessons from the Pacific coastal ecoregion. New York: Springer-Verlag. Naiman, R. J. and Décamps, H. (1997). The ecology of interfaces—riparian zones. Annual Review of Ecology and Systematics 28: 621-658. Naiman RJ, Bilby RE, and Bisson PA. 2000. Riparian ecology and management in the Pacific Coastal rain forest. BioScience 50: 996–1011. Pettit NE and Naiman PJ. 2005. Flood deposited wood debris and its contribution to heterogeneity and regeneration in a semi-arid riparian landscape. Oecologia. Reeves, G. H., Hall, J. D., and Gregory, S. V. (1997). The impact of land-management activities on coastal cutthroat trout and their freshwater habitats. In: Bisson, P. A.; Gresswell, R. E., editors. Sea-run cutthroat trout: biology, management, and future conservation. Corvallis, OR: American Fisheries Society; 138-144. Schuett-Hames, D. A. E. Pleus, Ward, K., Fox, M., and Light, J. (1999). TFW Monitoring Program Method Manual for the Large Woody Debris Survey. Prepared for the Washington State Department of Natural Resources under Timber, Fish, and Wildlife Agreement. TFW-AM9-99-004. DNR #106. Thomas, J. W., Raphael, M. G., Anthony, R. G., Forsman, E. D., Gunderson, A. G., and Holthavsen, R. S. (1993). Viability assessments and management considerations for species associated with late-successional and old-growth forests of the Pacific Northwest: the report of the scientific analysis team. Portland, OR: USDA Forest Service. Tschlapinski, P. J. and Hartman, G. F. (1983). Winter distribution of juvenile coho salmon (Oncorhynchus kisutch) before and after logging in Carnation Creek, British Columbia, and some implications for overwinter survival. Canadian Journal of Fisheries and Aquatic Sciences 40: 452-461. Wondzell S.M. & Swanson F.J. (1999) Floods, channel change and the hyporheic zone. Water Resources Research, 35: 555–567. Read More