The alteration of floodplains has followed human settlement of river corridors worldwide, but human degradation of river-floodplain complexes has been




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НазваниеThe alteration of floodplains has followed human settlement of river corridors worldwide, but human degradation of river-floodplain complexes has been
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A. Abstract


The alteration of floodplains has followed human settlement of river corridors worldwide, but human degradation of river-floodplain complexes has been unprecedented in the U.S. In the Yankee Fork Salmon River (YFSR), located in Idaho’s Upper Salmon River Subbasin, dredge-mining impacts have altered a 9-km segment of stream and its associated floodplain. Alterations disconnected the stream from its floodplain, simplified in-stream habitat, and restricted riparian vegetation to the dredge/stream interface. Aerial photograph analysis (1945 and 2004 imagery) and ground surveys indicate that the channel was shortened and widened. As a result of the mining activity and subsequent channel responses, the channel bed and banks are armored with coarse sediments. The existing stream-floodplain complex consists of unconsolidated and un-vegetated dredge tailings that offer little habitat for aquatic species. Historically, the YFSR supported large spawning populations of chinook salmon, steelhead, and bull trout, but surveys indicate that the impacted segment now sustains far fewer fish than the upstream reaches and tributaries less affected by mining. For example, both chinook redd counts and parr abundance have been reduced by the dredge-mining activities, and current smolt production (5,000 smolts/yr) for the YFSR is about 5% of its estimated potential (90,000 smolts/yr). This high potential for production, combined with a large amount of existing data, ongoing studies, and cultural significance to the Shoshone-Bannock Tribes, makes the YFSR an ideal candidate for restoration. Here we propose to design a restoration plan to restore natural sediment and hydraulic regimes in the YFSR by redistributing and removing dredge tailing piles, thus reconnecting the river with its floodplain. A conceptual design will allow us to progress towards restoring connectivity that will re-establish critical ecosystem processes, including water, sediment, energy, and nutrient transfers. These processes create habitat and maintain the trophic base for production of endangered salmonids, as well as other native fish and wildlife, in this critical subbasin.


B. Technical and Scientific Background


The alteration of floodplains has followed human settlement of river corridors worldwide, but the degradation of rivers and floodplains in developed nations like the U.S. has been unprecedented (Benke 1990, Tockner and Stanford 2002). Because broad, unconfined floodplains associated with low gradient reaches of rivers were most attractive for development, rivers were straightened or diked to minimize the threat of flooding; these modifications led to the disconnection of rivers from their floodplains. The loss of longitudinal, lateral, and vertical connectivity through channel and flow alteration has diminished the biophysical complexity and ecological processes that historically made these habitats regional “hot-spots” of diversity and production (Stanford 1998, Ward 1998). In particular, severing these floodplain connections has sacrificed critical ecosystem processes, including water, sediment, energy, and nutrient transfers that create habitat and maintain the trophic base for production of endangered salmonids, as well as other native fish and wildlife (Junk et al. 1989, Bunn and Arthington 2002, Baxter et al. 2005).

Mining has been particularly damaging to floodplains and riparian corridors (Kondolf 1994, Moore et al. 1991). Church et al. (1998) suggested that more than 40% of the headwaters of western Rocky Mountain watersheds have been affected by historic mining activities. In particular, gold mining has not only disturbed sediment-water regimes in watersheds, but also led to the contamination of ground and surface waters. Aitken (1997) stated that every cyanide heap mine ever built in the state of Montana eventually leaked and directly polluted aquatic resources. Dredge-mining was a common practice throughout the western U.S., and frequently resulted in entire valley segments being “turned upside down” in the search for gold. Where dredging occurred, rivers were straightened and effectively channelized by dredge pilings, usually composed of coarse alluvium that cannot be moved by the power of the present-day river. Consequently, the river habitat is incised and simplified, and connections to the former floodplain are precluded. Such degradation has occurred in river systems of the Pacific Northwest, where its negative effects of endangered salmonids are of special concern.


T


Figure 1. Location of Yankee Fork Salmon River in central Idaho. A aerial photograph blended with a map inset shows the YFSR watershed. Mining activities associated with Jordan Creek are visible from the aerial photograph


here are few places in the Pacific Northwest where mining impacts on river-floodplain habitat are more apparent than in the Yankee Fork Salmon River (YFSR), located in central Idaho in the upper Salmon River subbasin (Figure 1). A 9-km segment of this river and its floodplain were extensively dredge-mined for gold in the 1930’s and 1950’s (Richards et al. 1992). Once complete, the combined effect of dredge-mining and associated road building disconnected the river from its floodplain, simplified in-stream habitat, and restricted riparian vegetation to the dredge piling/stream interface (Overton et al. 1999). Aerial photograph analysis (1945 and 2004 imagery) and ground surveys indicate that the channel was shortened and widened. As a result of the mining activity and subsequent channel responses, the channel bed and banks are armored with coarse sediments. The existing river-floodplain complex consists of unconsolidated and un-vegetated dredge tailings that offer little habitat for aquatic species (Photo 1). Historically, the YFSR supported large spawning populations of chinook salmon (Oncorhynchus tshawytscha; SYRFS), summer steelhead (O. mykiss; SRUMA-s), and bull trout (Salvelinus confluentus; UPS), but surveys indicate that the impacted segment now sustains far fewer fish than the upstream reaches and tributaries less affected by mining (Overton et al. 1999, Ray and Bacon 2005). For example, both chinook redd counts and parr abundance have been reduced by the dredge-mining activities, and recent smolt production (5,000 smolts/yr) for the YFSR is about 5% of its estimated potential (90,000 smolts/yr; Reiser and Ramey 1987).


Photo 1. Yankee Fork Salmon River dredge tailings





Floodplain segments like those affected by mining on the YFSR have been identified as habitats with high restoration potential and have been targeted as priorities for habitat improvement throughout the Columbia Basin, including the Salmon River subbasin (SSMP 2004, SHIPPUS 2005). Though the science of river restoration is still in its adolescence, reconnecting rivers and their floodplains has been identified as a critical step to restoring the ecosystem processes that lead toward re-expression of habitat capacity (Ebersole et al. 1997, NPPC 2000). Recent examples of this type of work include the North Fork John Day River where dredge tailing piles were removed to reestablish natural floodplain connections (North Fork John Day River Dredge Tailings Restoration BPA Project Proposal No. 199605300; Sanchez 2002) and Resurrection Creek in Alaska’s Kenai Peninsula where over 100,000 m3 of dredge tailing were redistributed and a new channel created; the new channel included the construction of new meanders and adjustment of the floodplain gradient over a 1.6 km reach in 2005 (MacFarlane 2006). On the North Fork of the John Day River, channel form and floodplain elevation were restored at the project site by redistributing over 6,000 m3 of tailings. Removal of tailings allowed the river, isolated for many decades between tailing piles, to once again access a floodplain surface at flows above bankfull. Observations from this project indicated that turbidity quickly returned to reference conditions following the removal and redistribution of the dredge piles. Further, chinook salmon were documented using the redistributed substrate to construct redds just weeks after in-channel work was completed (McKinney and Calame 1994). This river currently sustains 70% of the wild chinook salmon within the John Day basin, and managers are hopeful that this restoration action will enhance production in the subbasin (Sanchez 2002).


Here we propose a similar effort to design a restoration plan for the YFSR that would support natural sediment and hydraulic regimes by redistributing and removing dredge tailing piles and reconnecting the river with its floodplain. The YFSR has high restoration potential, and has been identified as a priority area for habitat improvements (SSMP 2004, SHIPPUS 2005). The size of the drainage, along with the historic presence of floodplain habitat, probably predisposed it to some of the highest salmonid production in the upper Salmon River (Richards et al. 1992, Reeves and Sedell 2001). For example, Buffington et al. (in review) estimated that the YFSR historically provided 10-1


Figure 2. 2004 aerial photo of the lower Yankee Fork Salmon River. The photo indicates the location of the channel (estimated from its center line) in 1945, obtained from 19 September 1945 aerial photography, and 2004. The reference line was used to estimate valley length for sinuosity estimates.
5% of the available chinook salmon spawning habitat in the upper Salmon Basin and the dredged segment of the YFSR also accounts for approximately 75% of the historical chinook spawning habitat in the YFSR and fragments the remaining quality habitat (Overton et al. 1999). Historically, chinook salmon used the YFSR and its tributaries in great numbers. Moreover, it was an important fishery for the Shoshone-Bannock, whose members camped at the mouth of Ramey Creek every summer to harvest spawning salmon (Richards et al. 1992). Walker (1993) ranked the YFSR among the principal traditional fisheries for the Lemhi Shoshone-Bannock. Principal traditional fisheries yielded as many as 60,000 fish per year (Walker 1993). Steelhead also represented a traditional resource in the drainage, and the Shoshone-Bannock Tribes Lower Snake River Compensation Plan Program actively conducts a major supplementation effort to sustain steelhead production (current target 330,000 smolts/year) in the YFSR and it tributaries. The YFSR also provides habitat for fluvial migratory bull trout; a recent radio telemetry study conducted by Idaho Fish and Game (Schoby 2006) showed more than half of the bull trout monitored in the upper Salmon River spent summer months in the drainage. It is likely that the lack of habitat in low-gradient floodplain segments is an important factor limiting production for any of these endangered salmonids (Reeves and Sedell 2001) in the upper Salmon basin. Such habitats are naturally rare in the subbasin, making their restoration all the more important.


We believe that habitat restoration in the YFSR has strong potential to enhance production of endangered salmonids in the upper Salmon basin through a number of processes. Buffington et al. (in review) previously estimated that the YFSR watershed historically provided 10 to 15% of the available chinook spawning habitat within the entire Upper Salmon Subbasin (4th HUC), and 25 to 30% of the spawning habitat (substrate size, channel type) typical to the chinook salmon phenotype (time of spawning, size of spawner) utilizing stream sections in the main Salmon River downstream of Valley Creek down to and including the East Fork Salmon River drainage. Therefore, the future restoration of river-floodplain connectivity in the YFSR would increase the availability and quality of physical habitat for these fish.


Little information is available on pre-settlement channel morphology, but comparison of 1945 and 2004 aerial photographs show the extent of channel relocation, shortening, and loss of sinuosity (Figure 2). The dredged segment currently possesses a large pool density of 0.38 pools/100 m (K. Bacon, SBT, unpublished), which is well below the reference conditions (0.90 pools/100 m for the upper reach; Overton et al. 1999) for this essential habitat feature (e.g., Torgersen et al. 1999). In addition, the channel of the YFSR is presently composed of material significantly coarser than that preferred for Chinook salmon spawning. The observed median particle size is approximately 77 mm, with a geometric mean of approximately 65 mm (Buffington and Barry unpublished data), far greater than the sizes selected by spawning chinook in the Salmon River basin (7-20 mm; Platts et al. 1979). As long as the channel remains confined by dredge piles, the power of the river at high flows winnows away any fine materials, and no spawning-sized substratum can accumulate. Restoring floodplain connectivity would allow deposition of gravels and fine sediments, which would also reduce the turbidity and fine sediment load, both factors that contributed to the YFSR being categorized as impaired and included on the U.S. Environmental Protection Agency’s 303(d) list. Finally, water temperatures in the dredged portion of the YFSR are 3 to 5ºC warmer than upper reaches of the YFSR and West Fork Yankee Fork, with maximum temperatures periodically exceeding criteria for salmonids (Meyer 1996). Overton et al. (1999) attributed high water temperatures in the lower YFSR to below average flows, and increased surface water exposure to solar radiation in widened and poorly vegetated reaches. Restoring floodplain connectivity would lead to greater riparian shading, as well as increased river-groundwater interactions, both of which would buffer the system against temperature increases. Cooler mainstem temperatures, as well as the presence of off-channel habitat influenced by floodplain groundwater, would also benefit salmonids (e.g., Torgersen et al. 1999, Baxter and Hauer 2000, Ebersole et al. 2003).


In addition to the future goal of restoring physical habitat, design outcomes of this project would advance towards re-establishing fluxes of energy and nutrients that are critical to the productivity of linked river-floodplain systems. Loss of floodplain connectivity reduces the exchange of organic matter between terrestrial and aquatic ecosystems (Cummins et al. 1989). Because multiple trophic levels in stream food webs depend on terrestrial carbon sources, this can significantly diminish in-stream productivity (Wallace et al. 1997). Severing stream-riparian connections can also reduce inputs of terrestrial invertebrate prey, which are known to play an important role in the diets and energy budgets of salmonid fishes (Baxter et al. 2005). Moreover, critical nutrient storage and transformation is known to occur on and within floodplains (Stanford and Ward 1993), including retention and processing of marine-derived nutrients (via salmon carcasses; Gende et al. 2002). The proposed restoration on the YFSR would help re-establish all of these important terrestrial-aquatic food web linkages.


Beyond improving habitat characteristics and re-establishing food web linkages, it is possible to generate some estimates of expected increases in endangered salmonid fish production that may result from this restoration effort. Principal among the objectives for the Yankee Fork Salmon River Dredge Tailings Restoration Project (YFSRDTRP; described in detail in Section F) is a goal to enhance populations of anadromous and resident salmonids to their biologic potential. For example, future restoration work would seek to increase chinook salmon smolt production in the YFSR by over an order of magnitude to their estimated potential (90,000 smolts annually; Reiser and Ramey 1987). Similarly, the estimated potential for natural steelhead production is 59,000 smolts annually (Kiefer et al. 1990). While potentials for the estimated increase in bull trout use of the drainage have not been predicted, they are expected based on their current use patterns (Schoby 2006) and given the marginal condition of existing habitat impacted by mining (Upper Salmon Interagency Technical Advisory Team 1998). Accomplishing specific goals for chinook salmon and steelhead and our overall goal of reconnecting the river and floodplain will require a sound understanding of streams and stream processes, the science of ecological restoration, and the needs of the focal species. The working group has extensive experience in all of these components and will adopt the recommended strategies necessary for successful river restoration (see Palmer et al. 2005 and Reeve et al. 2006) and adaptively integrate successful approaches using local (e.g. Richards et al. 1992) and regional (Sanchez 2002 and MacFarlane 2006) restoration examples and recommendations. Further, the use of minimally disturbed reference streams and historic information (e.g. Torgersen et al. 1999, Overton et al. 1999) have guided us in developing our restoration objectives.


The restoration approach, like that of Crooked River, Idaho, proposed here has the potential to serve as an example for similar efforts that may follow in other regions of the Pacific Northwest. Our goal to restore production in the YFSR will use strategies and approaches that reflect the current paradigm shift in river restoration from hard engineering approaches to the restoration of the natural sustainable processes characteristic of healthy functioning ecosystems (Ebersole et al. 2003, Palmer et al. 2005, Reeve et al. 2006). Collectively, our project team has amassed a large dataset documenting conditions in the YFSR and reference watersheds in central Idaho and this existing information is the basis for our assessment and post restoration monitoring program. The success of our project is rooted in this effort and our work will be benefit from the opportunities that this data provides, including the ability to asses responses at the watershed scale using within and paired watershed comparisons. Palmer et al. (2005) emphasized the need for effective post-restoration monitoring in successful river restoration. Surprisingly, Bernhardt et al. (2005) found that such monitoring was associated with only 10% of the 3700 river restoration projects included their global review of river restoration. The synergy with other projects/programs in this drainage, the extensive dataset, and historical and cultural significance of the YFSR, make this watershed a strong candidate for restoration. Our ultimate goal is to disseminate our findings widely and use the lessons learned from this endeavor to inform similar efforts around the region.

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