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29 Historical channel modification and floodplain forest decline: implications for conservation and restoration of a large floodplain river – Willamette River, Oregon
Gravel-Bed Rivers VI: From Process Understanding to River Restoration H. Habersack, H. Pie´gay, M. Rinaldi, Editors r 2008 Elsevier B.V. All rights reserved.
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29 Historical channel modification and floodplain forest decline: implications for conservation and restoration of a large floodplain river – Willamette River, Oregon Stanley Gregory
Abstract Trajectories of change in channel structure and riparian plant communities have been documented for the 273-km mainstem of the Willamette River from Eugene to Portland, OR, USA. We also map current human systems (population density, buildings and roads, public lands, land values, land use) as measures of social opportunities and constraints. We use this channel-change detection and human systems analysis as a basis for spatially explicit prioritization of potential restoration efforts. Priorities for conservation of relatively functional reaches are based on current conditions of the channel and floodplain forest along the river. We also measured the consequences in future alternatives as described by stakeholders in the Willamette River basin. Scenarios of change from 2000 to 2050 were developed for current policies and practices, development alternatives, and conservation options. We compare patterns of recent floods to historical channels to provide estimates of the potential for natural flood processes to restore biophysical structure and function in floodplain rivers. These quantitative evaluations of historical changes and future trajectories of ecological properties of the Willamette River will be used to identify potential strategies for restoration of a large river during a period of rapid population growth. 1.
Introduction
Human history is written in changing landscapes – forests converted to villages and fields, grasslands converted to agriculture, villages coalescing into cities, rivers straightened and contained (Petts, 1990; Hulse and Gregory, 2001). The societies that create such change also depend on these same landscapes for natural resources and a livable environment. Use of certain resources (e.g., gravel, trees, water) leads to the loss of other resources (e.g., fish, macroinvertebrates, amphibians, birds, mammals). E-mail address: [email protected] ISSN: 0928-2025
DOI: 10.1016/S0928-2025(07)11163-9
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As rivers flow by communities, resource managers, politicians, and individual citizens must look to a wide array of specialists for information and advice on their future options for creating a landscape in which they can meet their needs for food, water, land, commerce, transportation, and recreation. Inevitably, competing needs lead to challenging decisions about conserving existing landscape features and restoring landscape elements lost through either natural processes or past human decisions (Petts, 1990; Gregory et al., 1998; Baker et al., 2004). This paper describes historical changes in channel geomorphology in the Willamette River, a large anastomosing river in the Pacific Northwest region of the United States (Gregory et al., 2002a,d; Hulse et al., 2002). The history of channel change and loss of islands will be used to illustrate potential quantitative approaches for designing and communicating opportunities for conservation and restoration of a large river. The management of floodplain rivers commonly focuses on navigation, flood control, water consumption, protection of adjacent property, and channel confinement (Naiman and Decamps, 1990; IFMRC, 1994). Geomorphologists, civil engineers, hydrologists, ecologists, economists, rural land owners, urban land owners, and regional planners often find themselves either representing conflicting interests in decisions that are largely irreversible or being left out of the decision-making process all together (Hyman and Leibowitz, 2000; Baker et al., 2004). Our existing landscapes are mosaics of the outcomes of innumerable past actions by individuals, communities, regional and national government agencies, most with little attention to the overall form and function of a river ecosystem. One of the most common causes of river simplification is the loss of side channels and islands. Several processes can convert reaches with multiple channels to single channels (Petts and Foster, 1985; Gregory, 1992; Gurnell and Petts, 1995). Islands composed of alluvium can be eliminated, either through natural processes of channel erosion or through human removal. Gravel mining is a common practice that reduces or eliminates alluvial islands, but dredging for navigation or flow modification also can cause sediment scour and loss of bars and islands. In this case, the land that once existed as an island no longer exists. A second major process that eliminates islands in rivers is the closing of side channels and hardening of the banks in the vicinity of the channel closure. In this case, most of the area of the island remains but now functions as a river bank contiguous with the adjacent terrestrial ecosystem. Both mechanisms of island loss decrease the hydraulic and geomorphic complexity of the river, but they have much different outcomes in terms of area of floodplain and associated riparian habitat. The major processes of island loss change the hydraulic resistance to flow and geomorphic bedforms (Schumm, 1968; Gurnell and Petts, 1995). The modifications of the river channel have important consequences for ecological processes and aquatic ecosystems (Naiman et al., 1988; NRC, 1992; Van Sickle et al., 2004). Changes in habitat structure alter community composition and population abundance of plants, invertebrates, fish, and other vertebrates. These alterations also change floodplain plant communities and the riparian processes that influence aquatic ecosystems. Changes in hydraulics and bedform also modify hyporheic exchange, the interaction of surface water and subsurface flow (Gregory and Gurnell, 1988; Swanson et al., 1998). This may alter the thermal heterogeneity of the river.
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Examples of biological and thermal patterns in the Willamette River illustrate these potential effects of channel modification. The Willamette River flows north across the Willamette Valley for 273 km before entering the larger Columbia river close to Portland (Gregory et al., 2002a). Local landforms, such as volcanic cones, ridges, deposits of glacial outburst floods, block and confine the river along its length. The flat topography of the valley floor was caused by sediment deposition from a series of floods that inundated much of the Willamette Valley floor during the end of the last glacial period. The upstream portion of the Willamette Valley Eugene to Albany contains an anastomosing channel (Fig. 29.1). The channel in this upper portion of the mainstem remains the most complex reach of the river, but it has been simplified in the last 150 years through channel modification. Eight major tributaries (Middle Fork Willamette River, Coast Fork Willamette River, McKenzie River, Long Tom River, Marys River, Calapooia River, Santiam River, and Luckiamute River) have transported alluvial sediment into a depositional basin created by the blockages of the Salem hills. The low gradient and extensive alluvial deposits result in the anastomosing pattern of this reach of the river. The middle reach of the Willamette River extends from Albany to Newberg. Several large hills and ridgelines from the adjacent Coast Range to the west and Cascade Range to the east exert varying influences on the channel along this reach. As a result, the channel form exhibits a mixture of anastomosing channels and single thread channels. The downstream end of the Willamette River is a simple meandering river channel with lower gradients than the upper river section. The Willamette Falls exert a major control on the river, and the 45 miles (72 km) below Willamette Falls is extremely low gradient and controlled by the backwatering effect of the Columbia River. Complex braided channels are more localized, and lateral changes in the river channel are limited. Most islands in this reach are volcanic outcrops with far less alluvial deposits as in the islands and bars of upper reaches. In this study, we assess the channel change and loss of islands in the Willamette River from 1850 to 2000 and relate differences in channel dynamics to the geology of the basin and spatial patterns of human modification. Transitions from island to lateral floodplain are identified and implications for ecological processes are discussed.
2.
Methods
Channel change from 1850 to 2000 was measured for the 273-km mainstem of the Willamette River from Eugene, OR to its confluence with the Columbia River in Portland, OR. The Willamette River and its major tributaries were surveyed by the U.S. General Land Office (GLO) in the decade after 1850. Engineers surveyed in a grid-based system using a longitudinal axis along the west coast of the United States (the Willamette meridian). The surveyors delineated changes in vegetation, stream channels, and wetlands along monumented section lines and sketched the features within a one-square mile section of land. The Nature Conservancy transcribed the engineering coordinates and notes and we transferred the maps into a GIS database
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Figure 29.1. Map of the 30,000-km2 Willamette River basin, Oregon, USA.
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(1:24,000 scale). The Willamette River network and its riparian vegetation also were mapped in 1895 and 1932 by the U.S. Army Corps of Engineers. These two river surveys were conducted for the entire length of the Willamette River for navigation purposes. We used satellite imagery (Landsat) and digital orthophotographs to construct a map of the Willamette River in 1995 (recently updated for 2000). The river channels were classified as primary channel (unbraided, or the portion of the channel with the most flow), side channel (channels connected to the mainstem at both ends; this includes the small channels that form islands), and alcoves or sloughs. The latter are ‘‘blind’’ channels, i.e., they are connected only at one end to a primary, side, or secondary channel. Islands and gravel bars were identified from these surveys and represented in the GIS layers for each year. Channel change was quantified by estimating both length and area of the major channel types between years. This paper will focus on the overall change in the river channel and islands from 1850 to 1995. Change in island extent was measured first as an overall change in area of islands. This measure of island change cannot differentiate between sources of island loss (i.e., elimination of the area of the island or the elimination of a side channel converting the island into the terrestrial margin or riverbank). To distinguish these two processes, we visually examined each island along the 273-km length of the river and classified it into five major classes: (i) unchanged – original island largely the same as 1850, (ii) remnant – portion of the original island still existing, (iii) lost from remnant – portion of remnant that was lost, (iv) extinct – island no longer present and original area occupied by river, and (v) transformed – island no longer present but land area now adjacent to river. The data were converted to percentage of the original area of islands in 1850. No new islands were observed. Results are presented for the entire river and the three major geomorphic reaches. Thermal patterns of the Willamette River currently are being investigated and we present an example from a recent study to illustrate the consequence of island and channel change on the thermal heterogeneity of a large river. Thermal data loggers were deployed for 1 week at more than 50 locations in an island reach of the upper Willamette in late July 2005. Daily maxima for all stations were determined and used to identify areas of likely exchange of water from the hyporheos to the river.
3. 3.1.
Results Changes in channel and island area
From 1850 to 1995, total area of river channels decreased by 22% (Table 29.1, Gregory et al., 2002a,b) and total length of all channels decreased by 26%. More than 30% of alcoves and sloughs were lost by 1995. Islands diminished more than any other channel type. Total area of islands decreased by 63% over the 145-year interval. These changes differed along the length of the Willamette River. The upstream reach from Albany to Eugene experienced the greatest change in channel structure and loss of islands. This reach flows through broad alluvial and
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Table 29.1. Area of channel types and islands in 1850 and 1995 reported in hectares for the three reaches of the Willamette River and the combined length of the mainstem river. River section 1850 Upper Middle Lower Total mainstem 1995 Upper Middle Lower Total mainstem Percent change Upper Middle Lower Total mainstem
Primary channel
Secondary channel
Alcoves
Islands
Total area of channel
1946 2406 1473 5825
1059 309 110 1478
181 81 10 272
6897 1946 122 8965
10,083 4742 1715 16,540
1533 2115 1406 5054
280 209 169 658
103 71 2 176
1413 1777 117 3307
3329 4172 1694 9195
–73.5 –33.1 53.6 –55.6
–41.5 –15.5 –80.0 –35.1
–21.3 –12.1 –4.6 –13.3
–79.5 –8.6 –3.0 –63.1
–39.8 –14.5 –1.1 –22.3
Source: Modified from Gregory et al. (2002). Note: Percent change from 1850 to 1995 is reported following the areas for 1850 and 1995 (negative values for percent change represent loss of area). Total area of channel includes areas of the wetted channel types plus area of islands.
glacial flood deposits. During the period from 1850 to 1995, total area of river channels and islands (combined) in this reach decreased by 40% (Table 29.1). The total length of all channels decreased from 340 to 185 km (Gregory et al., 2002a; http://oregonstate.edu/dept/pnw-erc). More than 70% of the side channels and 40% of the alcoves were lost. Approximately 80% of the islands in this reach have been eliminated or converted to floodplain banks. The middle reach from Albany to Newberg exhibited variable patterns of channel change and was intermediate in loss of channels and islands. Basaltic outcrops and ridges from the foothills caused local constraint and channel control in the middle reach. The channel of the Willamette River in this reach has been simplified, though not to the extent observed in the upper river. Total area of channels and islands combined decreased by approximately 15%. Most of that change occurred through loss of primary channels and islands, though the proportional change was greatest for side channels (Table 29.1). Total length of channels did not change. The lower reach from Newberg to the mouth in Portland changed very little from 1850 to 1995. Lava flows from central Oregon created a basaltic trench in the lower reach more than 10 million years ago and make it less vulnerable to natural and human alteration. Though proportional changes were substantial, changes in area of major channel features were minor. Total area of channels and islands combined decreased by 22%, but even in 1850 the area of these geomorphic features was substantially less than the upper two reaches. Total length of channels increased from 69 to 76 km.
Historical channel modification and floodplain forest decline 3.2.
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Loss of Islands
Our analysis of islands in the Willamette River in 1995 revealed that very few islands from 1850 remain intact and unchanged today (Table 29.2). For the total Willamette River mainstem, only 1% of the area of islands remains unchanged. One-third of the island area now exists as a remnant of an island of 1850. The portion of those remnant islands that has been lost accounts for 14% of the island area in 1850. Almost half of the islands have become the banks of the river along the floodplain. However, only 5% of the islands of 1850 have been totally eliminated. This pattern of island change differs greatly for the three major reaches of the Willamette River (Table 29.2). In the upper river, where most of the overall change in islands has been observed, most of the island area has become riverbank. None of the original islands remain unchanged. Remnant islands account for almost 20% of the 1850 island extent, and almost 25% of the island area has been truly lost (extinct plus loss from remnant islands). In sharp contrast, the middle reach of the river predominantly (84%) exists as remnants of the islands of 1850. Only 11% of the 1850 island extent has been converted to floodplain riverbank. Less than 2% of the area of islands in 1850 has been truly lost in the middle reach. The lower reach, where relatively little channel change occurred, exhibits a totally different pattern, with a substantial proportional creation of new islands (16% of the area of 1850 islands). Over the same interval, 44% of the 1850 island area has been totally eliminated and another 13% has been lost from remnant islands. These patterns of island change indicate that analysis of channel change requires closer inspection than simple change in island extent. Human activity and natural geomorphic processes can cause markedly different processes of island alteration to occur. In the Willamette River, most of the change in islands did not occur in the areas of greatest human density, though human activities were responsible for much of the observed change. Portland and Salem (located in the lower and middle reaches, respectively) are the largest cities along the Willamette River. These reaches exhibited only 3 and 9% loss of island area since 1850. The upper reach has been shaped by depositions of sediments from glacial outburst floods 15,000–20,000 years ago and subsequent deposition of alluvial sediment. This section of the river contained the greatest length of anastomosing channels and associated islands. The major land use in this reach of river is agriculture with small cities distributed along its length. Secondary channels have been blocked, some levees have been built, and Table 29.2.
Sources of change in island extent from 1850 to 1995.
Reach
New
Extinct Transformed
Unchanged
Remnant
Loss from remnant
Upper Middle Lower Total mainstem
1.1 4.4 15.5 2.1
5.3 1.2 43.7 5.3
0.0 3.6 8.3 0.9
19.2 84.0 35.5 33.3
18.3 0.4 12.5 14.3
57.3 10.7 0.0 46.1
Note: Changes are expressed as percent of the total area of islands in 1850.
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numerous banks have been hardened with riprap. We mapped the locations where banks had been hardened with boulders and found that only 26% of the length of the mainstem river had been hardened. However, these structures have been placed in the most dynamic locations along the channel, occurring on 67% of the major meanders in the river (Gregory et al., 2002c). The geomorphic structure of the river in both 1850 and 1995 largely reflects the template created by the fluvial geomorphology and local influences of valley landforms in the different reaches. Humans have greatly altered the geomorphology of the Willamette River but not primarily in the areas of highest population density of human populations.
3.3.
Prioritization of future conservation and restoration actions
In 1850, the 273-km distance along the mainstem Willamette River from Eugene, OR downstream to Portland, OR contained 571 km of channels (e.g., side channel, sloughs, or alcoves) (Gregory et al., 2002a). By 1995, the length of channels over this distance had decreased to 424 km, a 25% loss of channel complexity. The floodplain forest along the river was changed even more by land conversion. In 1850, more than 4360 ha of floodplain forest surrounded the 273-km distance along the mainstem. By 1990, the extent of floodplain forest had been reduced to 1197 ha, a loss of 73% of the historical floodplain forest area. These changes in the river and its floodplain potentially alter the abundance and distribution of fish communities in the mainstem river. Our field studies in the Willamette River have demonstrated that fish abundance is approximately 50% greater in reaches with multiple channels or tributary junctions. In addition, we find approximately 20 fish species in a 1-km reach of the Willamette River that contains multiple channels or tributary junctions, but we find less than 17 species in the single-thread channels. We incorporated these relationships into an analysis of potential for future restoration in the Willamette River. We created a spatial framework for the assessment of the river and its floodplain by dividing the area of river that has been inundated between 1850 and the present into 1-km slices along the central axis of the channel (Fig. 29.2). We evaluated the biophysical potential for increased channel complexity, floodplain forest, and flood storage (through removal of revetments) based on the difference between 1850 and 1995 for each 1-km slice (Hulse and Gregory, 2004). We identified areas of socioeconomic obstacles for the same areas based on population density, buildings and roads, property value, and public land. Based on these two constraints on restoration success, we created a template to illustrate areas of higher potential for ecological restoration along the Willamette River (Fig. 29.3). This framework is now being used by the Willamette Partnership and other agencies and citizens groups to guide conservation and restoration efforts in the Willamette Valley.
3.4.
Consequences for thermal heterogeneity in river water
Channel avulsion and floodplain coalescence can alter the extent, position, and function of islands (Pie´gay and Bravard, 1997; Pie´gay, 1998). The distinction between
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slice 0
Portland Metro
Oregon
slice 50 slice 150 Albany slice 100
Salem/Keizer
Corvallis
Willamette River Basin Albany
Peoria
Corvallis
slice 200
slice 200
Harrisburg
Eugene/ Springfield
Figure 29.2. Longitudinal framework for analysis of conditions within the historical floodplain in 1-km ‘‘slices’’ perpendicular to floodplain axis. For scale, distances between lines along the floodplain axis are 1 km and numbering begins near the confluence of the Willamette River and the Columbia River and extends 227 km upstream to above the confluence with the McKenzie River. These 1-km slices were used for prioritization of locations for river restoration. (Reproduced with permission from Hulse and Gregory, 2004, Springer.)
true loss of island area and transformation of islands to riverbanks has important geomorphic and ecological implications. To explore the impact of physical heterogeneity on thermal patterns in the river water, we placed temperature data loggers around island features in the upper Willamette River. Most of the habitats associated with the primary channel exhibited maximum temperatures ranging from 19.1 to 19.81C. Temperature did not differ greatly with depth (o11C), indicating turbulent flow and thorough mixing. However, several springs were observed emerging from the floodplain and gravel bar that exhibited maximum temperatures of 14.6, 15.4, and 15.81C. We investigated five additional reaches during 2005 and all reaches contained cold water habitats with temperatures ranging from 2.0 to 8.01C colder than the mainstem. These colder springs most likely represent hyporheic exchange with the surface water and are created by flow through the gravel bar and transport through the alluvium of islands and gravel bars. In the Willamette River, such subsurface exchange provides important cold water refuges for coldwater species, such as Cutthroat trout (Oncorhynchus clarki clarki), Chinook salmon (Oncorhynchus tshawytscha), and Steelhead (Oncorhynchus mykiss). Loss of island features is likely to reduce the frequency and thermal influence of the subsurface
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90 100 110 120 130 140
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190 200 210
Demographic and Economic Constraints Index
1.00
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Slice 198 0.50
Slice 190 0.00
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Figure 29.3. Illustration of high priority sites for restoration based on potential improvement on (1) increased channel complexity, (2) increased area of floodplain forest, (3) increased non-structural flood storage. (Reproduced with permission from Hulse and Gregory, 2004, Springer.) Dark gray bands indicate areas with high ecological potential for restoration and low socioeconomic obstacles. White bands are areas with high ecological potential for restoration and high socioeconomic obstacles. Light gray bands indicate areas with low ecological potential for restoration and low socioeconomic obstacles, and medium gray bands are areas with low ecological potential for restoration and high socioeconomic obstacles. For scale, distances between lines along the floodplain axis are 1 km.
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exchange and lead to thermal simplification and warmer overall river temperatures (Gregory and Bisson, 1996).
4.
Discussion and conclusions
Human modification of the river and its basin has simplified the Willamette River from 1850 to the present. Land owners and agencies directly blocked side channels with boulders, wood pilings, and fill in an attempt to prevent channel change and loss of property along the river. In addition, 26% of the length of the river has been hardened by lining the bank with boulders (riprap) on either one bank or both banks. In addition, gravel has been mined from the river since the late 1800s and aggregate industries continue to mine gravel bars in the main channel. Straightening the channel, eliminating side channels, and removing gravel increase channel degradation further isolate the active channel from its floodplain and accelerates the loss of side channels and islands. Klingeman (1973) analyzed changes in the relationship between staff gage elevation and discharge at gaging stations along the Willamette River. He concluded that the mainstem Willamette River was changing rapidly and was not in a dynamic equilibrium hydrologically. Most sites exhibited streambed degradation, though lateral channel adjustment was noted at some sites. The rate of channel degradation or downcutting for the mainstem river was estimated to be approximately 0.3 m/decade. Klingeman (1973) cited upstream dams, gravel mining in the river, and streambank hardening (i.e., riprap) as likely mechanisms responsible for the observed channel degradation, which is consistent with our observations of channel change in the Willamette River since 1850. Two additional factors – changes in discharge and sediment delivery to the mainstem river – are potential mechanisms for channel simplification in the Willamette River over the last 150 years. Flood-control dams alter the frequency of floods that exceed the bankfull channel (Magilligan et al., 2003) and reduce sediment delivery into downstream rivers (Ligon et al., 1995). Both of these processes could contribute to channel incision and loss of lateral channel complexity in the Willamette River. Flood management currently reduces the peak discharges and maintains high flows at close to bankfull discharge but less than floodplain inundation discharges. This focuses the stream power within the active channel and may lead to channel incision. Likewise, reduction of sediment input into the mainstem river reduces the gravel load and causes continued erosion of older gravels deposits within the mainstem river. While dams may currently contribute to channel simplification, they could only account for a minor portion of historical channel change because they were not built until after 1945. Channel simplification in the Willamette River was clearly evident by 1895 and 1932, thus human alteration or other mechanisms were influencing the channel morphology prior to flood control dams. One final factor that could influence the changes in the Willamette River is regional climate change. Peak discharges in the Willamette River have declined since the late 1800s (Fig. 29.4), but mean annual discharge has remained relatively constant. Flood-control reservoir were built in the basin after 1945, therefore the decline in peak flows between 1860 and 1945 cannot be attributed to dams. After 1965, flood
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Mean Daily Discharge (cms)
6500
5500
4500
3500
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1500 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 Year Figure 29.4. Daily mean discharges during floods greater than 1982 cms at the Albany, OR gaging station from 1895 to 2006 (data from USGS gaging station 14174000). The estimated 2-year recurrence discharge under flow regulation is 1982 cms; the 2-year unregulated flow is estimated to be 3200 cms. (Data from Portland, OR Office of USACE). All daily mean discharges that exceeded the 2-year regulated discharge are plotted.
control reservoirs reduce most peak flows in the mainstem Willamette River by 30–40%. Flow regulation has decreased 2-year recurrence flows at the Albany gaging station from 3200 to 1980 cms (hydrologic record from 1895 to 2006). Reduced peak flows over the last 150 years could increase the potential for incision and channel simplification (as previously discussed). One of the major tools in floodplain restoration is the re-establishment of hydrologic and geomorphic processes to the extent possible and in river reaches where the outcomes are likely and the ecological benefits are significant (NRC, 1992; Hulse and Gregory, 2004). Restoration of complex braiding or anastomosing channels can be accomplished by reducing lateral human-created constraints along the banks, increasing delivery of sediments, restoring hydrologic regimes, and reconnecting historical channels. The degree of success will depend on the local hydraulic and geomorphic processes and the degree to which human modifications can be reversed or reduced. There is a tendency to consider human manipulations in restoring river complexity, but natural flood processes may achieve such restoration more consistently and effectively than costly and time-consuming engineering approaches. Passive restoration through natural hydrologic and geomorphic processes obviously requires the reduction of human constraints, but it also requires the acceptance of dynamic and relatively unplanned and undirected channel change. This latter requirement commonly is the most difficult to achieve. Scientists who study gravel-bed rivers appreciate the
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efficiency and effectiveness of using natural geomorphic and ecological processes in river restoration, but incorporation of these approaches into river management will require new and more effective methods of illustration and communication between collaboration of river scientists, river managers, and regional communities. One possible interpretation of the findings of this study could be that the total area of floodplain has not been greatly diminished even though 63% of the total area islands has been lost. Many of the islands became floodplains adjacent to the mainstem river, and the secondary channels were blocked and filled. Total elimination of islands occurred primarily in the lower, simpler reach of the river near Portland (44% became extinct). In the more complex upper reach of the river, 57% of the islands in 1850 were transformed to current adjacent floodplains. These areas where islands became floodplains still support many potential floodplain functions (e.g., flood detention, soil deposition, wildlife habitat, riparian forests), though land use has converted many of these floodplains to crops, residences, or urban areas and altered the potential floodplain functions. Ecological functions of complex anastomosing or braided river channels extend far beyond the functions of islands as areas of floodplain. These channels provide extensive edge habitat with variable depths and velocities, secondary channels with greater roughness and reduced velocity, numerous gravel bars, alcoves, and sloughs. Such areas exhibit greater hyporheic exchange and provide mosaics of cold and warm water habitats. In addition, the greater roughness of such complex channels dissipates energy during floods and provides critical refuges for aquatic communities. Loss of channel complexity, secondary channels and islands potentially diminishes the species richness and population abundances of aquatic organisms in large rivers. Restoration of such complexity cannot be achieved simply by carving a few alcoves or side channels in an artificially hardened river. Restoration of river complexity requires a broader perspective of the river in which it is allowed to meander within its floodplain, erode and deposit alluvial sediments, and recruit wood and sediment dynamically from adjacent forests during floods. Communities must minimize channel hardening where possible and avoid development of costly buildings and roads in the most dynamic portions of the landscape. This can be best accomplished by protecting existing floodplain functions to the greatest extent possible and taking advantage of river changes that occur to major flood events to back away from the river and allow it to restore itself through geomorphic processes.
Acknowledgements This work was funded by STAR grant R825797 between the U.S. Environmental Protection Agency and Oregon State University. Data on historical river changes and land use/land cover were developed as part of cooperative agreement CR824682 between USEPA and OSU. Supplementary information and the digital datasets referenced herein are available via the Internet at http://www.orst.edu/dept/ pnw-erc/
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References Baker, J.P., Hulse, D.W., Gregory, S.V., et al., 2004. Alternative futures for the Willamette river basin. Ecol. Appl. 14, 313–324. Gregory, K.J., Gurnell, A.M., 1988. Vegetation and river channel form and process. In: Viles, H.A. (Ed.), Biogeomorphology. Basil Blackwell, Oxford, pp. 11–42. Gregory, K.J., 1992. Vegetation and river channel process interactions. In: Boon, P.J., Calow, P., Petts, G.E. (Eds), River Conservation and Management. John Wiley and Sons, Chichester, UK, pp. 255–269. Gregory, S., Ashkenas, L., Oetter, D., et al., 2002a. Mainstem river. In: Hulse, D.W., Gregory, S.V., and Baker, J.P. (Eds), Willamette River Basin: Trajectories of Environmental and Ecological Change. Oregon State University Press, Corvallis, OR, pp. 112–113. Gregory, S., Ashkenas, L., Oetter, D., et al., 2002b. Longitudinal patterns – channel. In: Hulse, D.W., Gregory, S.V., and Baker, J.P. (Eds), Willamette River Basin: Trajectories of Environmental and Ecological Change. Oregon State University Press, Corvallis, OR, pp. 134–135. Gregory, S., Ashkenas, L., Oetter, D., Wildman, K., 2002c. Revetments. In: Hulse, D.W., Gregory, S.V., and Baker, J.P. (Eds), Willamette River Basin: Trajectories of Environmental and Ecological Change. Oregon State University Press, Corvallis, OR, pp. 138–139. Gregory, S.V., Ashkenas, L.R., Minear, P., 2002d. Application of analysis of historical channel change in the restoration of large rivers. Verhandlungen der Internationale Vereinigung Limnologie 27 (7), 4077–4086. Gregory, S.V., Bisson, P.A., 1996. Degradation and loss of anadromous salmonid habitat in the Pacific Northwest. In: Stouder, D. and Naiman, R.J. (Eds), Pacific Salmon and Their Ecosystems: Status and Future Options. Chapman Hall, pp. 277–314. Gregory, S.V., Hulse, D.W., Landers, D.H., Whitelaw, E., 1998. Integration of biophysical and socioeconomic patterns in riparian restoration of large rivers. In: Wheater, H. and Kirby, C. (Eds), Proceedings of the British Hydrological Society International Conference, Exeter: Hydrology in a Changing Environment. Wiley, Chichester, UK, pp. 231–247. Gurnell, A., Petts, G., 1995. Changing River Channels. Wiley, Chichister, UK, 442pp. Hulse, D., Gregory, S.V., 2001. Alternative futures as an integrative framework for riparian restoration of large rivers. In: Dale, V.H. and Haeuber, R. (Eds), Applying Ecological Principles to Land Management. Springer-Verlag, New York, NY, pp. 194–212. Hulse, D., Gregory, S.V., 2004. Integrating resilience into floodplain restoration. Urban Ecosyst. 7, 295–314. Hulse, D., Gregory, S.V., Baker, J. (Eds), 2002. Willamette River Basin Planning Atlas: Trajectories of Environmental and Ecological Change. Oregon State University Press, Corvallis, OR, p. 178. Hyman, J.B., Leibowitz, S.G., 2000. A general framework for prioritizing land units for ecological protection and restoration. Environment. Manage. 25 (1), 23–35. Interagency Floodplain Management Review Committee (IFMRC), 1994. Sharing the challenge: floodplain management into the 21st century. Washington, DC. Klingeman, P.C., 1973. Indications of streambed degradation in the Willamette Valley. Project completion report (OWRR Project A-016-ORE) submitted to Office of Water Research, U.S. Department of Interior. 99pp. Ligon, F.K., Dietrich, W.E., Trush, W.J., 1995. Downstream ecological effects of dams. BioScience 45 (3), 183–192. Magilligan, F., Nislow, K.H., Graber, B.E., 2003. Scale-independent assessment of discharge reduction and riparian disconnectivity following flow regulation by dams. Geology 31 (7), 569–572. Naiman, R.J., Decamps, H., 1990. The Ecology and Management of Aquatic–Terrestrial Ecotones. UNESCO-MAB, Paris. Naiman, R.J., Decamps, H., Pastor, J., Johnston, C.A., 1988. The potential importance of boundaries to fluvial ecosystems. J. N. Am. Benthol. Soc. 7, 289–306. National Research Council (NRC), 1992. Restoration of Aquatic Ecosystems. National Academy Press, Washington, DC. Petts, G. and Foster, I., 1985. Channel morphology. Pages 140–174 In: River and Landscape. Edward Arnold Publishers, London, 275pp.
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Petts, G.E., 1990. The role of ecotones in aquatic landscape management. In: Naiman, R.J. and Decamps, H. (Eds), The Ecology and Management of Aquatic–Terrestrial Ecotones. UNESCO, Paris/Parthenon, Carnforth, UK, pp. 227–261. Pie´gay, H., 1998. Interactions between large woody debris and meander cutoff (example of the Mollon Site on the Ain River, France). Z. Geomorphol. 42, 187–208. Pie´gay, H., Bravard, J.P., 1997. Response of a Mediterranean riparian forest to a 1 in 400 year flood, Ouveze River, Drome-Vaucluse, France. Earth Surf. Process. Landf. 22 (1), 31–43. Schumm, S.A., 1968. River adjustment to altered hydrological regime – Murrumbidgee River and paleochannels, Australia. United States Geological Survey, Professional Paper 598. 62pp. Swanson, F.J., Johnson, S.L., Gregory, S.V., Acker, S.A., 1998. Flood disturbance in a forested mountain landscape. BioSci. 48 (9), 681–689. Van Sickle, J., Baker, J., Herlihy, A., et al., 2004. Projecting the biological condition of streams under alternative scenarios of human land use. Ecol. Appl. 14, 368–380.
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29 Historical channel modification and floodplain forest decline: implications for conservation and restoration of a large floodplain river – Willamette River, Oregon Stanley Gregory
Abstract Trajectories of change in channel structure and riparian plant communities have been documented for the 273-km mainstem of the Willamette River from Eugene to Portland, OR, USA. We also map current human systems (population density, buildings and roads, public lands, land values, land use) as measures of social opportunities and constraints. We use this channel-change detection and human systems analysis as a basis for spatially explicit prioritization of potential restoration efforts. Priorities for conservation of relatively functional reaches are based on current conditions of the channel and floodplain forest along the river. We also measured the consequences in future alternatives as described by stakeholders in the Willamette River basin. Scenarios of change from 2000 to 2050 were developed for current policies and practices, development alternatives, and conservation options. We compare patterns of recent floods to historical channels to provide estimates of the potential for natural flood processes to restore biophysical structure and function in floodplain rivers. These quantitative evaluations of historical changes and future trajectories of ecological properties of the Willamette River will be used to identify potential strategies for restoration of a large river during a period of rapid population growth. 1.
Introduction
Human history is written in changing landscapes – forests converted to villages and fields, grasslands converted to agriculture, villages coalescing into cities, rivers straightened and contained (Petts, 1990; Hulse and Gregory, 2001). The societies that create such change also depend on these same landscapes for natural resources and a livable environment. Use of certain resources (e.g., gravel, trees, water) leads to the loss of other resources (e.g., fish, macroinvertebrates, amphibians, birds, mammals). E-mail address: [email protected] ISSN: 0928-2025
DOI: 10.1016/S0928-2025(07)11163-9
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As rivers flow by communities, resource managers, politicians, and individual citizens must look to a wide array of specialists for information and advice on their future options for creating a landscape in which they can meet their needs for food, water, land, commerce, transportation, and recreation. Inevitably, competing needs lead to challenging decisions about conserving existing landscape features and restoring landscape elements lost through either natural processes or past human decisions (Petts, 1990; Gregory et al., 1998; Baker et al., 2004). This paper describes historical changes in channel geomorphology in the Willamette River, a large anastomosing river in the Pacific Northwest region of the United States (Gregory et al., 2002a,d; Hulse et al., 2002). The history of channel change and loss of islands will be used to illustrate potential quantitative approaches for designing and communicating opportunities for conservation and restoration of a large river. The management of floodplain rivers commonly focuses on navigation, flood control, water consumption, protection of adjacent property, and channel confinement (Naiman and Decamps, 1990; IFMRC, 1994). Geomorphologists, civil engineers, hydrologists, ecologists, economists, rural land owners, urban land owners, and regional planners often find themselves either representing conflicting interests in decisions that are largely irreversible or being left out of the decision-making process all together (Hyman and Leibowitz, 2000; Baker et al., 2004). Our existing landscapes are mosaics of the outcomes of innumerable past actions by individuals, communities, regional and national government agencies, most with little attention to the overall form and function of a river ecosystem. One of the most common causes of river simplification is the loss of side channels and islands. Several processes can convert reaches with multiple channels to single channels (Petts and Foster, 1985; Gregory, 1992; Gurnell and Petts, 1995). Islands composed of alluvium can be eliminated, either through natural processes of channel erosion or through human removal. Gravel mining is a common practice that reduces or eliminates alluvial islands, but dredging for navigation or flow modification also can cause sediment scour and loss of bars and islands. In this case, the land that once existed as an island no longer exists. A second major process that eliminates islands in rivers is the closing of side channels and hardening of the banks in the vicinity of the channel closure. In this case, most of the area of the island remains but now functions as a river bank contiguous with the adjacent terrestrial ecosystem. Both mechanisms of island loss decrease the hydraulic and geomorphic complexity of the river, but they have much different outcomes in terms of area of floodplain and associated riparian habitat. The major processes of island loss change the hydraulic resistance to flow and geomorphic bedforms (Schumm, 1968; Gurnell and Petts, 1995). The modifications of the river channel have important consequences for ecological processes and aquatic ecosystems (Naiman et al., 1988; NRC, 1992; Van Sickle et al., 2004). Changes in habitat structure alter community composition and population abundance of plants, invertebrates, fish, and other vertebrates. These alterations also change floodplain plant communities and the riparian processes that influence aquatic ecosystems. Changes in hydraulics and bedform also modify hyporheic exchange, the interaction of surface water and subsurface flow (Gregory and Gurnell, 1988; Swanson et al., 1998). This may alter the thermal heterogeneity of the river.
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Examples of biological and thermal patterns in the Willamette River illustrate these potential effects of channel modification. The Willamette River flows north across the Willamette Valley for 273 km before entering the larger Columbia river close to Portland (Gregory et al., 2002a). Local landforms, such as volcanic cones, ridges, deposits of glacial outburst floods, block and confine the river along its length. The flat topography of the valley floor was caused by sediment deposition from a series of floods that inundated much of the Willamette Valley floor during the end of the last glacial period. The upstream portion of the Willamette Valley Eugene to Albany contains an anastomosing channel (Fig. 29.1). The channel in this upper portion of the mainstem remains the most complex reach of the river, but it has been simplified in the last 150 years through channel modification. Eight major tributaries (Middle Fork Willamette River, Coast Fork Willamette River, McKenzie River, Long Tom River, Marys River, Calapooia River, Santiam River, and Luckiamute River) have transported alluvial sediment into a depositional basin created by the blockages of the Salem hills. The low gradient and extensive alluvial deposits result in the anastomosing pattern of this reach of the river. The middle reach of the Willamette River extends from Albany to Newberg. Several large hills and ridgelines from the adjacent Coast Range to the west and Cascade Range to the east exert varying influences on the channel along this reach. As a result, the channel form exhibits a mixture of anastomosing channels and single thread channels. The downstream end of the Willamette River is a simple meandering river channel with lower gradients than the upper river section. The Willamette Falls exert a major control on the river, and the 45 miles (72 km) below Willamette Falls is extremely low gradient and controlled by the backwatering effect of the Columbia River. Complex braided channels are more localized, and lateral changes in the river channel are limited. Most islands in this reach are volcanic outcrops with far less alluvial deposits as in the islands and bars of upper reaches. In this study, we assess the channel change and loss of islands in the Willamette River from 1850 to 2000 and relate differences in channel dynamics to the geology of the basin and spatial patterns of human modification. Transitions from island to lateral floodplain are identified and implications for ecological processes are discussed.
2.
Methods
Channel change from 1850 to 2000 was measured for the 273-km mainstem of the Willamette River from Eugene, OR to its confluence with the Columbia River in Portland, OR. The Willamette River and its major tributaries were surveyed by the U.S. General Land Office (GLO) in the decade after 1850. Engineers surveyed in a grid-based system using a longitudinal axis along the west coast of the United States (the Willamette meridian). The surveyors delineated changes in vegetation, stream channels, and wetlands along monumented section lines and sketched the features within a one-square mile section of land. The Nature Conservancy transcribed the engineering coordinates and notes and we transferred the maps into a GIS database
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Figure 29.1. Map of the 30,000-km2 Willamette River basin, Oregon, USA.
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(1:24,000 scale). The Willamette River network and its riparian vegetation also were mapped in 1895 and 1932 by the U.S. Army Corps of Engineers. These two river surveys were conducted for the entire length of the Willamette River for navigation purposes. We used satellite imagery (Landsat) and digital orthophotographs to construct a map of the Willamette River in 1995 (recently updated for 2000). The river channels were classified as primary channel (unbraided, or the portion of the channel with the most flow), side channel (channels connected to the mainstem at both ends; this includes the small channels that form islands), and alcoves or sloughs. The latter are ‘‘blind’’ channels, i.e., they are connected only at one end to a primary, side, or secondary channel. Islands and gravel bars were identified from these surveys and represented in the GIS layers for each year. Channel change was quantified by estimating both length and area of the major channel types between years. This paper will focus on the overall change in the river channel and islands from 1850 to 1995. Change in island extent was measured first as an overall change in area of islands. This measure of island change cannot differentiate between sources of island loss (i.e., elimination of the area of the island or the elimination of a side channel converting the island into the terrestrial margin or riverbank). To distinguish these two processes, we visually examined each island along the 273-km length of the river and classified it into five major classes: (i) unchanged – original island largely the same as 1850, (ii) remnant – portion of the original island still existing, (iii) lost from remnant – portion of remnant that was lost, (iv) extinct – island no longer present and original area occupied by river, and (v) transformed – island no longer present but land area now adjacent to river. The data were converted to percentage of the original area of islands in 1850. No new islands were observed. Results are presented for the entire river and the three major geomorphic reaches. Thermal patterns of the Willamette River currently are being investigated and we present an example from a recent study to illustrate the consequence of island and channel change on the thermal heterogeneity of a large river. Thermal data loggers were deployed for 1 week at more than 50 locations in an island reach of the upper Willamette in late July 2005. Daily maxima for all stations were determined and used to identify areas of likely exchange of water from the hyporheos to the river.
3. 3.1.
Results Changes in channel and island area
From 1850 to 1995, total area of river channels decreased by 22% (Table 29.1, Gregory et al., 2002a,b) and total length of all channels decreased by 26%. More than 30% of alcoves and sloughs were lost by 1995. Islands diminished more than any other channel type. Total area of islands decreased by 63% over the 145-year interval. These changes differed along the length of the Willamette River. The upstream reach from Albany to Eugene experienced the greatest change in channel structure and loss of islands. This reach flows through broad alluvial and
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Table 29.1. Area of channel types and islands in 1850 and 1995 reported in hectares for the three reaches of the Willamette River and the combined length of the mainstem river. River section 1850 Upper Middle Lower Total mainstem 1995 Upper Middle Lower Total mainstem Percent change Upper Middle Lower Total mainstem
Primary channel
Secondary channel
Alcoves
Islands
Total area of channel
1946 2406 1473 5825
1059 309 110 1478
181 81 10 272
6897 1946 122 8965
10,083 4742 1715 16,540
1533 2115 1406 5054
280 209 169 658
103 71 2 176
1413 1777 117 3307
3329 4172 1694 9195
–73.5 –33.1 53.6 –55.6
–41.5 –15.5 –80.0 –35.1
–21.3 –12.1 –4.6 –13.3
–79.5 –8.6 –3.0 –63.1
–39.8 –14.5 –1.1 –22.3
Source: Modified from Gregory et al. (2002). Note: Percent change from 1850 to 1995 is reported following the areas for 1850 and 1995 (negative values for percent change represent loss of area). Total area of channel includes areas of the wetted channel types plus area of islands.
glacial flood deposits. During the period from 1850 to 1995, total area of river channels and islands (combined) in this reach decreased by 40% (Table 29.1). The total length of all channels decreased from 340 to 185 km (Gregory et al., 2002a; http://oregonstate.edu/dept/pnw-erc). More than 70% of the side channels and 40% of the alcoves were lost. Approximately 80% of the islands in this reach have been eliminated or converted to floodplain banks. The middle reach from Albany to Newberg exhibited variable patterns of channel change and was intermediate in loss of channels and islands. Basaltic outcrops and ridges from the foothills caused local constraint and channel control in the middle reach. The channel of the Willamette River in this reach has been simplified, though not to the extent observed in the upper river. Total area of channels and islands combined decreased by approximately 15%. Most of that change occurred through loss of primary channels and islands, though the proportional change was greatest for side channels (Table 29.1). Total length of channels did not change. The lower reach from Newberg to the mouth in Portland changed very little from 1850 to 1995. Lava flows from central Oregon created a basaltic trench in the lower reach more than 10 million years ago and make it less vulnerable to natural and human alteration. Though proportional changes were substantial, changes in area of major channel features were minor. Total area of channels and islands combined decreased by 22%, but even in 1850 the area of these geomorphic features was substantially less than the upper two reaches. Total length of channels increased from 69 to 76 km.
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Loss of Islands
Our analysis of islands in the Willamette River in 1995 revealed that very few islands from 1850 remain intact and unchanged today (Table 29.2). For the total Willamette River mainstem, only 1% of the area of islands remains unchanged. One-third of the island area now exists as a remnant of an island of 1850. The portion of those remnant islands that has been lost accounts for 14% of the island area in 1850. Almost half of the islands have become the banks of the river along the floodplain. However, only 5% of the islands of 1850 have been totally eliminated. This pattern of island change differs greatly for the three major reaches of the Willamette River (Table 29.2). In the upper river, where most of the overall change in islands has been observed, most of the island area has become riverbank. None of the original islands remain unchanged. Remnant islands account for almost 20% of the 1850 island extent, and almost 25% of the island area has been truly lost (extinct plus loss from remnant islands). In sharp contrast, the middle reach of the river predominantly (84%) exists as remnants of the islands of 1850. Only 11% of the 1850 island extent has been converted to floodplain riverbank. Less than 2% of the area of islands in 1850 has been truly lost in the middle reach. The lower reach, where relatively little channel change occurred, exhibits a totally different pattern, with a substantial proportional creation of new islands (16% of the area of 1850 islands). Over the same interval, 44% of the 1850 island area has been totally eliminated and another 13% has been lost from remnant islands. These patterns of island change indicate that analysis of channel change requires closer inspection than simple change in island extent. Human activity and natural geomorphic processes can cause markedly different processes of island alteration to occur. In the Willamette River, most of the change in islands did not occur in the areas of greatest human density, though human activities were responsible for much of the observed change. Portland and Salem (located in the lower and middle reaches, respectively) are the largest cities along the Willamette River. These reaches exhibited only 3 and 9% loss of island area since 1850. The upper reach has been shaped by depositions of sediments from glacial outburst floods 15,000–20,000 years ago and subsequent deposition of alluvial sediment. This section of the river contained the greatest length of anastomosing channels and associated islands. The major land use in this reach of river is agriculture with small cities distributed along its length. Secondary channels have been blocked, some levees have been built, and Table 29.2.
Sources of change in island extent from 1850 to 1995.
Reach
New
Extinct Transformed
Unchanged
Remnant
Loss from remnant
Upper Middle Lower Total mainstem
1.1 4.4 15.5 2.1
5.3 1.2 43.7 5.3
0.0 3.6 8.3 0.9
19.2 84.0 35.5 33.3
18.3 0.4 12.5 14.3
57.3 10.7 0.0 46.1
Note: Changes are expressed as percent of the total area of islands in 1850.
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numerous banks have been hardened with riprap. We mapped the locations where banks had been hardened with boulders and found that only 26% of the length of the mainstem river had been hardened. However, these structures have been placed in the most dynamic locations along the channel, occurring on 67% of the major meanders in the river (Gregory et al., 2002c). The geomorphic structure of the river in both 1850 and 1995 largely reflects the template created by the fluvial geomorphology and local influences of valley landforms in the different reaches. Humans have greatly altered the geomorphology of the Willamette River but not primarily in the areas of highest population density of human populations.
3.3.
Prioritization of future conservation and restoration actions
In 1850, the 273-km distance along the mainstem Willamette River from Eugene, OR downstream to Portland, OR contained 571 km of channels (e.g., side channel, sloughs, or alcoves) (Gregory et al., 2002a). By 1995, the length of channels over this distance had decreased to 424 km, a 25% loss of channel complexity. The floodplain forest along the river was changed even more by land conversion. In 1850, more than 4360 ha of floodplain forest surrounded the 273-km distance along the mainstem. By 1990, the extent of floodplain forest had been reduced to 1197 ha, a loss of 73% of the historical floodplain forest area. These changes in the river and its floodplain potentially alter the abundance and distribution of fish communities in the mainstem river. Our field studies in the Willamette River have demonstrated that fish abundance is approximately 50% greater in reaches with multiple channels or tributary junctions. In addition, we find approximately 20 fish species in a 1-km reach of the Willamette River that contains multiple channels or tributary junctions, but we find less than 17 species in the single-thread channels. We incorporated these relationships into an analysis of potential for future restoration in the Willamette River. We created a spatial framework for the assessment of the river and its floodplain by dividing the area of river that has been inundated between 1850 and the present into 1-km slices along the central axis of the channel (Fig. 29.2). We evaluated the biophysical potential for increased channel complexity, floodplain forest, and flood storage (through removal of revetments) based on the difference between 1850 and 1995 for each 1-km slice (Hulse and Gregory, 2004). We identified areas of socioeconomic obstacles for the same areas based on population density, buildings and roads, property value, and public land. Based on these two constraints on restoration success, we created a template to illustrate areas of higher potential for ecological restoration along the Willamette River (Fig. 29.3). This framework is now being used by the Willamette Partnership and other agencies and citizens groups to guide conservation and restoration efforts in the Willamette Valley.
3.4.
Consequences for thermal heterogeneity in river water
Channel avulsion and floodplain coalescence can alter the extent, position, and function of islands (Pie´gay and Bravard, 1997; Pie´gay, 1998). The distinction between
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Oregon
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Figure 29.2. Longitudinal framework for analysis of conditions within the historical floodplain in 1-km ‘‘slices’’ perpendicular to floodplain axis. For scale, distances between lines along the floodplain axis are 1 km and numbering begins near the confluence of the Willamette River and the Columbia River and extends 227 km upstream to above the confluence with the McKenzie River. These 1-km slices were used for prioritization of locations for river restoration. (Reproduced with permission from Hulse and Gregory, 2004, Springer.)
true loss of island area and transformation of islands to riverbanks has important geomorphic and ecological implications. To explore the impact of physical heterogeneity on thermal patterns in the river water, we placed temperature data loggers around island features in the upper Willamette River. Most of the habitats associated with the primary channel exhibited maximum temperatures ranging from 19.1 to 19.81C. Temperature did not differ greatly with depth (o11C), indicating turbulent flow and thorough mixing. However, several springs were observed emerging from the floodplain and gravel bar that exhibited maximum temperatures of 14.6, 15.4, and 15.81C. We investigated five additional reaches during 2005 and all reaches contained cold water habitats with temperatures ranging from 2.0 to 8.01C colder than the mainstem. These colder springs most likely represent hyporheic exchange with the surface water and are created by flow through the gravel bar and transport through the alluvium of islands and gravel bars. In the Willamette River, such subsurface exchange provides important cold water refuges for coldwater species, such as Cutthroat trout (Oncorhynchus clarki clarki), Chinook salmon (Oncorhynchus tshawytscha), and Steelhead (Oncorhynchus mykiss). Loss of island features is likely to reduce the frequency and thermal influence of the subsurface
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Figure 29.3. Illustration of high priority sites for restoration based on potential improvement on (1) increased channel complexity, (2) increased area of floodplain forest, (3) increased non-structural flood storage. (Reproduced with permission from Hulse and Gregory, 2004, Springer.) Dark gray bands indicate areas with high ecological potential for restoration and low socioeconomic obstacles. White bands are areas with high ecological potential for restoration and high socioeconomic obstacles. Light gray bands indicate areas with low ecological potential for restoration and low socioeconomic obstacles, and medium gray bands are areas with low ecological potential for restoration and high socioeconomic obstacles. For scale, distances between lines along the floodplain axis are 1 km.
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exchange and lead to thermal simplification and warmer overall river temperatures (Gregory and Bisson, 1996).
4.
Discussion and conclusions
Human modification of the river and its basin has simplified the Willamette River from 1850 to the present. Land owners and agencies directly blocked side channels with boulders, wood pilings, and fill in an attempt to prevent channel change and loss of property along the river. In addition, 26% of the length of the river has been hardened by lining the bank with boulders (riprap) on either one bank or both banks. In addition, gravel has been mined from the river since the late 1800s and aggregate industries continue to mine gravel bars in the main channel. Straightening the channel, eliminating side channels, and removing gravel increase channel degradation further isolate the active channel from its floodplain and accelerates the loss of side channels and islands. Klingeman (1973) analyzed changes in the relationship between staff gage elevation and discharge at gaging stations along the Willamette River. He concluded that the mainstem Willamette River was changing rapidly and was not in a dynamic equilibrium hydrologically. Most sites exhibited streambed degradation, though lateral channel adjustment was noted at some sites. The rate of channel degradation or downcutting for the mainstem river was estimated to be approximately 0.3 m/decade. Klingeman (1973) cited upstream dams, gravel mining in the river, and streambank hardening (i.e., riprap) as likely mechanisms responsible for the observed channel degradation, which is consistent with our observations of channel change in the Willamette River since 1850. Two additional factors – changes in discharge and sediment delivery to the mainstem river – are potential mechanisms for channel simplification in the Willamette River over the last 150 years. Flood-control dams alter the frequency of floods that exceed the bankfull channel (Magilligan et al., 2003) and reduce sediment delivery into downstream rivers (Ligon et al., 1995). Both of these processes could contribute to channel incision and loss of lateral channel complexity in the Willamette River. Flood management currently reduces the peak discharges and maintains high flows at close to bankfull discharge but less than floodplain inundation discharges. This focuses the stream power within the active channel and may lead to channel incision. Likewise, reduction of sediment input into the mainstem river reduces the gravel load and causes continued erosion of older gravels deposits within the mainstem river. While dams may currently contribute to channel simplification, they could only account for a minor portion of historical channel change because they were not built until after 1945. Channel simplification in the Willamette River was clearly evident by 1895 and 1932, thus human alteration or other mechanisms were influencing the channel morphology prior to flood control dams. One final factor that could influence the changes in the Willamette River is regional climate change. Peak discharges in the Willamette River have declined since the late 1800s (Fig. 29.4), but mean annual discharge has remained relatively constant. Flood-control reservoir were built in the basin after 1945, therefore the decline in peak flows between 1860 and 1945 cannot be attributed to dams. After 1965, flood
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Mean Daily Discharge (cms)
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5500
4500
3500
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1500 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 Year Figure 29.4. Daily mean discharges during floods greater than 1982 cms at the Albany, OR gaging station from 1895 to 2006 (data from USGS gaging station 14174000). The estimated 2-year recurrence discharge under flow regulation is 1982 cms; the 2-year unregulated flow is estimated to be 3200 cms. (Data from Portland, OR Office of USACE). All daily mean discharges that exceeded the 2-year regulated discharge are plotted.
control reservoirs reduce most peak flows in the mainstem Willamette River by 30–40%. Flow regulation has decreased 2-year recurrence flows at the Albany gaging station from 3200 to 1980 cms (hydrologic record from 1895 to 2006). Reduced peak flows over the last 150 years could increase the potential for incision and channel simplification (as previously discussed). One of the major tools in floodplain restoration is the re-establishment of hydrologic and geomorphic processes to the extent possible and in river reaches where the outcomes are likely and the ecological benefits are significant (NRC, 1992; Hulse and Gregory, 2004). Restoration of complex braiding or anastomosing channels can be accomplished by reducing lateral human-created constraints along the banks, increasing delivery of sediments, restoring hydrologic regimes, and reconnecting historical channels. The degree of success will depend on the local hydraulic and geomorphic processes and the degree to which human modifications can be reversed or reduced. There is a tendency to consider human manipulations in restoring river complexity, but natural flood processes may achieve such restoration more consistently and effectively than costly and time-consuming engineering approaches. Passive restoration through natural hydrologic and geomorphic processes obviously requires the reduction of human constraints, but it also requires the acceptance of dynamic and relatively unplanned and undirected channel change. This latter requirement commonly is the most difficult to achieve. Scientists who study gravel-bed rivers appreciate the
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efficiency and effectiveness of using natural geomorphic and ecological processes in river restoration, but incorporation of these approaches into river management will require new and more effective methods of illustration and communication between collaboration of river scientists, river managers, and regional communities. One possible interpretation of the findings of this study could be that the total area of floodplain has not been greatly diminished even though 63% of the total area islands has been lost. Many of the islands became floodplains adjacent to the mainstem river, and the secondary channels were blocked and filled. Total elimination of islands occurred primarily in the lower, simpler reach of the river near Portland (44% became extinct). In the more complex upper reach of the river, 57% of the islands in 1850 were transformed to current adjacent floodplains. These areas where islands became floodplains still support many potential floodplain functions (e.g., flood detention, soil deposition, wildlife habitat, riparian forests), though land use has converted many of these floodplains to crops, residences, or urban areas and altered the potential floodplain functions. Ecological functions of complex anastomosing or braided river channels extend far beyond the functions of islands as areas of floodplain. These channels provide extensive edge habitat with variable depths and velocities, secondary channels with greater roughness and reduced velocity, numerous gravel bars, alcoves, and sloughs. Such areas exhibit greater hyporheic exchange and provide mosaics of cold and warm water habitats. In addition, the greater roughness of such complex channels dissipates energy during floods and provides critical refuges for aquatic communities. Loss of channel complexity, secondary channels and islands potentially diminishes the species richness and population abundances of aquatic organisms in large rivers. Restoration of such complexity cannot be achieved simply by carving a few alcoves or side channels in an artificially hardened river. Restoration of river complexity requires a broader perspective of the river in which it is allowed to meander within its floodplain, erode and deposit alluvial sediments, and recruit wood and sediment dynamically from adjacent forests during floods. Communities must minimize channel hardening where possible and avoid development of costly buildings and roads in the most dynamic portions of the landscape. This can be best accomplished by protecting existing floodplain functions to the greatest extent possible and taking advantage of river changes that occur to major flood events to back away from the river and allow it to restore itself through geomorphic processes.
Acknowledgements This work was funded by STAR grant R825797 between the U.S. Environmental Protection Agency and Oregon State University. Data on historical river changes and land use/land cover were developed as part of cooperative agreement CR824682 between USEPA and OSU. Supplementary information and the digital datasets referenced herein are available via the Internet at http://www.orst.edu/dept/ pnw-erc/
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References Baker, J.P., Hulse, D.W., Gregory, S.V., et al., 2004. Alternative futures for the Willamette river basin. Ecol. Appl. 14, 313–324. Gregory, K.J., Gurnell, A.M., 1988. Vegetation and river channel form and process. In: Viles, H.A. (Ed.), Biogeomorphology. Basil Blackwell, Oxford, pp. 11–42. Gregory, K.J., 1992. Vegetation and river channel process interactions. In: Boon, P.J., Calow, P., Petts, G.E. (Eds), River Conservation and Management. John Wiley and Sons, Chichester, UK, pp. 255–269. Gregory, S., Ashkenas, L., Oetter, D., et al., 2002a. Mainstem river. In: Hulse, D.W., Gregory, S.V., and Baker, J.P. (Eds), Willamette River Basin: Trajectories of Environmental and Ecological Change. Oregon State University Press, Corvallis, OR, pp. 112–113. Gregory, S., Ashkenas, L., Oetter, D., et al., 2002b. Longitudinal patterns – channel. In: Hulse, D.W., Gregory, S.V., and Baker, J.P. (Eds), Willamette River Basin: Trajectories of Environmental and Ecological Change. Oregon State University Press, Corvallis, OR, pp. 134–135. Gregory, S., Ashkenas, L., Oetter, D., Wildman, K., 2002c. Revetments. In: Hulse, D.W., Gregory, S.V., and Baker, J.P. (Eds), Willamette River Basin: Trajectories of Environmental and Ecological Change. Oregon State University Press, Corvallis, OR, pp. 138–139. Gregory, S.V., Ashkenas, L.R., Minear, P., 2002d. Application of analysis of historical channel change in the restoration of large rivers. Verhandlungen der Internationale Vereinigung Limnologie 27 (7), 4077–4086. Gregory, S.V., Bisson, P.A., 1996. Degradation and loss of anadromous salmonid habitat in the Pacific Northwest. In: Stouder, D. and Naiman, R.J. (Eds), Pacific Salmon and Their Ecosystems: Status and Future Options. Chapman Hall, pp. 277–314. Gregory, S.V., Hulse, D.W., Landers, D.H., Whitelaw, E., 1998. Integration of biophysical and socioeconomic patterns in riparian restoration of large rivers. In: Wheater, H. and Kirby, C. (Eds), Proceedings of the British Hydrological Society International Conference, Exeter: Hydrology in a Changing Environment. Wiley, Chichester, UK, pp. 231–247. Gurnell, A., Petts, G., 1995. Changing River Channels. Wiley, Chichister, UK, 442pp. Hulse, D., Gregory, S.V., 2001. Alternative futures as an integrative framework for riparian restoration of large rivers. In: Dale, V.H. and Haeuber, R. (Eds), Applying Ecological Principles to Land Management. Springer-Verlag, New York, NY, pp. 194–212. Hulse, D., Gregory, S.V., 2004. Integrating resilience into floodplain restoration. Urban Ecosyst. 7, 295–314. Hulse, D., Gregory, S.V., Baker, J. (Eds), 2002. Willamette River Basin Planning Atlas: Trajectories of Environmental and Ecological Change. Oregon State University Press, Corvallis, OR, p. 178. Hyman, J.B., Leibowitz, S.G., 2000. A general framework for prioritizing land units for ecological protection and restoration. Environment. Manage. 25 (1), 23–35. Interagency Floodplain Management Review Committee (IFMRC), 1994. Sharing the challenge: floodplain management into the 21st century. Washington, DC. Klingeman, P.C., 1973. Indications of streambed degradation in the Willamette Valley. Project completion report (OWRR Project A-016-ORE) submitted to Office of Water Research, U.S. Department of Interior. 99pp. Ligon, F.K., Dietrich, W.E., Trush, W.J., 1995. Downstream ecological effects of dams. BioScience 45 (3), 183–192. Magilligan, F., Nislow, K.H., Graber, B.E., 2003. Scale-independent assessment of discharge reduction and riparian disconnectivity following flow regulation by dams. Geology 31 (7), 569–572. Naiman, R.J., Decamps, H., 1990. The Ecology and Management of Aquatic–Terrestrial Ecotones. UNESCO-MAB, Paris. Naiman, R.J., Decamps, H., Pastor, J., Johnston, C.A., 1988. The potential importance of boundaries to fluvial ecosystems. J. N. Am. Benthol. Soc. 7, 289–306. National Research Council (NRC), 1992. Restoration of Aquatic Ecosystems. National Academy Press, Washington, DC. Petts, G. and Foster, I., 1985. Channel morphology. Pages 140–174 In: River and Landscape. Edward Arnold Publishers, London, 275pp.
Historical channel modification and floodplain forest decline
777
Petts, G.E., 1990. The role of ecotones in aquatic landscape management. In: Naiman, R.J. and Decamps, H. (Eds), The Ecology and Management of Aquatic–Terrestrial Ecotones. UNESCO, Paris/Parthenon, Carnforth, UK, pp. 227–261. Pie´gay, H., 1998. Interactions between large woody debris and meander cutoff (example of the Mollon Site on the Ain River, France). Z. Geomorphol. 42, 187–208. Pie´gay, H., Bravard, J.P., 1997. Response of a Mediterranean riparian forest to a 1 in 400 year flood, Ouveze River, Drome-Vaucluse, France. Earth Surf. Process. Landf. 22 (1), 31–43. Schumm, S.A., 1968. River adjustment to altered hydrological regime – Murrumbidgee River and paleochannels, Australia. United States Geological Survey, Professional Paper 598. 62pp. Swanson, F.J., Johnson, S.L., Gregory, S.V., Acker, S.A., 1998. Flood disturbance in a forested mountain landscape. BioSci. 48 (9), 681–689. Van Sickle, J., Baker, J., Herlihy, A., et al., 2004. Projecting the biological condition of streams under alternative scenarios of human land use. Ecol. Appl. 14, 368–380.