RIVERBED DIGITAL ELEVATION MODELS AS A TOOL FOR HOLISTIC RIVER MANAGEMENT: MOTUEKA RIVER,NELSON, NEW ZEALAND

River management in New Zealand's laterally active gravelly rivers has permitted floodplain development and protection of agricultural resources and infrastructure. Management of these dynamic systems has been hailed as a success for the approaches adopted, namely straightening and confining the river using bank protection and managing riverbed levels by gravel extraction. However, this activity also impacts river morphological/habitat diversity and potential gravel resource, by replacing broad riparian corridors with narrower channels and reducing lateral connectivity with the floodplain. This paper quantifies river behaviour in three laterally confined reaches in the upper Motueka River over a 7‐year period, using annual high‐resolution ground surveys to address the nature of morphological change and associated sediment flux in these reaches with a view to informing management of the gravel resource. Surveys between 2004 and 2010 acquired data to construct digital elevation models (DEMs) of the active riverbed in three ~1‐km‐long reaches. Morphological budgeting based on differencing between successive DEM surfaces reveals complex spatial and temporal patterns of erosion and deposition, demonstrating complex reach dynamics. Overall, volumetric changes suggest these narrowed reaches have been net exporters of sediment, associated with continued channel degradation. This has left bar features, traditionally the focus of gravel extraction in the reaches, relatively isolated from all but extreme flows, limiting replenishment of the gravel resource. The paper demonstrates the utility of riverbed DEMs as a potential tool to frame river character and behaviour at the reach scale in gravel‐bed rivers, thereby providing an important contribution to holistic river management in these systems. Copyright © 2012 John Wiley & Sons, Ltd.     

LONG PROFILE RESPONSES OF ALPINE BRAIDED RIVERS IN SE FRANCE

Channel elevation change since the early 20th century was characterized for 31 alpine braided river reaches (total length of 129 km), representing about 20% of the braided river network length in SE France. Present‐day long profiles were surveyed in the field and compared with historical profiles measured between 1894 and 1930. Forty‐four cross sections of active channels including terrace boundaries were also surveyed to study morphological signatures of braided channels characterized by different channel responses (aggradation or degradation). Results show that 56% of the sampling river length is characterized by degradation, 24% by stability and 20% by aggradation. Gravel mining is recognized as the main factor explaining degradation of braided channels. It is also demonstrated that aggrading braided channels are those where sediment supply from active torrents and bank erosion have been best preserved. The comparison of morphological signatures associated with aggrading and degrading channels reveals significant differences in cross‐section morphometry. A normalized bed relief index is proposed as a reliable indicator of the channel response. Field observations show that long‐term aggraded braided channels have a lower bed relief index than degraded channels, as already established by some laboratory experiments. This index can be used to determine the position of the braided channel in the cycle of degradation‐recovery following the cessation of gravel mining. It is also established that aggraded channels develops wider active channels than degraded channels for a same drainage area. This study provides new insights for the assessment of spontaneous recovery potential of braided channels impacted by gravel mining. Copyright © 2012 John Wiley & Sons, Ltd.     

A GIS‐based approach to mapping probabilities of river bank erosion: regulated River Tummel,Scotland

This study explores the use of Geographic Information Systems (GIS) techniques for mapping river channel planform change and bank erosion probability. The method used is primarily based on an approach developed by Graf (Graf, W.L. 1984. ‘A probabilistic approach to the spatial assessment of river channel instability’, Water Resour. Res., 20 (7), 953–962), which proposed that bank erosion probability could be determined for any given cell on a floodplain by taking into account (i) its distance laterally and in the upstream direction to the active river channel, and (ii) a value representing flood magnitudes for the given period. In this study, Graf's method is refined by using a GIS approach and by incorporating the influence of geomorphic variables, such as river bank morphology, sediment type and floodplain vegetation, on bank erosion rates. In addition, the technique is applied to a wandering gravel‐bed river of roughly 80 m width, representing a different type and size of river to that used in Graf's study. Thus, the wider applicability of the technique is tested. In addition, the analysis here covers only a short time scale (1988–1994) compared with that used by Graf. The high temporal resolution of this study is enabled by the use of aerial photography and also by the substantial channel changes that occurred within this time period as a result of several high magnitude flood events. The results of the study indicate the usefulness and validity of the approach, particularly with regard to floodplain erosion hazard mapping and the assessment of the effects of altered flood regimes and land use. Copyright © 2000 John Wiley & Sons, Ltd.     

Turbulence structure and bank erosion process in a dredged channel

Riverbank erosion has significant geomorphological as well as anthropogenic consequences. The geomorphological impacts include form changes such as lateral channel migration, meanders, channel expansion, etc. The anthropogenic effects include the threat to floodplain human habitation, agricultural land, and stability of instream hydraulic structures and buried pipelines. Channel dredging for the extraction of sand and gravel has seen a multi-fold rise in the last few decades. Therefore, riverbank erosion response to channel mining gains importance in river basin management. Sandpits dredged in the riverbeds can significantly impact the downstream riverbank stability. In order to assess these impacts, we conducted a series of experiments at a laboratory scale in a recirculating water flume. Three riverbank slopes, 25° (gentle), 31° (equal to the angle of repose of the bank sediments), and 40° (steeper than the angle of repose), were tested along with a sandpit. Remarkable changes in the turbulence structure of riverbank flow were found due to the channel pit. Pit excavation directly impacts the fluvial erosion characteristics of the riverbank. Pit action increases the Reynolds shear stress fields in the near-bank flow, which causes progressive fluvial erosion of the berm at the bank toe. The erosivity of the main channel flow in the riverbank also leads to channel degradation, which increases the exposed height of the bank slope. Pit dredging leads to the generation of stronger ejection bursts which provide a mechanism for berm sediment mobility and erosion. The hydro morphological response of the riverbank due to sand mining was analysed, and process understandings are presented in the paper.     

Floodplain forest dynamics: Half‐century floods enable pulses of geomorphic disturbance and cottonwood colonization along a prairie river

In dry ecoregions, trees are restricted to river valley floodplains where river water supplements the limited local precipitation. Around the Northern Hemisphere, cottonwoods, riparian poplars, are often predominant trees in floodplain forests and these ecological specialists require floods that create and saturate sand and gravel bars, enabling seedling recruitment. By pairing the interpretation of aerial photographs at approximately decade intervals with dendrochronology, we explored the coordination between river floods, geomorphic disturbance and colonization of plains cottonwoods (Populus deltoides) over eight meanders along the Red Deer River in the semi‐arid prairie of western Canada. This river has a relatively natural flow regime and minimal human alteration through the World Heritage Site of Dinosaur Provincial Park. We found that the 50‐year flood of 1954 increased channel migration and produced extensive accretion with downstream expansion of meander lobes and some channel infilling, which was followed by prolific cottonwood colonization. Those processes accompanied the major flood, while bank erosion and cottonwood losses were more gradual and continuous over the past half‐century. Results indicated even greater floodplain and woodland development after an earlier 100‐year flood in 1915. Each flood produced an arcuate band of mature cottonwoods and there were five to seven progressively older woodland bands across the floodplain, with each cottonwood age grouping increasing by about a half‐century. The 700 m wide floodplain was progressively reworked by the river through pulses of channel movement and floodplain and woodland development over approximately 250 years and correspondingly, the oldest cottonwoods were about 250 years old.     

The effect of the shortage of gravel sediment in midstream river channels on riparian vegetation cover

The vegetation cover in the steep, gravelly reaches of rivers was originally low. However, significant vegetation colonization can currently be observed in these riparian areas, and understanding the cause of this colonization is important for management. The objective of this study was to assess the effect of sediment deposition and erosion on vegetation colonization in gravel bed rivers. The delay in colonization by herbs and trees after sediment deposition or erosion was investigated using aerial photos of 6 Japanese rivers. A field study was conducted before and after a large flood at depositional and erosional locations. The colonization of vegetation after flushing was substantially delayed at sites where gravels were deposited compared with that at erosional sites, and it was faster in sandy reaches compared with gravelly reaches. Little tree colonization was observed at the depositional sites of gravelly sediments, whereas at erosional sites, new shoots sprouted from the collapsed live trees in the following spring, achieving a rapid increase in tree density. The nutrient and moisture contents of the sediment were significantly higher at the erosional sites. Gravels are deposited after washing and being segregated from fine sediment during floods. This coarse‐sediment layer is low in moisture and nutrients compared with the erosional sites, at which the underlying sediments are exposed by flooding. However, moisture and nutrients are the primary requirements for vegetation colonization. Therefore, the reduction in gravelly sediments due to gravel mining, river regulation, and modification of river basins can have a substantial effect on vegetation colonization.     

Reconstruction of the characteristics of a natural alluvial river–floodplain system and hydromorphological changes following human modifications: the Danube River (1812–1991)

Based on detailed historical surveys from 1812, the natural riverine landscape of a 10.25‐km‐long reach of the Danube River in the Austrian Machland region prior to channelization is analysed. Anthropogenically induced changes of fluvial dynamics, hydrological connectivity and aquatic habitat composition are discussed, comparing the situations following channelization (1925) and flow regulation (1991). In 1812 the alluvial river–floodplain system of the Danube River comprised a highly complex channel network, numerous gravel bars and extensive islands, with the main channel and side arms (eupotamon) representing about 97% of the entire water surface at low flow. The floodplain was characterized by relatively flat terrain and numerous natural trenches (former active channels) connected to the main channel. These hydromorphological conditions led to marked expansion/contraction of the water surface area at water level fluctuations below bankfull (‘flow pulse’). The high degree of hydrological connectivity enabled intensive exchange processes and favoured migrations of aquatic organisms between the river and floodplain habitats over a period of approximately 90 days per year. Overall in 1812, 57% of the active zone (active channels and floodplain) was inundated at bankfull water level. Channelization and construction of hydropower plants resulted in a truncated fluvial system. Consequently, eupotamal water bodies decreased by 65%, and gravel/sand bars and vegetated islands decreased by 94% and 97%, respectively, whereas the area of the various backwaters doubled. In 1991 the former ‘flow pulse’ was halved due to artificial levees and embankments, greatly diminishing hydrological connectivity and decoupling large areas of the floodplain from the main channel. Active overflow, formerly playing an important role, is now replaced by backwater flooding and seepage inflow in isolated water bodies. Copyright © 2003 John Wiley & Sons, Ltd.     

Valley floor landscape change following almost 100 years of flood embankment abandonment on a wandering gravel‐bed river

Critical to restoring the nature conservation value of many river corridors is an understanding of how alluvial landscapes will respond to cessation of river management and land use practices that have previously degraded the environment. This paper analyses changes in valley floor landforms and vegetation patch dynamics, in relation to fluvial disturbance, over a period of almost 100 years following flood embankment abandonment on a wandering gravel‐bed river, namely the River Tummel, Scotland. Such rivers were once typical of many draining upland areas of northern maritime Europe. Prior to abandonment the valley floor landscape was agriculturally dominated and the river for the most part was single thread confined between flood embankments. The pattern of landform change and vegetation patch development over time following a decision in 1903 not to maintain embankments was tracked by geomorphic and land cover mapping utilizing successive sets of aerial photography for the period 1946 to 1994. A historical context for these changes was also feasible because the channel planform in 1900 and earlier channel planform changes dating back to 1753 were known due to the availability of old maps and earlier geomorphic studies. The land cover mapping was validated by comparison of results produced from the interpretation work on the 1994 aerial photographs with the field‐based UK National Vegetation Classification protocol. The findings of the study illustrate that bordering the River Tummel fluvial landforms and vegetation patch mosaics, presumably resembling those that occurred before valley floor land use intensification, evolved in less than 50 years after flood embankment abandonment with a resultant increase in habitat diversity. The change relates primarily to flood‐induced channel planform change and moderate levels of fluvial disturbance. The general significance of this change to plant species diversity on the valley floor of the River Tummel and elsewhere is discussed as is possible implications of the upstream impoundment and scenarios for climatically induced changes in flood frequency and magnitude. The overall outcome is the strong possibility that simple changes in river management and land use practices could result in re‐establishment of the nature conservation value of similar river corridors in Europe over the medium term without active restoration efforts. Copyright © 2002 John Wiley & Sons, Ltd.     

Quantifying the effect of riparian forest versus agricultural vegetation on river meander migration rates,central Sacramento River,California, USA

Riparian forest vegetation is widely believed to protect riverbanks from erosion, but few studies have quantified the effect of riparian vegetation removal on rates of river channel migration. Measured historical changes in a river channel centreline, combined with mapped changes in floodplain vegetation, provide an opportunity to test how riparian vegetation cover affects the erodibility of riverbanks. We analysed meander migration patterns from 1896 to 1997 for the central reach of the Sacramento River between Red Bluff and Colusa, using channel planform and vegetation cover data compiled from maps and aerial photography. We used a numerical model of meander migration to back‐calculate local values for bank erodibility (i.e. the susceptibility of bank materials to erosion via lateral channel migration, normalized for variations in near‐bank flow velocities due to channel curvature). A comparison of migration rates for approximately 50 years before and after the construction of Shasta dam suggests that bank migration rates and erodibility increased roughly 50%, despite significant flow regulation, as riparian floodplains were progressively converted to agriculture. A comparison of migration rates and bank erodibilities between 1949 and 1997, for reaches bordered by riparian forest versus agriculture, shows that agricultural floodplains are 80 to 150% more erodible than riparian forest floodplains. An improved understanding of the effect of floodplain vegetation on river channel migration will aid efforts to predict future patterns of meander migration for different river management and restoration scenarios. Copyright © 2004 John Wiley & Sons, Ltd.     

长江砂石料开采量初步分析

开采长江砂石料作建筑材料之用，已成为８０年代以来的新兴行业，砂石料开采用主要位于河床成型堆积体、弯道深槽和矶头冲刷坑下游堆积区、床沙输移区。砂石料的开采除了可能给防洪和航运安全带来不利影响外，对河道冲淤和河势也会带来重大影响，因此，应严格限制砂石开采区和开采量，长江上川江的卵石年开采最近期宜控制在２００万吨左右，中下游的砂石年开采量近期宜控制在５０００万吨以内。     

Effects of Damming on Long‐Term Development of Fluvial Islands,Elbe River (N Czechia)

Damming and water impoundment have fundamental influences on the geomorphology and ecological processes of lotic systems. Although these engineering projects affect all segments of the river channel, fluvial (mid‐channel, river) islands are among the most threatened features because of their link to both hydrostatic and hydrodynamic effects of damming. In this study, we used historical maps (1843, 1852) and aerial photos (1954, 2014), as well as other written and iconographic documentary sources, to document the long‐term development of the fluvial islands and channel planform in the Lower Labe (Elbe) River area (Northern Czechia) over the past ~170 years. Our results indicate the decrease of fluvial islands from 16 (1843), resp. 20 (1852) in the mid‐19^th century to eight in 1954, and finally to five in 2014. Most islands have disappeared because of the construction of dams and lock chambers for the purpose of river navigation in the first half of the 20th century. The possible processes responsible for island extinction in individual river segments include sediment starvation (downstream of the dam), erosion by overflow (near upstream of the dam) and decreased flow in inter‐island branches (far upstream of the dam). The islands most susceptible to extinction are those with a smaller size and elliptical or irregular shape. Based on visual evaluation of historical photos and survey of present day temporary islands, the medium and fine sedimentary fraction and absence of a vegetation cover seem to be another predictor of island extinction. Finally, we stress the relevance of our findings for the current discussion on the construction of new lock chambers downstream of the study area. Copyright © 2016 John Wiley & Sons, Ltd.     

Management of vegetation encroachment by natural and induced channel avulsions: A physical model

Flow regulation and water abstractions may change the complex relationship between river hydraulics, morphology, and riparian vegetation. As a result, rivers are likely to decrease their dynamics, increase the amount of vegetation, and modify their habitat structure. Flood events provide a natural mechanism for removal of invasive vegetation and recreation of natural floodplain habitats. This work aims at evaluating and quantifying how gravel‐bed braided rivers naturally control vegetation encroachment through morphological processes and the impact of both naturally occurring and induced avulsions. Flume experiments were conducted in a 24‐m‐long x 1.6‐m‐wide channel filled with well‐sorted sand and constant longitudinal gradient at 0.01 m/m. Once a braided network developed, the flume was seeded with Eruca sativa at a density of 1.5 seeds/cm² and grown until an approximate height of 1.1 cm. Experiments evaluated low‐, medium‐, and large‐flood events and documented morphological changes and impacts to vegetation at four intervals during the experiments. High‐resolution images captured approximately 3 m above the flume were used to produce accurate Structure‐from‐Motion‐derived topography and orthoimagery (average errors 2 mm). Vegetation dynamics were observed to be highly variable and depend on local morphological changes and bank erosion. Discharge is the first‐order control on vegetation removal, but our results show that occurrence of avulsions significantly increases vegetation removal. The experiments highlight that a relatively small amount of sediment relocation can be an effective tool to induce avulsions and reduce vegetation encroachment on regulated rivers.     

Channel forms and vegetation adjustment to damming in a Mediterranean gravel‐bed river (Serpis River,Spain)

This paper focuses on the analysis of changes observed in channel morphology in the Serpis River (Alicante, Spain), a gravel‐bed river dammed since 1958. The paper analyses flow series and several aerial images, prior and subsequent to dam construction, to analyse changes in channel morphology and vegetation colonisation using Geographical Information Systems (GIS) techniques. Results show a concatenation of morphological changes throughout an adjustment sequence (60 years), which started with the transformation from wandering to single thread channel pattern, was followed by a slow vegetation encroachment, and culminated with the stabilization of channel migration. The role of vegetation (particularly Salicaceae species) has been critical in controlling floods' effectiveness, reducing river mobility and shifting, and consolidating a channel planform model adapted to the post‐dam flow conditions.     

Assessing the Relationship Between River Mobility and Habitat

Human interventions that limit channel mobility such as bank stabilization are frequent in riparian zones in urban or agricultural environments. This is potentially problematic because channel mobility is an important geomorphological and ecological agent that structures natural instream and riparian ecosystems. This study aims to (1) quantify the relationship between mobility and three types of habitat‐related features, namely bars, oxbow lakes and log jams, for a 54‐km‐long reach of the Yamaska Sud‐Est River (Quebec, Canada), which runs from the forested Appalachian Mountains to the agricultural St‐Lawrence lowlands, and (2) evaluate the impact of human interventions and geomorphological characteristics on these three features. Channel mobility was measured from historical aerial photos for the period 1950–2009. A combination of high‐resolution aerial photos, LiDAR digital elevation model, and field observations was used to measure and map sediment bars, oxbow lakes and log jams, as well as several geomorphological characteristics (channel width, slope, sinuosity and floodplain width). A strong link between the mobility and the presence of habitat features is revealed, but local geomorphological contexts result in different mobility patterns responsible for specific habitats. Floodplain to channel width ratio appears as the best geomorphological factor predicting habitat diversity. Human intervention, mostly through bank stabilization, also appears to be a key factor limiting mobility and its related habitats. These results highlight the importance of defining a protected mobility corridor along rivers where geomorphic processes such as bank erosion can freely occur, as it is an essential process that should be integrated in land use planning and river management. Copyright © 2015 John Wiley & Sons, Ltd.     

Floodplain management strategies for flood attenuation in the river Po

This paper analyses the effects of different floodplain management policies on flood hazard using a 350 km reach of the river Po (Italy) as a case study. The river Po is the longest Italian river, and the largest in terms of streamflow. The middle‐lower Po flows East some 350 km in the Pianura Padana (Po Valley), a very important agricultural region and industrial heart of Northern Italy. This portion of the river consists of a main channel (200–500 m wide) and a floodplain (overall width from 200 m to 5 km) confined by two continuous artificial embankments. Floodplains are densely cultivated, and a significant portion of these areas is protected against frequent flooding by a system of minor dykes, which impacts significantly the hydraulic behaviour of the middle‐lower Po during major flood events. This study aims at investigating the effects of the adoption of different floodplain management strategies (e.g. raising, lowering or removing the minor dyke system) on the hydrodynamics of the middle‐lower Po and, in particular, on flood‐risk mitigation. This is a crucial task for institutions and public bodies in charge of formulating robust flood risk management strategies for the river Po. Furthermore, the results of this study are of interest for other European water‐related public bodies managing large river basins, in the light of the recent European Directive 2007/60/EC on the assessment and management of flood risks. The analysis is performed by means of a quasi‐2D hydraulic model, which has been developed on the basis of a laser‐scanning DTM and a large amount of calibration data recorded during the significant flood event of October 2000. Copyright © 2010 John Wiley & Sons, Ltd.     

RECRUITMENT AND SUCCESSIONAL DYNAMICS DIVERSIFY THE SHIFTING HABITAT MOSAIC OF AN ALASKAN FLOODPLAIN

We measured plant recruitment patterns, successional dynamics, and biophysical processes on laterally expansive floodplains of the Kwethluk River, Alaska. The main channel of this gravel‐bed river frequently avulses and possesses an anabranching plan form. Fluvial processes were interactive with life history processes of riparian plants in determining initial stages of primary succession. Reproductive strategies and herbivory became important factors later in succession. We conducted plant age and compositional surveys to assess patterns of propagule deposition. We found that dispersal strategies of species were an important factor influencing recruitment patterns. Flood‐dispersed seeds resulted in even‐aged cohorts of species persistently segregated in space, based on age data. Sediment characteristics and inundation potential had little influence on seedling distributions. Recruitment was also segregated on the basis of dispersal strategy. At the heads of bars, where vegetative propagules (live drift wood) were entombed during floods, the distributions of species were random. This size‐selective nature of recruitment persisted through time. Vegetation age and distribution patterns were further diversified by the river's legacy of gravel deposition that diversified the primary successional pathway: one associated with ridges and another with swales. Interactions between these pathways and beaver herbivory initiated secondary succession. We used satellite imagery to quantify the extent of floodplain influenced by herbivory and to assess the importance of this driver of secondary succession. We also used high‐resolution aerial imagery and randomly selected sites to provide an unbiased analysis. We classified this imagery to quantify the spatial extent of herbivory and its influence on the initiation of secondary succession. The results showed, in addition to recruitment and successional dynamics, the flood‐plain habitat mosaic was diversified by the initiation of secondary succession. Patch and species composition within the vegetation mosaics were significantly different than those portions of the floodplain engaged in primary succession. Copyright © 2012 John Wiley & Sons, Ltd.     

Historical changes in hydrology,geomorphology, and floodplain vegetation of the Willamette River,Oregon

Historical trends in hydrology, geomorphology, and floodplain vegetation provide fundamental contexts for designing future management of large rivers, an area of fluvial research extensively informed by studies of historical channel dynamics. Changes in hydrology, channel structure, floodplain forests, and large wood were documented for the 273‐km main stem of the Willamette River from 1850 to present. Reduced sediment supply and frequency and magnitude of floods have decreased channel mobility and incised channels, leading to fewer gravel bars, islands, and side channels. Human alteration of channel morphology, vegetation, and bank hardening has exacerbated channel simplification caused by reductions in floods, sediment supply, and inputs of wood. A substantial number of floodplain channels reoccupied remnants of previous active channels inundated during recent floods, demonstrating functional but often forgotten role of historical geomorphic structure in modern floodplains and flood processes. In most reaches, area of floodplain forests in 1990 was only 10% to 25% of the area of forests in 1850. Abundance of wood in the wetted channel was generally greater in reaches with higher abundances of floodplain forests. Future trajectories will be influenced by legacies of the historical river but increasingly will reflect evolution of a new river shaped by human development, changing climate, and emerging hydrogeomorphic and vegetation processes. Understanding historical characteristics and anticipating future rates and patterns of ecosystem change provide fundamental contexts for restoring biophysical processes and structure in a large floodplain river.     

Influences of valley form and land use on large river and floodplain habitats in Puget Sound

In this paper, we use a system‐wide census of large river and floodplain habitat features to evaluate influences of valley form and land use on salmon habitats along 2,237 km of river in the Puget Sound region of Washington State, USA. We classified the study area by geomorphic process domains to examine differences in natural potential to form floodplain habitats among valley types, and by dominant land cover to examine land use influences on habitat abundance and complexity. We evaluated differences in aquatic habitat among strata in terms of metrics that quantify the length of main channels, side channels, braid channels, and area of wood jams. Among geomorphic process domains, habitat metrics standardized by main channel length were lowest in canyons where there is limited channel migration and less potential to create side channels or braids, and highest in post‐glacial and mountain valleys where island‐braided channels tend to form. Habitat complexity was lower in glacial valleys (generally meandering channels) than in post‐glacial valleys. Habitat abundance and complexity decreased with increasing degree of human influence, with all metrics being highest in areas classified as forested and lowest in areas classified as developed. Using multiple‐year aerial photography, we assessed the ability of our methods to measure habitat changes through time in the Cedar and Elwha Rivers, both of which have recent habitat restoration activity. We were able to parse out sources of habitat improvement or degradation through time, including natural processes, restoration, or development. Our investigation indicates that aerial photography can be an effective and practical method for regional monitoring of status and trends in numerous habitats.     

PAST AND PREDICTED FUTURE CHANGES IN THE LAND COVER OF THE UPPER MISSISSIPPI RIVER FLOODPLAIN,USA

This study provides one historical and two alternative future contexts for evaluating land cover modifications within the Upper Mississippi River (UMR) floodplain. Given previously documented changes in land use, river engineering, restoration efforts and hydro‐climatic changes within the UMR basin and floodplain, we wanted to know which of these changes are the most important determinants of current and projected future floodplain land cover. We used Geographic Information System data covering approximately 37% of the UMR floodplain (3232 km²) for ca 1890 (pre‐lock and dam) and three contemporary periods (1975, 1989 and 2000) across which river restoration actions have increased and hydro‐climatic changes have occurred. We further developed two 50‐year future scenarios from the spatially dependent land cover transitions that occurred from 1975 to 1989 (scenario A) and from 1989 to 2000 (scenario B) using Markov models. Land cover composition of the UMR did not change significantly from 1975 to 2000, indicating that current land cover continues to reflect historical modifications that support agricultural production and commercial navigation despite some floodplain restoration efforts and variation in river discharge. Projected future land cover composition based on scenario A was not significantly different from the land cover for 1975, 1989 or 2000 but was different from the land cover of scenario B, which was also different from all other periods. Scenario B forecasts transition of some forest and marsh habitat to open water by the year 2050 for some portions of the northern river and projects that some agricultural lands will transition to open water in the southern portion of the river. Future floodplain management and restoration planning efforts in the UMR should consider the potential consequences of continued shifts in hydro‐climatic conditions that may occur as a result of climate change and the potential effects on floodplain land cover. Published 2011. This article is a U.S. Government work and is in the public domain in the USA.     

Downstream channel changes after a small dam removal: Using aerial photos and measurement error for context; Calapooia River,Oregon

Dam removal is often implemented without adequate baseline monitoring to distinguish background variability from channel changes due to the removal. This study evaluated aerial photos as substitutes for multiple‐year pre‐removal field data to assess downstream channel changes associated with a small dam removal. The Brownsville Dam, a 2.1 m tall concrete dam on the Calapooia River, Oregon, was removed in 2007. We mapped bars and the low flow channel downstream from the dam and in an upstream control reach using aerial photos (1994–2008) and in the field prior to (2007) and following (2008) removal. The locations and magnitudes of changes in bar area and wetted width, relative to errors, indicate that downstream channel changes were a result of the removal. The maximum changes (?3520 ± 1460 m² for bar area, 32 ± 8 m for wetted width) observed prior to dam removal with aerial photos were far downstream. In contrast, the maximum changes after removal were immediately below the dam (200 ± 90 m² for bar area, ?11 ± 3 m for wetted width), and small in the upstream control (?150 ± 130 m² for bar area, 9 ± 4 m for wetted width). The dominant errors were photo specific: exposure error for spring to summer comparisons, position error for photos not processed for this study and identification error for small scale photos not scanned from film. We found aerial photos to be an acceptable but coarse substitute for multi‐year pre‐removal field data, and suggest best practices to minimize errors. Copyright © 2009 John Wiley & Sons, Ltd.