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.     

Putting down roots: Afforestation and bank cohesion of Icelandic Rivers

Riparian vegetation is widely recognized as a critical component of functioning fluvial systems. Human pressures on woody vegetation including riparian areas have had lasting effects, especially at high latitude. In Iceland, prior to human settlement, native downy birch woodlands covered approximately 15%–40% of the land area compared to 1%–2% today. Afforestation efforts include planting seedlings, protecting native forest remnants, and acquiring land areas as national forests. The planted and protected nature of vegetation along rivers within forests provides a unique opportunity to evaluate the various taxa within riparian zones and the channel stabilizing characteristics of the vegetation used in afforestation. We investigated bank properties, sediment textures, and root characteristics within riparian zones along four rivers in forests in Iceland. Bank sediment textures are dominantly sandy loam overlying coarser textures. Undercut banks are common because of erosion of the less cohesive subsurface layer. Quantitative root data indicate that the woody taxa have greater root densities, rooting depths, and more complex root structures than forbs or graminoids. The native downy birch has the highest root densities, with <1 mm roots most abundant. Modeling of added bank cohesion indicates that willow provides up to six times and birch up to four times more added cohesion to the coarse sediment textures comprising stream banks compared to no vegetation. We conclude that planting and protecting the native birch and willow helps to reduce bank erosion, especially where long-term grazing exclusion can be maintained.     

Desnagging to resnagging: new directions in river rehabilitation in southeastern Australia

Extensive desnagging (removal of large woody debris and living riparian vegetation) and associated river improvement works were conducted in rivers of southeastern Australia (Victoria and New South Wales) between at least 1886 and 1995. Swamp drainage, large woody debris removal and vegetation clearing were strongly supported by legislation, government funding and institutional arrangements in both states. As a result, large amounts of large woody debris were removed from rivers, regenerating indigenous vegetation was cleared from within designed alignment widths and, ironically, huge numbers of exotic trees, especially willows, were planted. The environmental impacts of desnagging have only been documented on a few impacted rivers but have included increased flow velocity, spatially extensive bed degradation, massive channel enlargement and loss of fish habitat. Recognition of the need for more integrated land and water management, and new research on the hydraulic, geomorphic, biogeographic and ecological significance of large woody debris and the values of indigenous riparian vegetation during the 1980s led to a major shift in river rehabilitation. We have drawn on our own and other published research to further develop a set of guidelines for the incorporation of large woody debris into river rehabilitation plans. Our guidelines extend those recently prepared for southeastern Australia and address site selection, where to place timber, the amount to be introduced, how to distribute it, techniques of introduction and woody debris sources. However, in the long term, riparian vegetation rehabilitation within the potential recruitment zone is essential to supply large woody debris. Given that our results demonstrate that very large woody debris makes a significant contribution to the total loading, it will be a very long time (>100 years) before natural recruitment can be recreated. Copyright © 2003 John Wiley & Sons, Ltd.     

Comparison of Direct and Indirect Boundary Shear Stress Measurements along Vegetated Streambanks

Estimates of boundary shear stress along vegetated streambanks are needed to predict streambank fluvial erosion. Because fluvial shear stress cannot be directly measured in the field, reliable estimation techniques using field instrumentation are needed. This study evaluated local bank shear stress estimation methods applicable to sloping, vegetated streambanks. Two reaches of a second order stream were modelled in a flume using a fixed‐bed Froude‐scale modelling technique. One reach was dominated by dense shrubs while the other reach was located in a mature forest. Direct measurements of local bank shear stress using a hot‐film anemometer were compared to estimates based on velocity measurements (logarithmic method, Reynolds stresses, and turbulent kinetic energy). For channels with no or widely spaced vegetation, the velocity‐based estimates underestimated the bank shear stress due to secondary flow contributions. For banks with dense vegetation, Reynolds stresses and turbulent kinetic energy estimates were statistically similar to direct measurements on average, but substantial error occurred when making point comparisons. Velocity‐based estimates generally over predicted bank stress in areas of high shear at the vegetation edge and underpredicted stress within dense vegetation. Ultimately, results suggest that none of tested techniques can be broadly applied to streambanks, and flow structure is critical in selecting the appropriate estimation technique. Copyright © 2016 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.     

In‐channel wood‐related hazards at bridges: A review

In‐channel wood is a key component in fluvial ecosystems; however, transport of in‐channel wood during floods can create hazards in urbanized areas. Among the main problems is wood accumulation at bridges, which reduces flow openings, causes blockage and inundation of nearby areas and, eventually, results in structures collapsing. Increasing awareness of the importance of the ecological role of wood in rivers calls for a compromise between the preservation of river ecosystems and management strategies for the prevention of wood‐related hazards. In recent years, knowledge related to in‐channel wood dynamics and hazards has notably increased, and a significant body of valuable information can be found in an extensive number of studies. This review provides a comprehensive summary of the most relevant advances regarding in‐channel wood‐bridge interactions. We review the factors controlling wood accumulation formation and summarize the different approaches used to analyse this process, namely, physical and numerical modelling. Finally, we conclude by highlighting the most important knowledge gaps, addressing particularly underresearched fields and stressing the remaining challenges.     

Sedimentology of New Fluvial Deposits on the Elwha River,Washington, USA,Formed During Large‐Scale Dam Removal

Removal of two dams 32 m and 64 m high on the Elwha River, Washington, USA, provided the first opportunity to examine river response to a dam removal and controlled sediment influx on such a large scale. Although many recent river‐restoration efforts have included dam removal, large dam removals have been rare enough that their physical and ecological effects remain poorly understood. New sedimentary deposits that formed during this multi‐stage dam removal result from a unique, artificially created imbalance between fluvial sediment supply and transport capacity. River flows during dam removal were essentially natural and included no large floods in the first two years, while draining of the two reservoirs greatly increased the sediment supply available for fluvial transport. The resulting sedimentary deposits exhibited substantial spatial heterogeneity in thickness, stratal‐formation patterns, grain size and organic content. Initial mud deposition in the first year of dam removal filled pore spaces in the pre‐dam‐removal cobble bed, potentially causing ecological disturbance but not aggrading the bed substantially at first. During the second winter of dam removal, thicker and in some cases coarser deposits replaced the early mud deposits. By 18 months into dam removal, channel‐margin and floodplain deposits were commonly >0.5 m thick and, contrary to pre‐dam‐removal predictions that silt and clay would bypass the river system, included average mud content around 20%. Large wood and lenses of smaller organic particles were common in the new deposits, presumably contributing additional carbon and nutrients to the ecosystem downstream of the dam sites. Understanding initial sedimentary response to the Elwha River dam removals will inform subsequent analyses of longer‐term sedimentary, geomorphic and ecosystem changes in this fluvial and coastal system, and will provide important lessons for other river‐restoration efforts where large dam removal is planned or proposed. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

A LIDAR‐DERIVED EVALUATION OF WATERSHED‐SCALE LARGE WOODY DEBRIS SOURCES AND RECRUITMENT MECHANISMS: COASTAL MAINE,USA

In‐channel large woody debris (LWD) promotes quality aquatic habitat through sediment sorting, pool scouring and in‐stream nutrient retention and transport. LWD recruitment occurs by numerous ecological and geomorphic mechanisms including channel migration, mass wasting and natural tree fall, yet LWD sourcing on the watershed scale remains poorly constrained. We developed a rapid and spatially extensive method for using light detection and ranging data to do the following: (i) estimate tree height and recruitable tree abundance throughout a watershed; (ii) determine the likelihood for the stream to recruit channel‐spanning trees at reach scales and assess whether mass wasting or channel migration is a dominant recruitment mechanism; and (iii) understand the contemporary and future distribution of LWD at a watershed scale. We utilized this method on the 78‐km‐long Narraguagus River in coastal Maine and found that potential channel‐spanning LWD composes approximately 6% of the valley area over the course of the river and is concentrated in spatially discrete reaches along the stream, with 5 km of the river valley accounting for 50% of the total potential LWD found in the system. We also determined that 83% of all potential LWD is located on valley sides, as opposed to 17% on floodplain and terrace surfaces. Approximately 3% of channel‐spanning vegetation along the river is located within one channel width of the stream. By examining topographic and morphologic variables (valley width, channel sinuosity, valley‐side slope) over the length of the stream, we evaluated the dominant recruitment processes along the river and often found a spatial disconnect between the location of potential channel‐spanning LWD and recruitment mechanisms, which likely explains the low levels of LWD currently found in the system. This rapid method for identification of LWD sources is extendable to other basins and may prove valuable in locating future restoration projects aimed at increasing habitat quality through wood additions. Copyright © 2011 John Wiley & Sons, Ltd.     

汶川地震引发的流域管理新课题

汶川地震引发了大量的山体滑坡和崩塌.巨型滑坡夷平了山谷,创造了平缓坡地.这些新生地非常不稳定,易引发泥石流.尽快稳定和开发新生地是一个新问题.滑坡造成的堰塞湖引起剧烈河床演变,对河流地貌和稳定产生长远和深刻的影响.大量的崩塌体形成大石块堆积体,表面没有土壤,如何修复植被是一个严峻的挑战.本文通过野外测量和试验,对上述问题进行了探索并取得了初步成果.成果表明,采用阶梯-深潭结构稳定滑坡体,可以控制或减轻泥石流;发生于崩塌和滑坡体上的泥石流含水量小且大石块多,不是黏性泥石流而是两相泥石流,运动速度仅为黏性泥石流的1/10;堰塞坝发育成尼克点会显著改变河流纵剖面,稳定岸坡减少滑坡崩塌灾害;在花岗岩崩塌体上引种紫萼藓能显著加快植被演替,绿化裸露的石块堆积体.     

Recent changes in riparian vegetation: possible consequences on dead wood processing along rivers

Little is known about changes in the composition of dead wood jams along rivers and the possible consequences of any such changes on the river ecosystem. Although tree zonation along the upstream‐to‐downstream continuum is weak and highly variable from a system to another, a clear transition appears in the piedmont zone, which is reflected by transitions in dead wood sources as well as for dead wood transport, storage and decomposition processes. In this paper, we focus on large lowland rivers of southwestern France, where riparian vegetation is increasingly fragmented, reduced in area and/or is entirely replaced by planted forests (poplar plantations). The amount and the potential role of dead wood is practically unknown in these rivers. One reason is that French legislation obliges landowners and public service managers to remove all material from the stream in order to maintain unobstructed river flows. The other reason is that unlike pristine streams in northern regions, these rivers have been regulated for several decades (Adour River) or even for several centuries (Garonne River). The vegetation component of the managed riparian landscape has changed in particular as a result of i) a decrease in stream dynamics, ii) the replacement of natural forests by planted ones, and iii) the invasion of natural communities by introduced woody species. The possible consequences of biological invasions on the role of dead wood jams are discussed in light of: i) a local study of wood jams along a moderately modified system; ii) changes observed in the composition of trees along the Adour River over the past 10 years; iii) a regional case study involving two chosen species. Whereas white willow populations are declining along streams in southwestern France, the box‐elder, introduced from the United States, has spread extensively in the last two decades. Statistical models would suggest that competitive pressures are limited between these two species, boxelder is expected to replace white willow in the near future as a consequence of an increase in river regulation and global warming. This can be expected to have important consequences on dead wood dynamics, and on the management of woody debris, especially since trends indicate a replacement of softwood species by hardwood species. Copyright © 2003 John Wiley & Sons, Ltd.     

Bank erosion in a macrotidal tropical river: Exploring the relative impact of boat wash on riverbank erosion

Macrotidal tropical rivers are dynamic systems where wet‐season floods and tidal flows cause significant riverbank erosion and sediment transport. This study aimed to explore patterns of riverbank erosion and deposition in a large, tropical, macrotidal river in Northern Australia; the Daly River. In particular, we aimed to determine if recreational boat use was impacting bank erosion in this dynamic river. Erosion pins were installed at multiple levels on both banks at 10 sites along a 34 km reach of the river. Measurements were made every four to six weeks during the low water dry season, and opportunistically during the wet season (flooding period) and seasonal transition periods. A bank geotechnical assessment, riverbed cross‐sections and site bathymetry were undertaken. Whilst the wet season was a period of substantial erosion (mean rate of 0.64 mm day^?1), the highest mean erosion rate (3.6 mm day^?1) was observed in the early dry season (April to May), a period of stabilizing water levels but greatest boat traffic. Bank erosion at this time was measured on both sides of the river and the inside of meander bends, which is atypical of normal riverine bank erosion patterns, and indicative of erosion due to boat wash. As the dry season progressed, significant spatial differences in erosion rates were evident, where erosion was observed at sites upstream of a large shallow sand bar, while sites downstream from the sand bar gained material through the deposition of tidally transported sediment. This study highlights the importance of understanding the significance and interaction of various erosive factors in tropical tidal rivers and has demonstrated that boat wash may be an important contributor to dry season bank erosion in these systems. We encourage management agencies to consider the role of boats in any future river management program in these systems.     

SPATIAL PATTERN,DENSITY AND CHARACTERISTICS OF LARGE WOOD IN CONNECTICUT STREAMS: IMPLICATIONS FOR STREAM RESTORATION PRIORITIES IN SOUTHERN NEW ENGLAND

Many streams have been modified so extensively that river managers do not have clear reference conditions to frame targets for stream restoration. Large woody debris (LWD) has long been recognized as an important influence on both geomorphic and ecologic processes in stream channels; however, there have been few studies of LWD dynamics in New England. Although this region is heavily forested today, the forest is predominantly young (70–90 years old) regrowth following a historical episode of severe deforestation. This study presents the results of an extensive census of LWD and associated stream characteristics in over 16 river kilometres of northeastern Connecticut streams and represents the first reported inventory of wood loading and sorting in Southern New England. Results of this study indicate that wood loading and jam frequencies in the study region are low: 2.5–17.8 and 0.5–5.51 per 100 m, respectively. Orientation of LWD is predominantly parallel to flow, an indication that these streams are not retaining organic matter or sediment, which has important geomorphic and ecologic implications. Results imply that stream recruitment of LWD is still lagging from the massive forest conversions of the 18th and 19th centuries. Given the low wood loadings observed in the study reaches, manual wood addition and continued forest regeneration would likely improve both habitat diversity and organic matter and fine sediment retention in these systems. Copyright © 2011 John Wiley & Sons, Ltd.     

Geomorphological adjustment of a newly engineered upland sinuous gravel-bed river diversion: Evan Water,Scotland

Following construction of a sinuous diversion on an upland gravel-bed river two flood events within three days of each other, with estimated recurrence intervals of between 1·5 and 2·0 years, caused marked geomorphological adjustment to the channel. The floods resulted in bank erosion, point bar formation, scour on the outside of meander bends and particle sorting, but the overall stability of the river diversion, which is to flow adjacent to a new motorway embankment was not affected. Moreover, the channel adjustment brought about beneficial changes in terms of increasing morphological diversity: probably enhancing fish habitat, although spawning gravels placed at the head of riffles in the river diversion were scoured. The extent and nature of the geomorphological changes relate principally to the high stream power and bedload transport capacity of the river. Geomorphological principles, critical stream powers and bedload transport rates can account for the size and redistribution of bed material transport. Other factors accounting for the adjustment include limited vegetation colonization of geotextile-covered river banks and construction of overly narrow and high channel constrictions to promote flow convergence and acceleration at the head of downstream pools. The case study provides lessons for future construction of river diversions on high energy gravel-bed rivers. © 1997 John Wiley & Sons, Ltd.     

The role of riparian vegetation and woody debris in the development of macroinvertebrate assemblages in streams

Riparian vegetation development and macroinvertebrate assemblages were studied in 16 streams formed between 35 and 230 years ago, following glacial recession in Glacier Bay National Park, southeast Alaska. Riparian vegetation established most rapidly in streams where flow variation in downstream reaches was buffered by a lake. Riparian vegetation development was positively correlated with lower bank stability, but was independent of stream age. Roots and branches of riparian vegetation trailing into streams (trailing riparian habitat—TRH) were shown to be an important habitat for a number of macroinvertebrate taxa. In young and unstable streams, TRH was colonized mainly by Plecoptera whereas in more stable lake‐influenced streams Simuliidae dominated. Significant coarse woody debris (CWD) accumulations were not observed until after approximately 130 years of stream development had occurred when certain channel features, such as gravel bars, were stabilized by dead wood. Where dead wood was present, opportunistic wood taxa were abundant, even in the younger streams. However, a xylophagous species, Polypedilum fallax, was not recorded until streams were over 100 years old. Two‐way indicator species analysis (TWINSPAN) using presence/absence of macroinvertebrate taxa on TRH, initially divided streams into lake and non‐lake systems, but subsequent divisions were consistent with differences in stream age. TWINSPAN of macroinvertebrate assemblages on dead wood again highlighted differences in stream age. Canonical correspondence analysis indicated that bed stability and stream age were the most important environmental variables influencing macroinvertebrate distribution on TRH. Trailing riparian habitat was most abundant in moderately unstable streams where it facilitates invertebrate colonization. CWD contributes markedly to channel stabilization, provides habitat for invertebrate xylophages, and confers additional habitat complexity. Maximum levels of CWD are predicted to occur in non‐lake streams after approximately 300 years, but at least a further 100 years will be required in stable streams below lakes where dead wood entrainment is not enhanced by flooding, channel migration and bank undercutting. A conceptual model summarizing the role of TRH and CWD on stream development in Glacier Bay is presented. Copyright © 2005 John Wiley & Sons, Ltd.     

Vegetation community response to hydrologic and geomorphic changes following dam removal

Dam removal can restore fish passage, natural flow regimes, sediment transport in streams, dispersal of organic matter, and drift of aquatic insects. However, dam removal also impacts the riparian vegetation, with both immediate and delayed responses. In this study, we measure vegetation change at the Merrimack Village Dam site on the Souhegan River in Merrimack, NH, USA. The August 2008 removal caused a ~3‐m drop in water level and rapid erosion of impounded sediment, with ~50% removed in the first 3 months. Terrace, floodplain, and wetland communities were surveyed in summer 2007, 2009, 2014, and 2015. Temporal change was quantified using Analysis of Similarity on the Bray–Curtis dissimilarity matrix. Only herbaceous vegetation closest to the river channel and in the off‐channel wetland changed significantly. The herbaceous plots directly adjacent to the impoundment eroded to bare sand in 2009, but by 2014, the original riparian fringe community had re‐established in the newly developed floodplain. Between 2007 and 2014, the off‐channel wetland area changed from aquatic species to a stable terrestrial community that persisted without significant change in 2015. The vegetation response was greatest in areas with the largest geomorphic and hydrologic change. These included the channel margin where erosion and bank slumping created an unstable scarp. The mid‐channel island and off‐channel wetland were strongly affected by the lowered water table. However, large unvegetated areas never persisted nor did the areal coverage of invasive species expand, which are two frequent concerns of dam removal stakeholders.     

河流泥沙学科几个方面发展跟踪

河流泥沙学科是一门综合性的基础技术科学,它是研究泥沙在水体中的输移、冲刷和淤积的规律,是水利学科的理论基础之一,包括泥沙运动力学、河床演变学、工程泥沙、航道与港口治理、高含沙水流与泥石流等多方面的内容。本文在查阅浏览大量研究资料基础上,着重从泥沙运动、河床演变、水库泥沙和数值模拟技术等几个方面对泥沙学科的历史进程、重要成果和发展动态进行了跟踪,对比分析了国内外研究水平,提出了今后发展的新方向和值得关注的议题,对我国河流泥沙在国际上发挥更大的作用具有参考意义。     

Is it Worth the Money? The Functionality of Engineered Shallow Stream Banks as Habitat for Juvenile Fishes in Heavily Modified Water Bodies

Heavily modified water bodies (HMWB) are characterized by monotonous and straightened channel morphologies with high degrees of bank enforcement. They often lack shallow bank habitats, which are considered important for critical life stages of fishes. In this study, three principle options to engineer shallow stream zones were assessed concerning the value of the created habitats for larval, juvenile and adult stages of fishes in 30 sites from three HMWB. The construction scheme of the juvenile habitats comprised different degrees of embankment ranging from rip‐rap structures with steep bank angles to almost nature‐like construction schemes with very flat river‐banks and sparing usage of structural enrichment such as boulders and dead wood. In general, the differences between the three habitat types were more pronounced in density of different life stages than in the presence or absence of species or certain life stages. A steep bank angle and a high degree of engineering such as placement of rip‐rap embankment, boulders or dead wood structure in the habitats were hardly accepted by early larval and juvenile stages of rheophilic fishes. In contrast, the construction scheme of a nature‐like habitat with a flat bank angle (<10%), low water depth (mean = 24 cm) and a sparing usage of coarse woody debris (CWD) and boulders had the highest success. Other investigated habitat types did not provide additional benefit, neither in terms of supporting additional species and life stages, nor in high individual numbers and should thus only be implemented when land for restoration is scarce and nature‐like habitats cannot be realized. Copyright © 2016 John Wiley & Sons, Ltd.     

CONTROLS ON THE LONGITUDINAL DISTRIBUTION OF CHANNEL‐SPANNING LOGJAMS IN THE COLORADO FRONT RANGE,USA

Channel‐spanning logjams completely span the active channel and create longitudinal discontinuities of the water surface and stream bed across at least two‐thirds of the channel width. These jams disproportionately affect channel process and form relative to smaller jams that do not span the entire channel width. We analyze a spatially extensive dataset of 859 channel‐spanning jams distributed along 124 km of 16 distinct rivers on the eastern side of Rocky Mountain National Park, Colorado, USA, with drainage areas spanning 2.6 to 258 km² and diverse valley geometry and forest stand age. We categorized valley geometry in terms of lateral confinement (confined, partly confined, or unconfined), which correlates with gradient. Jams exhibit substantial downstream variability in spacing at channel lengths of 10²–10³ m. The number of jams within a reach is explained by a statistical model that includes drainage area, valley type (lateral confinement), and channel width. Longitudinal spacing of jams drops substantially at drainage areas greater than ~20 km², although jam spacing exhibits tremendous variability at smaller drainage areas. We interpret the lack of jams at larger drainage areas to reflect increasing transport capacity for instream wood. We interpret the variability in jam spacing at small drainage areas to reflect local controls of valley geometry and associated wood recruitment and fluvial transport capacity. Our results suggest that management of instream wood designed to facilitate the formation of channel‐spanning jams can be most effectively focused on smaller drainage areas where these jams are most abundant in the absence of management that alters instream wood recruitment or retention. Unmanaged streams in the study region with drainage area <60 km² have ~1.1 channel‐spanning jams per 100 m length of stream. The cumulative effects of these jams on instream storage of sediment and organic matter, hyporheic exchange, instream habitat, stream metabolism, and channel–floodplain connectivity are likely to be enormous. Copyright © 2012 John Wiley & Sons, Ltd.