Bank stabilization,riparian land use and the distribution of large woody debris in a regulated reach of the upper Missouri River,North Dakota,USA

Large woody debris (LWD) is an important component of ecosystem structure and function in large floodplain rivers. We examined associations between LWD distribution and riparian land use, bank stabilization (e.g. riprap revetment), local channel geomorphology, and distance downriver from the dam in the Garrison Reach, a regulated reach of the upper Missouri River in North Dakota, USA. We conducted a survey of shoreline‐associated LWD in the reach during typical summer flow conditions. Reach‐wide LWD density was 21.3 pieces km^?1 of shoreline, of which most pieces (39% ) were ‘beached’ between the waterline and the bankfull level, 31% of pieces had evidence of originating at their current location (anchored), 18% of pieces were in deep water (>1 m), and 13% were in shallow water. LWD density along unstabilized alluvial (sand/silt) shorelines (27.3 pieces km^?1) was much higher than along stabilized shorelines (7.2 pieces km^?1). LWD density along forested shorelines (40.1 pieces km^?1) was higher than along open (e.g. rangeland, crop land; 9.2 pieces km^?1) or developed (e.g. residential, industrial; 7.8 pieces km^?1) shorelines. LWD density was highest overall along unstabilized, forested shorelines (45 pieces km^?1) and lowest along open or developed shorelines stabilized with a blanket‐rock revetment (5.5 pieces km^?1). Bank stabilization nearly eliminated the positive effect of riparian forest on LWD density. A predicted longitudinal increase in LWD density with distance from the dam was detected only for deep LWD (including snags) along unstabilized alluvial shorelines. Partial resurvey in the summer following the initial survey revealed a reduction in total LWD density in the reach that we attribute to an increase in summer flow between years. Changes in riparian management and land use could slow the loss of LWD‐related ecosystem services. However, restoration of a natural LWD regime in the Missouri River would require naturalization of the hydrograph and modification of existing bank stabilization and channel engineering structures. Copyright © 2004 John Wiley & Sons, Ltd.     

Impact of a 100‐year flood on vegetation,benthic invertebrates,riparian fauna and large woody debris standing stock in an alpine floodplain

This paper investigates the impact of a 100‐year flood in May 1999 on community composition and large woody debris standing stock in an alpine floodplain (Isar, Germany). Detailed pre‐flood data sampled from 1993 to 1998 are compared with the situation directly after the flood. In those parts of the Isar floodplain mainly covered with pioneer vegetation prior to flooding, the coverage of unvegetated gravel bars increased by 22% following the flood. However, the flood did not remove larger amounts of older successional vegetation stages (willow thickets, floodplain forest). No significant changes in the benthic invertebrate fauna were recorded. The lowest densities of riparian ground beetles (Carabidae) within the study were recorded one month after the flood. Two months later, the ground beetle densities increased to the highest values ever recorded, indicating the ground beetle's high potential for recolonization. These results highlight the degree of resilience of both the aquatic and the riparian invertebrate fauna. The flood also caused a significant increase in large woody debris standing stock; in one section the number of logs increased tenfold and the volume increased by a factor of 20, leading to the assumption that most woody debris in alluvial flood‐plains is provided by catastrophic events. Copyright © 2004 John Wiley & Sons, Ltd.     

Hydrogeomorphic and Biotic Drivers of Instream Wood Differ Across Sub‐basins of the Columbia River Basin,USA

Instream wood promotes habitat heterogeneity through its influence on flow hydraulics and channel geomorphology. Within the Columbia River Basin, USA, wood is vital for the creation and maintenance of habitat for threatened salmonids. However, our understanding of the relative roles of the climatic, geomorphic, and ecological processes that source wood to streams is limited, making it difficult to identify baseline predictions of instream wood and create targets for stream restoration. Here, we investigate how instream wood frequency and volume differ between seven sub‐basins of the interior Columbia River Basin and what processes shape these differences within these sub‐basins. We collected data on wood volume and frequency, discharge and stream power, and riparian and watershed forest structure for use in modelling wood volume and frequency. Using random forest models, we found that mean annual precipitation, riparian tree cover, and the individual watershed were the most important predictors of wood volume and frequency. Within sub‐basins, we used linear models, finding that some basins had unique predictors of wood. Discharge, watershed area, or precipitation often combined with forest cover, riparian conifer, and/or large tree cover in models of instream large wood volume and frequency. In many sub‐basins, models showed at least one hydrologic variable, indicative of transport competence and one ecological variable, indicative of the reach or upstream watershed's capability to grow measurable instream wood. We conclude that basin‐specific models yield important insights into the hydrologic and ecological processes that influence wood loads, creating tractable hypotheses for building predictive models of instream wood. Copyright © 2015 John Wiley & Sons, Ltd.     

The Influence of Logjams on Largemouth Bass (Micropterus Salmoides) Concentrations on the Lower Roanoke River,a Large Sand‐Bed River

This study examines the relation between logjams and largemouth bass (Micropterus salmoides) on the alluvial sand‐bed lower Roanoke River. Disparate data sets from previous bank erosion, fisheries, and large wood studies were used to compare the distribution of largemouth bass with logjam frequency. Logjams are related to the frequency of bank mass wasting increasing from near an upstream dam to the middle reach of the study segment and then decreasing as the river approaches sea level. The highest concentration of largemouth bass and logjams was in the middle reach (110 fish per hour and 21 jams per km). Another measure of largemouth bass distribution, fish biomass density (g h^?1), had a similar trend with logjams and was a better predictor of fish distribution versus logjams (R² = 0.6 and 0.8 and p = 0.08 and 0.02 for fish per hour and g h^?1 versus logjam, respectively). We theorize that the preference for adult bass to congregate near logjams indicates the use of the jams as feeding areas. The results of a principal component analysis indicate that fish biomass concentration is much more related to logjam frequency than channel geometry (width, depth, and bank height), bed grain size, bank erosion, or turbidity. The results of this research support recent studies on in‐channel wood and fisheries: Logjams appear to be important for maintaining, or increasing, both largemouth bass numbers and total biomass of fish in large eastern North American rivers. Persistent logjams, important as habitat, exist where relatively undisturbed river reaches allow for bank erosion inputs of wood and available anchoring locations. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.     

Experimental reintroduction of woody debris on the Williams River,NSW: geomorphic and ecological responses

A total of 436 logs were used to create 20 engineered log jams (ELJs) in a 1.1 km reach of the Williams River, NSW, Australia, a gravel‐bed river that has been desnagged and had most of its riparian vegetation removed over the last 200 years. The experiment was designed to test the effectiveness of reintroducing woody debris (WD) as a means of improving channel stability and recreating habitat diversity. The study assessed geomorphic and ecological responses to introducing woody habitat by comparing paired test and control reaches. Channel characteristics (e.g. bedforms, bars, texture) within test and control reaches were assessed before and after wood placement to quantify the morphological variability induced by the ELJs in the test reach. Since construction in September 2000, the ELJs have been subjected to five overtopping flows, three of which were larger than the mean annual flood. A high‐resolution three‐dimensional survey of both reaches was completed after major bed‐mobilizing flows. Cumulative changes induced by consecutive floods were also assessed. After 12 months, the major geomorphologic changes in the test reach included an increase in pool and riffle area and pool depth; the addition of a pool–riffle sequence; an increase by 0.5–1 m in pool–riffle amplitude; a net gain of 40 m³ of sediment storage per 1000 m² of channel area (while the control reach experienced a net loss of 15 m³/1000 m² over the same period); and a substantial increase in the spatial complexity of bed‐material distribution. Fish assemblages in the test reach showed an increase in species richness and abundance, and reduced temporal variability compared to the reference reach, suggesting that the changes in physical habitat were beneficial to fish at the reach scale. Copyright © 2004 John Wiley & Sons, Ltd.     

The role of the hyporheic zone in the nitrogen dynamics of a semi‐arid gravel bed stream located downstream of a heavily polluted reservoir (Tafna wadi,Algeria)

Nitrogen retention was measured along the Tafna wadi downstream of a heavily polluted reservoir in North‐West Algeria to understand the role of the hyporheic zone (HZ) in nitrogen dynamics. Nutrient concentrations were measured monthly for 2 years within the bed sediments of a 300 m reach located 20 km downstream from the dam. Due to strong hydrological fluctuations hyporheic water was analysed during natural low and high water (HW) periods, and during water reservoir releases. Nutrient concentrations in surface water (SW) increased during water releases and in the HZ during the low water (LW) periods. Surface/hyporheic water interactions were characterized by determining the vertical hydraulic gradient (VHG) and the chemical signature of the ground water (GW). The latter was obtained from regional GW monitoring. Hyporheic chemistry was strongly influenced by patterns of surface flow. Hyporheic and SWs had similar chloride concentrations during high flow when they were significantly lower than those of the regional GW. GW was generally richer in nitrates and nitrites, but was lower in ammonium concentrations than interstitial and river waters. Nitrates decreased significantly from upstream to downstream within the HZ throughout the hydrological period even though temporal fluctuations were high. Ammonium concentrations in interstitial water (IW) were significantly higher than in SW and generally increased from upstream to downstream. This study demonstrates the importance of the HZ in altering the dissolved inorganic nitrogen composition and concentrations of heavily polluted arid streams. The study is of interest because it documents a large ‘natural experiment’ that being the effect of periodic water release from a reservoir with serious water quality problems on the water quality dynamics (particularly nitrogen) of subsurface and SWs downstream. Copyright © 2008 John Wiley & Sons, Ltd.     

Invertebrate communities in Compensation Creek,a man‐made stream in boreal Newfoundland: The influence of large woody debris

Benthic invertebrate communities were examined in Compensation Creek, a man‐made stream in south‐central Newfoundland, Canada. Samples taken in September 2006 and September 2007 from large woody debris (LWD) were compared with samples from benthic environments to determine whether LWD supported a more diverse and abundant invertebrate community. Benthic habitats in a nearby natural stream were also sampled. Taxa composition was similar between the man‐made and the natural stream, highlighting successful colonization for the majority of taxa. Within Compensation Creek, taxa richness was higher in benthic habitats than on LWD, likely influenced by the successional age of the stream and surrounding habitat. The more complex benthic substrate provided refugia and allowed for the accumulation of fine detritus as a food source. Scrapers were almost completely absent from LWD and collector‐gatherer abundance was greater in the benthos. Collector‐filterer abundance was more than six times greater near the pond outflow than farther downstream when discharge was high, but abundances were almost equal when discharge was reduced. Riparian vegetation has not fully established around the man‐made stream, whereas it is overhanging and extensive at the natural stream, leading to more leaf‐litter input for shredders. As the morphology of Compensation Creek changes, the invertebrate community will continue to develop and likely increase utilization of accumulated detritus at LWD. Copyright © 2009 John Wiley & Sons, Ltd.     

Patterns in groundwater nitrogen concentration in the riparian zone of a large semi‐arid river (River Murray,Australia)

Lateral exchanges of surface water between river channels and their floodplains are important for vegetation health and aquatic food‐web productivity in semi‐arid ecosystems. However, the significance of the lateral connectivity via sub‐surface pathways in these systems is not as well understood. Patterns in nitrogen concentration in groundwater and in the unsaturated zone were used to infer the sub‐surface biogeochemistry of N in the riparian zone of a large semi‐arid floodplain (Hattah‐Kulkyne National Park) of the River Murray, Australia. The riparian zone plays a special role in this system as it is an area of transition between fresh surface waters and saline floodplain groundwater. The river was losing water to the floodplain during baseflow conditions but gradients were temporarily reversed following floods. In general, the redox conditions were sub‐oxic to anoxic in riparian groundwater and the main forms of N present were NH and dissolved organic N. There was a gradient in NH concentration from the river to the floodplain, suggesting that the main source of NH was from the decomposition of organic matter in fluvial sediments. Elevated concentrations of NO were occasionally found in shallow groundwater away from the river following floods but tended not to persist. The source of the NO appeared to be unsaturated‐zone NO displaced to the water table during floods. Assuming that denitrification was the main attenuation process, this displacement of unsaturated zone NO to anoxic groundwater could be a significant N removal process from the ecosystem (estimated at 18 kg N ha^?1 for the largest flood during the study). Understanding the impact of river regulation on floodplain nutrient cycles in River Murray floodplains will be challenging because the changes in floodplain hydrology are complex and coincide with salinization of soils and groundwater. Copyright © 2005 John Wiley & Sons, Ltd.     

Spatial distribution of large wood jams in streams related to stream‐valley geomorphology and forest age in Northern Michigan

Geomorphology at the scale of stream valleys influences smaller scale processes that give rise to spatially distributed patches, including large wood jams (LWJ) in streams. Understanding the spatial distribution of LWJ along streams with reference to large‐scale geomorphology is valuable for understanding stream and riparian interactions, and may be critical for effective stream management and restoration. We surveyed the locations of LWJ along 18 stream segments within study areas defined by stream‐valley geomorphology. The objective of this study was to test the prediction that LWJ in this system will be aggregated in areas defined by stream‐valley geomorphology, but be randomly distributed at smaller scales. The spatial distribution of LWJ was analysed by a one‐dimensional K‐function analysis capable of detecting aggregated, random and segregated patterns at different scales. The prediction that LWJ aggregate in areas defined by stream‐valley geomorphology was supported: LWJ aggregated at scales up to several kilometres in three streams. LWJ also was segregated at smaller scales in two of these streams; this was detectable when several stream valley segments were considered together. The prediction that LWJ would be randomly distributed at smaller scales was supported at most smaller scales for most streams. In fact, 40% of individual stream valley segments contained LWJ that were randomly distributed at all scales. Twenty‐seven per cent of individual stream valley segments showed segregated LWJ distributions. Large‐scale aggregation of LWJ evidences the need to select reference reaches that encompass several geomorphic patches at the scale of the stream valley. Segregated patterns of LWJ distributions evidence opportunities to better understand the relationships between hydraulic systems, material transport dynamics and riparian forests. Copyright © 2009 John Wiley & Sons, Ltd.     

The influence of river regulation and land use on floodplain forest regeneration in the semi‐arid upper Colorado River Basin,USA

Flow regulation effects on floodplain forests in the semi‐arid western United States are moderately well understood, whereas effects associated with changes in floodplain land use are poorly documented. We mapped land cover patterns from recent aerial photos and applied a classification scheme to mainstem alluvial floodplains in 10 subjectively selected 4th order hydrologic units (subbasins) in the Upper Colorado River Basin (UCRB) in order to document land use patterns (floodplain development) and assess their effects on Fremont cottonwood forest (CF) regeneration. Three of the mainstem rivers were unregulated, five were moderately regulated and two were highly regulated. We classified polygons as Undeveloped (with two categories, including CF) and Developed (with five categories). We ground‐truthed 501 randomly selected polygons (4–28% of the floodplain area in each subbasin) to verify classification accuracy and to search for cottonwood regeneration, defined as stands established since regulation began or 1950, whichever is most recent. From 40% to 95% of the floodplain area remained undeveloped, but only 19–70% of the floodplain area was classified as forest. Regeneration occupied a mean of 5% (range 1–17%) of the floodplain. The likelihood of the presence of regeneration in a polygon was reduced 65% by development and independently in a complex manner by flow regulation. Our analyses indicate that floodplain forests may be in jeopardy on both regulated and unregulated rivers and that information on historical forest extent is needed to better understand their current status in the UCRB. Conservation efforts need to be coordinated at a regional level and address the potentially adverse affects of both flow regulation and floodplain development. Published in 2007 by John Wiley & Sons, Ltd.