Geomorphic Effects Research Articles

Hydrologic and geomorphic effects on riparian plant species occurrence and encroachment: Remote sensing of 360 km of the Colorado River in Grand Canyon

AbstractA common impact on riparian ecosystem function following river regulation is the expansion and encroachment of riparian plant species in the active river channels and floodplain, which reduces flow of water and suspended sediment between the river, riparian area and upland ecosystems. We characterised riparian plant species occurrence and quantified encroachment within the dam‐regulated Colorado River in Grand Canyon, Arizona, USA. We mapped 10 riparian species with high‐resolution multispectral imagery and examined effects of river hydrology and geomorphology on the spatial distribution of plant species and open sand. Analysis spanned an image time series from 2002 to 2009 to 2013, a period when plant species and sand were spatially dynamic and operations of Glen Canyon Dam included daily hydro‐peaking and small episodic controlled flood releases. Plant species occurrence and encroachment rates varied with hydrology, geomorphology and local species pool. Encroachment was greatest on surfaces frequently inundated by hydro‐peaking. Seep willow (Baccharis spp.), tamarisk (Tamarix spp.) and arrowweed (Pluchea sericea) were the primary encroaching woody species. Common reed (Phragmites australis) and horsetail (Equisetum xferrissii) were the primary encroaching herbaceous species. Encroachment composition from 2002 to 2009 was similar to the entire riparian landscape, whereas encroachment from 2009 to 2013 primarily consisted of seep willow and early colonising herbaceous species. Emergence of seep willow and arrowweed after burial by sand deposited by controlled floods indicated that those species were resilient to this form of disturbance. Describing patterns of species encroachment is an important step towards designing flow regimes that favour riparian species and ecosystem functions valued by stakeholders.

Tectonic geomorphology of an active slow-moving, intrabasinal fault: The Galera Fault (Guadix-Baza Basin, central Betic Cordillera, southern Spain)

In this work, we prove the usefulness of morphometric analyses, typically applied to basin-border faults, to define the tectonic geomorphology of a slow-moving, intrabasinal structure: the Galera Fault (Guadix-Baza Basin, southern Spain). The Galera Fault is a 30 km-long, oblique-slip fault with major left-lateral and minor vertical slip components. Through geological and structural analyses, we define for the first time the detailed surface geometry of the fault, which is characterized by features typical of left-lateral strike-slip faults. The morphometric analysis indicates that a combination of slow slip rates and the high erodibility of the juxtaposed basin infill deposits favours a rapid landscape response to fault activity that erases many landscape effects related to active tectonics. This masking is more effective on features generated by strike-slip displacement, leaving only subtle evidence, such as local stream deflections and upstream widening of catchments. In contrast, geomorphic effects related to vertical displacement are better preserved, including the control of the geometry of the main rivers and morphological differences in the drainage network between the two fault blocks. On the upthrown fault block, streams are generally shorter and steeper and have greater valley incision, leading to the development of a badland landscape. Moreover, the vertical deformation of a Middle Pleistocene glacis surface (ca. 90 m) demonstrates the important role of this slow-moving intrabasinal fault in the generation of relief in the Betic Cordillera during recent Quaternary time. Although the impact of this fault on relief building is very low in comparison with oblique-slip, basin-border faults in the mountain range, it has a key control on the Quaternary landscape evolution.

Geomorphic effect of debris-flow sediments on the Min River, Wenchuan Earthquake region, western China

Geomorphic effect of debris-flow sediments on the Min River, Wenchuan Earthquake region, western China

Geomorphic history of Lake Manix, Mojave Desert, California: Evolution of a complex terminal lake basin

Pluvial lake deposits in the Basin and Range province have long been exploited for records of hydrologic and climatic change. To obtain the most accurate reconstructions, a thorough understanding of the geomorphic evolution of such lake basins must be grounded in field stratigraphy and mapping, which are often lacking or are not well integrated with core studies. We conducted extensive stratigraphic investigation, mapping, and dating of exposed strata in the Lake Manix basin, former terminus of the Mojave River, California, during the Pleistocene to provide such a background and to supplement interpretations from a 45-m-long core.In this paper, we emphasize the geomorphic and tectonic processes that shaped the evolution of the Manix basin after the arrival of the Mojave River in the early-middle Pleistocene. The main processes include (1) sedimentary infilling of the lake basin, (2) interaction among the subbasins as controlled by internal sills and sill failures, (3) tectonics, and (4) climate change. From about 500 to 190 ka, Lake Manix was confined to its western subbasins, and fluctuated in response to climate change and perhaps to brief diversions of the river into the upstream Harper Lake basin. During this time, about 24 m of sediment accumulated near the confluence of the Mojave River and Manix Wash, and a 15-km-long clastic wedge built into the western basins, gradually filling the accommodation space.Two basin-integration events subsequently occurred, both caused by failure of sills that lay on sheared deposits along the Manix fault zone. When Lake Manix rose at the onset of glacial marine isotope stage 6, the sill on the south flank of Buwalda Ridge failed and triggered a catastrophic flood that entered the new, lower elevation, previously endorheic Afton subbasin. Subsequent lake-level changes affected both subbasins, but the thickest deposits after the flood are preserved in the Afton subbasin. The area upstream of Buwalda Ridge was only submerged during relatively high lake levels. Thus, sediment records interpreted from the core site and nearby outcrops in part reflect a geomorphic event (basin integration) rather than climatic conditions. The sill of Lake Manix after the integration event lay at the northeast end of the Afton subbasin, athwart strands of the Manix fault zone. At about 25 ka, this eastern sill was rapidly incised, and the river advanced to terminate in Lake Mojave in the Soda and Silver Lake basins. Recognition and dating of integration events, common for lake-river interactions in active tectonic settings, provide important context for interpreting paleoclimate records from sediments and provide temporal and spatial constraints on the biogeography of aquatic species.

Environmental associations and effects of disturbances by common wombats in alpine Tasmania

AbstractThe common wombat (Vombatus ursinus) is the largest extant fossorial mammal in Tasmania. Fossorial mammals have substantial edaphic and geomorphic effects, which, in turn can affect vegetation. We asked whether wombat disturbances were associated with particular environments in an alpine environment in Tasmania, and whether, and to what degree, they altered topography, soil or vegetation. Soil analyses, vegetation surveys and topographic transects were used to test for the effects of wombat activity. We contrasted toilets and controls. We compared adjacent burrows, mounds and paths. We also mapped burrows and animal trails. Wombat burrowing created distinct topography and was linked to feeding areas by path systems. There was no difference in bulk density, pH or phosphorus among burrow mounds, paths and undisturbed areas. Burrow mounds, usually used as toilets, were higher in nitrogen (mg L−1) (9. 8 ± 6.3) than either paths (2.3 ± 2.0) or controls (1.08 mg L−1 ± 1.5). Herbs were favoured by wombat disturbance, having a mean percentage cover of 27.8 ± 4.1 on burrow mounds compared to 14 ± 2.69 on controls, and a mean percentage cover of 51.6 ± 7.8 in toilets compared to 25.6 ± 4.8 in control plots. In contrast, shrubs have greater percentage cover in controls than on mounds (47.2 ± 12.0 cf. 12.7 ± 5.0) and toilets (37.3 ± 10 cf. 17.8 ± 6.5). Overall, wombat activity and modification of the alpine environment were concentrated in grassy vegetation above the winter water table, although paths, burrows and toilets extended into less grassy alpine communities. Our results imply that common wombats have a major role in engineering topography and soils with subsequent effects on vegetation, directly affecting at least 1.2% of the study area. Their reduction, or loss, is likely to lead to substantial changes in patterns of geodiversity and biodiversity.

Potential secondary effects of in‐stream wood structures installed for natural flood management: A conceptual model

AbstractThe use of in‐stream wood is one of the most commonly employed natural flood management (NFM) techniques. The effectiveness of NFM wood structures in reducing flood risks (i.e., their “primary” effect) has been relatively well documented. However, their additional or “secondary” effects on other natural processes have not been fully evaluated. These secondary effects can be inferred by reviewing previous studies that scrutinized natural wood accumulations or artificial wood structures constructed for purposes other than NFM. The degree of contact with base flows and the stream bed provides a broad classification of NFM wood structures. Having considered the similarities between NFM wood structures and other in‐stream wood types, it is suggested that the following geomorphic effects are common to all types of NFM wood structures: pool formation; accumulation of clasts immediately upstream; buffering against stream bed coarsening; and bank erosion, causing channel widening and the formation of floodplain channels. These geomorphic changes contribute to stream bed heterogeneity, potentially creating new niches for aquatic organisms such as macroinvertebrates. Moreover, NFM wood structures may retain benthic organisms accidentally flushed away during flood events, serving as sources of colonists during phases of recovery. Geomorphic changes induced by NFM wood structures may also contribute to spatial variation in rates of biogeochemical processing. Accumulation of fine sediments in some areas may provide more surfaces for the attachment of organic matter and micro‐organisms, hence increasing benthic metabolic rates. Stream bed scouring in other areas may lead to sediment instability, suppressing the growth of micro‐organisms and benthic metabolic rates.This article is categorized under: Water and Life > Conservation, Management, and Awareness Science of Water > Water Extremes Engineering Water > Sustainable Engineering of Water

First Chronological Constraints for the High Terraces of the Upper Ebro Catchment

The Cenozoic sedimentary basins in the Iberian Peninsula show a change from long-term basin infill to incision, a transition that indicates a period of major drainage reorganization that culminated in the throughflow of the networks to the Atlantic and Mediterranean oceans. Both the cause of the transition from aggradation to degradation and the linkages to tectonic, climatic, and geomorphic events hinge on the chronology of the fluvial network incision and excavation of the basin’s sedimentary fills. In this paper, we describe the first chronologic data on the highest fluvial terraces of the upper area of the Ebro River, one of the largest fluvial systems in the Iberian Peninsula, to determine the onset of incision and excavation in the basin. For this purpose, we combine electron spin resonance (ESR) and paleomagnetism methods to date strath terraces found at 140, 90, and 85 m above the current river level. Our results show ages of ca. 1.2 and 1.5 Ma for the uppermost river terraces in the upper Ebro catchment, constraining the minimum age of the entrenchment of the upper Ebro River.

Geomorphic effects of recurrent outburst superfloods in the Yigong River on the southeastern margin of Tibet

Landslide dam outburst floods have a significant impact on landform evolution in high mountainous areas. Historic landslide dams on the Yigong River, southeastern Tibet, generated two outburst superfloods > 105 m3/s in 1902 and 2000 AD. One of the slackwater deposits, which was newly found immediately downstream of the historic dams, has been dated to 7 ka BP. The one-dimensional backwater stepwise method gives an estimate of 225,000 m3/s for the peak flow related to the paleo-stage indicator of 7 ka BP. The recurrence of at least three large landslide dam impoundments and super-outburst floods at the exit of Yigong Lake during the Holocene greatly changed the morphology of the Yigong River. More than 0.26 billion m3 of sediment has been aggraded in the dammed lake while the landslide sediment doubles the channel slope behind the dam. Repeated landslide damming may be a persistent source of outburst floods and impede the upstream migration of river knickpoints in the southeastern margin of Tibet.

Badland landscape response to individual geomorphic events

Landscapes form by the erosion and deposition of sediment, driven by tectonic and climatic forcing. The principal geomorphic processes of badland – landsliding, debris flow and runoff erosion – are similar to those in full scale mountain topography, but operate faster. Here, we show that in the badlands of SW Taiwan, individual rainfall events cause quantifiable landscape change, distinct for the type of rainfall. Typhoon rain reduced hillslope gradients, while lower-intensity precipitation either steepened or flattened the landscape, depending on its initial topography. The steep topography observed in our first survey is inconsistent with the effects of any of the rainfall events. We suggest that it is due to the 2016 Mw 6.4 Meinong earthquake. The observed pattern in the badlands was mirrored in the response of the Taiwan mountain topography to typhoon Morakot in 2009, confirming that badlands offer special opportunities to quantify natural landscape dynamics on observational time scales.

Local geomorphic effects in the presence of accumulations of different densities

Mobile components of dynamic river systems are varied, and more than fundamental water and sediment interactions must be considered to assess morphodynamic changes. Here, the effects of instream wood and plastic accumulations on the riverbed are modelled and assessed in the presence of a hydraulic structure. We observe a recent tendency towards considering instream wood and plastics as integral riverine system components. Such shift is driven by a multidisciplinary interest in instream wood and riverine plastic dynamics, including those addressing flood hazards and risk assessments. Like sediment, instream wood and plastics can be transported along the river channels during floods, increasing flooding risks and hazards by accumulating near hydraulic structures. Collection racks, a commonly encountered hydraulic structure, are used for trapping and removing various accumulated material, mitigating any potential risks. However, accumulations at collection racks can cause other risks because of potential backwater rise and local scour development. Accumulations of different densities, such as a combined presence of instream wood and plastics, add further complexity to the process understanding. In this study, a physical model is used to explore the geomorphic effects of accumulations of two different densities at a collection rack. Local geomorphic changes (i.e., scour and deposition), their evolution and the spatial extent around a vertical collection rack were analysed with Structure from Motion photogrammetry. The results show a strong relationship between the accumulation density and the hydraulic and geomorphic changes. Light floating material, forming accumulations closer to the water surface, are associated with the largest geomorphic changes, in contrast to dense material, forming accumulations close to the riverbed. In the presence of different densities, intermediate geomorphic changes were observed. Finally, to discuss the observations, we present a conceptual flow field for different scenarios of accumulation densities. Ultimately, outcomes from this work are essential to improve design practices for river managers.

Hydrological and geomorphic effects upstream of the Cantillana Dam along the Guadalquivir River (southern Spain)

Hydrological and geomorphological impacts upstream dams have been less frequently investigated than those occurring downstream. Geomorphological mapping, multitemporal analysis of aerial photos, topographic survey comparison of the channel bed, and archives of information concerning flood events were used in this paper to document the effects that occurred upstream of the Cantillana Dam along the lower Guadalquivir River. The results show that the construction of the dam in 1956 caused progressive siltation that reduced either the water surface width (36.5% until 2010) or the reservoir volume (70% reduction until 2012). These changes decrease from the upstream boundary of the reservoir to the dam but show an increase in correspondence with the main tributary (Rivera de Huéznar) in the middle part. The aggradational process along the reservoir reach caused a reduction in the channel cross-sectional area and its flow capacity during floods. Consequently, higher river stages are reached for the same flow discharge, thereby producing an increased frequency and duration of inundation of the alluvial terraces adjacent to the reservoir.

Quantification of biogeomorphic interactions between small‐scale sediment transport and primary vegetation succession on proglacial slopes of the Gepatschferner, Austria

AbstractProglacial slopes provide suitable conditions for observing the co‐development of abiotic and biotic systems. The frequency and magnitude of geomorphic processes and plant composition govern this interplay, which is described in the model of biogeomorphic succession. In high mountain environments, this model has only been tested in a limited number of studies. The study aimed to quantify small‐scale sediment transport via erosion plots along a plant cover gradient and to investigate the influence of sediment transport on plant communities. We aimed to generate quantitative data to test existing biogeomorphic models. Small‐scale biogeomorphic interactions were investigated on 30 test plots of 2 × 3 m size on proglacial slopes of the Gepatschferner (Kaunertal) in the Austrian Alps during the snow‐free summer months over three consecutive years. The experimental plots were established on slopes along a plant cover gradient. A detailed vegetation survey was carried out to capture biotic conditions, and specific sediment yield was measured at each plot. Species abundance and composition at each site reflected successional stages. Additional environmental parameters, such as terrain age, geomorphometry, grain size distribution, soil nutrients, and precipitation, were also included in the analyses. We observed two pronounced declines in geomorphic activity on plots with both above 30% and above 75% plant cover. Nonmetric multidimensional scaling showed distinct clusters of vegetation composition that mainly followed a successional gradient. Sites that were affected by high‐magnitude geomorphic events showed different environmental conditions and species communities. Quantified process rates and observed species composition support the concept of biogeomorphic succession. The findings help to narrow down a biogeomorphic feedback window.

Influence of successive landslides on topographic changes revealed by multitemporal high-resolution UAS-based DEM

Landslides are recognized as dominant geomorphic events of morphological evolution in most mountainous and hilly landscapes. However, the lack of multitemporal high-resolution topographic data has resulted in a lack of quantitative estimates of topographic changes influenced by successive landslides. Taking a typical hillslope with successive loess landslides in the Heifangtai loess tableland, China, as an example, we conducted four unmanned aerial system (UAS) surveys and created corresponding high-resolution digital elevation models (HRDEMs) and orthophotos. We found that multitemporal UAS surveys have become a powerful new approach for addressing local topographic changes and evolution over a relatively long time series. Moreover, landslides can leave persistent geomorphic imprints on hillslope topography. The frequency distributions of topographic indexes are significantly influenced by successive landslides. The mean slope gradient, roughness and local relief decreased with successive landslide occurrences, whereas the mean topographic wetness index (TWI) increased. However, the mean slope aspect was almost unchanged by successive landslides. Furthermore, analysis of the coefficient of variation demonstrates that the frequency distribution of the slope gradient becomes more dispersed with landslide occurrences, while the slope aspect and TWI become more concentrated. The slope gradient changes with elevation. More broadly, this study provides new insights into the prediction of the local topographic feature changes and morphology evolution trends caused by successive landslides.

Geomorphic effects caused by heavy rainfall in the Corigliano-Rossano area (NE Calabria, Italy) on 12 August 2015

ABSTRACT The surroundings of Corigliano and Rossano (NE Calabria) were damaged by a short-lasting and localized rainstorm on 12 August 2015, when more than 230 mm of rain in sixteen hours caused damage exceeding 100 million euros. In the Map, shallow landslides triggered by the event are shown at 1:50,000 scale, together with areas affected by soil erosion and flooding, and most damaged sites; historical landslides are also mapped. The main slope-stability controlling factors are mapped at 1:200,000 scale, and an event-based geomorphological landslide inventory of the most damaged sector (Citrea basin) is provided at 1:10,000 scale. Isohyets (cumulative over 16 h), as obtained by interpolating rain-gauge and weather-radar data by Kriging techniques, are shown together with the density of event landslides. Estimated rain maxima (ca. 506 mm in 16 h) by far overcome measured ones, and ground effects well fit the sectors heavily stricken by the storm.

Geomorphologic Domination on Urban Sprawl of Southern Riyadh, Saudi Arabia Using Differential Interferometric Synthetic Aperture Radar (DInSAR)

This study tests the southern part of the Riyadh City growth domination by the Early Quaternary-Holocene trans-tensional Central Arabian graben system reactivation and the subsequent dissolution-induced collapses and karstification. This study utilizes Synthetic Aperture Radar (SAR) and Differential Interferometric Synthetic Aperture Radar (DinSAR) to examine the morphology of arid landscape, south of Riyadh. Eight Single Look Complex (SLC) amplitude images are calibrated, filtered, georeferenced and orthorectified at a resolution of 20 meters, and compared with one another by producing 17 diachronic images of the pairs at different intervals (1996, 2003-2005, 2008). The diachronic SAR intensity imageries suggest a downthrown displacement reaching 600 m and eastward tilting at the bottoms of the grabens. Also, the structurally-controlled valleys are developing an eastward-running drainage system towards the oasis of Al-Kharj and capturing an older hydrologic system. Moreover, a 12-year period (1996-2008) of the SAR data was obtained to examine the average annual rate of southern Riyadh’s urban sprawl, which is estimated at approximately 390 metres/year over the 12 years and constrained by geomorphological features towards the deformed area. DInSAR imageries show the primary results obtained from the 26 May 2004 and 31 Jan. 2005 pair of images, merged with 30 m resolution DEM-SRTM data for the arid region south of Riyadh to eliminate the influence of topography. DInSAR is applied in this study for its ability to detect small displacements at the centimetre scale (1/2 wavelength). Although the DInSAR’s coherence and phase imageries suggest a fairly stable region since the last tectonic and subsequent geomorphic events, erosional and artificial changes are observed, bounded within the valleys and depressions, primarily due to aeolian and fluvial processes and agriculture. It is highly recommended to preserve the area for sustainability and economy.

Geomorphic Effects of a Run-of-The-River Dam in a Multi-Driver Context: The Case of the Upper Garonne (Central Pyrenees)

Geomorphic Effects of a Run-of-The-River Dam in a Multi-Driver Context: The Case of the Upper Garonne (Central Pyrenees)

Lowland gravel-bed river recovery through former mining reaches, the key role of sand

Whereas the geomorphic effects of in-channel mining have been widely documented, very few studies have focused on recovery trajectories after mining ended. This paper describes and quantifies the channel adjustment of extended and over-widened mining pits located in a low energy gravel-bed river, and identifies the factors that control their recovery rate. The study was based on aerial photographs and LiDAR analysis, grain-size sampling, bedload computation and geophysical measurements made during ground penetrating radar (GPR) and electrical resistivity tomography (ERT) surveys. We describe the spatio-temporal changes in the morphology of former pits as well as the current surficial longitudinal grain-size pattern in pits and in neighbouring upstream and downstream reaches. Our results show a wide range of geomorphic readjustments. Between 23 and 47 yr after the end of mining, the mining reaches have still not yet fully recovered. The planimetric recovery rate is closely correlated with the ratio of the volume deposited in the pits to their original volume. Finally, the infilled material is only partly composed of bedload and the mainly sandy fine sediments play a primary role in the recovery process. The abundance of sand, controlled by the lithology of the watershed, explains the unexpectedly high recovery rates of some pits despite the low energy of the river.

Morphologic Response of a River Channel to Sand Mining in River Tyaa, Kitui County, Kenya

Over 40 billion tons of sand is mined worldwide every year which is estimated to be higher than the natural replacement rates. In Kenya, the rate of sand mining is raising concerns over its environmental effects since it is not regulated. This paper presents findings on the geomorphic effects of sand mining in the ephemeral River Tyaa channel in Kitui County. The study adopts the concept of feedback response mechanism of a natural geomorphic system. Through purposive sampling River Tyaa was selected for the study, where rampant sand mining was reportedly taking place. Random sampling on the five sand mining sites identified came up with a representative site namely Kanginga on which systematic sampling was applied while collecting data at both the active and control sites. Data on channel width, depth and slope angles was obtained through physical measurements while data on quantity of sand mined was obtained from Mwingi Sand Mining Cooperative. Multiple logistic regression analysis was used to analyse data whereby the model compared active and control sites. Test results indicated that sand mining had significantly increased river channel’s width (O.R. =1.531), depth (O.R. =1.527) and slope angles (O.R. =1.634) at active mining sites compared to control sites as deduced from the respective Odds Ratios. It concluded that sand mining had altered channel’s morphology resulting to adverse environmental effects such as loss of riparian vegetation and channel incision. It recommended curbing of illegal sand mining through licencing operators and reducing quantity of sand mined by closing some mines. Furthers, it recommended monitoring through regular Environmental Impact Assessment (E.I.A) and Audit (E.A) to inform protection of the river system from degrading.

Major geomorphic events and natural hazards during monsoonal precipitation 2018 in the Kali Gandaki Valley, Nepal Himalaya

Highest geomorphic activity in the central Nepal Himalayas is mainly driven by monsoonal precipitation. In contrast, the northern flanks of the Nepal Himalaya lay in the relative dry rain shadow of the mountain range.During the monsoon 2018, major floods and geomorphic events occurred in the Kali Gandaki (KG) valley impacting both the monsoon-affected and the dry parts of the catchments. We analyze the events and its triggers based on field observations, multiple satellite image interpretation, climatological analysis using Global Precipitation Measurement and MODIS snow cover data, hydrological analysis and media analysis.The hydro-meteorological triggers are complex. Exceptional precipitation in April and May 2018 occurred in the entire study area, followed by a rather dry period. Precipitation in August was exceptional in the northern part whereas below average in the South. We argue that dynamics of snow accumulation and delayed melting contributed significantly to flooding and increased geomorphic activity in the southern part in August whereas flooding in the northern part was mainly triggered by rainfalls. We thus define 2018 as an abnormal (pre-)monsoon year with less rainfall than average but being more catastrophic.Sediment dynamics in the study area are still controlled by the Dhampu rock avalanche dam and the braided river floodplain north of this knickpoint, where sediment pulses delivered from tributaries are rarely connected from the main river. During the monsoon floods 2018 sediment connectivity was given for most tributaries due to the steepness of the catchments.The study area is subject to major human impact. Mostly in the south, numerous hillslopes have been undercut by road construction, leading to higher geomorphic sensitivity. Severe landslides might thus be triggered in future even by less intense rainfall events.Magnitude and frequency of such abnormal (pre-)monsoon precipitation are highly relevant for sediment flux and natural hazards studies.

The effects of longitudinal variations in valley geometry and wood load on flood response

AbstractWe use field measurements and airborne LiDAR data to quantify the potential effects of valley geometry and large wood on channel erosional and depositional response to a large flood (estimated 150‐year recurrence interval) in 2011 along a mountain stream. Topographic data along 3 km of Biscuit Brook in the Catskill Mountains, New York, USA reveal repeated downstream alternations between steep, narrow bedrock reaches and alluvial reaches that retain large wood, with wood loads as high as 1261 m3 ha−1. We hypothesized that, within alluvial reaches, geomorphic response to the flood, in the form of changes in bed elevation, net volume of sediment eroded or aggraded, and grain size, correlates with wood load. We hypothesized that greater wood load corresponds to lower modelled average velocity and less channel‐bed erosion during the flood, and finer median bed grain size and a lower gradation coefficient of bed sediment. The results partly support this hypothesis. Wood results in lower reach‐average modelled velocity for the 2011 flood, but the magnitude of change in channel‐bed elevation after the 2011 flood among alluvial and bedrock reaches does not correlate with wood load. Wood load does correlate with changes in sediment volume and bed substrate, with finer grain size and smaller sediment gradation in reaches with more wood. The proportion of wood in jams is a stronger predictor of bed grain‐size characteristics than is total wood load. We also see evidence of a threshold: greater wood load correlates with channel aggradation at wood loads exceeding approximately 200 m3 ha−1. In this mountain stream, abundant large wood in channel reaches with alluvial substrate creates lower velocity that results in finer bed material and, when wood load exceeds a threshold, reach scale increases in aggradation. This suggests that reintroducing small amounts of wood or one logjam for river restoration will have limited geomorphic effects. © 2020 John Wiley & Sons, Ltd.