Low-gradient Channels Research Articles

The submerged Palaeo-Ems River in the Quaternary stratigraphic context of the German North Sea

During the Late Pleistocene, the Palaeo-Ems system (PES) was one of the major tributary channels feeding the Elbe Palaeovalley (EPV) and formed an important part of the drainage system of the continental northwestern Europe unglaciated hinterland. In this study, a detailed interpretation of high-resolution 2D sub-bottom profiler and Boomer grids were used to map the overall course of the submerged PES channel, as well as its spatial and stratigraphic relationships with other Quaternary geological units in the German North Sea. Mapping of buried tunnel valleys, a lacustrine-fill unit, straight channel structures was also carried out. The sedimentary record of the PES is represented by a low gradient and meandering channel which branched into two major pathways as it approached the EPV western flank. Its upstream to downstream trends, coupled with seaward decreasing gradient and sinuosity, are similar to those visible in present-day river deltas. Both the PES and EPV subsequently formed a combined depositional system, which was successively drowned due to the fast-rising relative sea level that overwhelmed the adaptation capabilities of the joint drainage system. For the first time, this study sheds light onto the PES/EPV morpho-stratigraphic relationships which played a key role in the coastal landscape architecture of the German North Sea since the Late Pleistocene.

Seasonal Catch Rates of the Endemic Olympic Mudminnow in Wetland Habitat

The Olympic mudminnow (Novumbra hubbsi) is a small freshwater fish, endemic to western Washington State. Although the species is listed as a Washington sensitive species, the lack of routine monitoring has resulted in poor understanding of population dynamics over time needed to support management and conservation actions. Olympic mudminnow commonly live in wetlands and associated low-gradient channels with tannic water and soft substrates, making conventional electrofishing and seine sampling approaches challenging and potentially inaccurate. Alternatively, minnow traps can easily be set in a wide range of depths and habitat types and allow for more systematic, repeated sampling. The study purpose was to conduct monthly surveys over a 19-month period to help develop standardized sampling methods. Sampling using baited Gee minnow traps set overnight was conducted in a wetland complex that flows into Eld Inlet in south Puget Sound, Washington. Catch rates of Olympic mudminnow were highest in late summer to early autumn (August through November) when water depths were low and water temperatures were decreasing. Trap mortality was relatively low for both Olympic mudminnow and amphibians but increased during warm spring and summer months. Eighty percent of the Olympic mudminnow captured were between 44 and 56 mm total length, and there was no difference in size between males and females. Gravid females were observed from October to June. Overall, minnow trapping appeared to be an effective method for monitoring Olympic mudminnow populations year round; however, sampling in the autumn appears to be advantageous because catch rates were high, and mortality was low.

Formation of low-gradient bedrock chutes by dry rockfall on planetary surfaces

AbstractChannel-like forms are ubiquitous on steep hillslopes on Earth, Mars, and other planetary bodies. On Earth and Mars, these landforms are commonly attributed to water activity, especially for slopes below the angle of repose (∼30°) where dry granular flows are considered ineffective. While the angle of repose characterizes loose sediment stability, it is unclear whether dry rockfall can traverse and erode channels in bedrock or cemented substrates. We used a large-scale experiment to show that bedrock chutes can form spontaneously at low gradients from dry rockfall. Our results, combined with observations of rocky outcrops and boulders on Mars, indicate that rockfall can be an important bedrock degradation process that can produce low-gradient channels in the absence of water.

Short-Term Stream and Riparian Responses to Beaver Dam Analogs on a Low-Gradient Channel Lacking Woody Riparian Vegetation

As ecosystem engineers, beavers (Castor canadensis) influence biogeomorphology and riparian vegetation. Streams with historic beaver removal and grazing often become incised, thereby altering stream and riparian processes. Anthropogenic structures that mimic beaver dams called beaver dam analogs (BDAs) may reverse incision by reintroducing historic processes. To understand BDAs as a process-based restoration tool, monitoring is needed across a range of watershed and stream conditions, but monitoring lags behind implementation. We constructed five BDAs in Central Oregon, on a low-gradient stream to test whether it may transport sufficient sediment to favor streambed aggradation behind BDAs. The stream also lacks woody riparian vegetation, and we examined how water temperatures and restoration plantings respond to BDAs where woody riparian vegetation is absent. We monitored structure integrity, aggradation, water temperature, groundwater, and vegetation for 1–2 yr after structures were installed. BDAs retained flows equally whether they were constructed from on-site juniper or off-site willow. During high flows, three structures failed and were rebuilt using improved design. After one year, aggradation of sediment above the two structures that survived flooding was negligible at one structure and high (33.7 m3) at the other. Groundwater levels rose 18–30 cm up to 135 m upstream of BDAs and 12 m into the floodplain. We found no evidence that BDAs raised stream temperatures. Within six months, willow cuttings planted near BDAs exhibited 1.3 times more growth than those on unimpounded locations. Overall, BDAs promoted a restoration trajectory on a low-gradient stream lacking woody riparian vegetation.

Barren Meandering Streams in the Modern Toiyabe Basin of Nevada, U.S.A., and Their Relevance To the Study of the Pre-vegetation Rock Record

Abstract Modern meandering streams devoid of plant life have long been ignored despite their relevance to the study of sedimentary landscapes in the long span of Earth's history that preceded the greening of continents. We present a sedimentologic and remote-sensing analysis of the McLeod Springs Wash, a small-scale, endorheic, and barren fluvial fan in the Toiyabe Basin of Nevada, USA. Fluvial channels therein undergo avulsion at the sub-decadal scale, and follow a morphodynamic cycle from low-sinuosity, higher-gradient, and sediment-starved streams into meandering, lower-gradient channels prone to overspilling. Sedimentation is purely clastic and, unlike in other arid-climate endorheic basins, water recharge maintains positive hydrologic budgets year-round such that any contribution to bank strength due to precipitation of evaporites is ruled out. Likewise, the unvegetated nature of the meandering channels excludes any direct biotic forcing on sedimentation, leaving an equilibrium between discharge, gradient, and bank strength aided by cohesive mud fractions as the main mechanism responsible for channel meandering. Point bars compose a volumetrically significant fraction of the McLeod Springs Wash's stratigraphic record, and consist of sub-meter-thick, fining-upward inclined ripple strata floored by intra-formational conglomerate. Their sedimentary record remains non-diagnostic unless morphodynamic analyses based on accretion and paleoflow vectors are integrated. Interpreting river planform from these deposits is complicated by swift channel avulsion and migration relative to alluvial aggradation, which make the preservation of entire channel-bar complexes exceptional. We conclude that a significant bias may exist against the interpretation and volumetric estimation of the pre-vegetation meandering-fluvial rock record, and that the application of classic point-bar models based on inclined heterolithic stratification to Ordovician and older rocks is intrinsically flawed.

Geomorphic constraints on uplift history in the Aspromonte Massif, southern Italy

The landscape morphology of the Aspromonte Massif, southern Italy, shows hanging low-relief surfaces that correlate with uplifted landsurfaces which are preserved all around the massif. The drainage network has deeply incised the high-standing low-relief landscapes, seen here as being the remnants of phases of relief smoothing prior to strong uplift that has affected the area since the Middle Pleistocene. GIS-aided estimation of the rock volumes eroded since the late Early Pleistocene indicate high variability in denudation rates between the gently rolling landscapes and the river valleys. In this framework, longitudinal river profiles are examined for 42 river channels to constrain multiple non-equilibrium conditions and to reconstruct the long-term uplift history of the massif. Most profiles consist of three/four segments separated by knickpoints and suggest a complex incision pattern. Longitudinal profiles reveal two upper, low gradient channel segments, a zone of steep channels further down the low relief landsurface margin, and a very steep lower channel segment, particularly in the western quadrant. Upper channel segments correspond to the Relict Landscape (RLandscape – a high-standing landsurface on top of the massif) and the Hanging Landscape (HLandscape – a landsurface surrounding the RLandscape), respectively. Steeper middle and lower segments are the result of two-phase river incision that is linked to changes in regional uplift rates, and, subordinately, to rock strength properties. Indeed, analysis of river longitudinal profiles could indicate, initially, relatively slow Aspromonte uplift during the early Pleistocene and, then, pulses of rapid uplift during the Middle-Late Pleistocene. Multi-segment river longitudinal profiles are, therefore, evidence of the transient landscapes of Aspromonte in response to pulsed massif uplift.

New insights into the fluvial geomorphology and its effects on paleofloods in the Black Hills, South Dakota

Identification and characterization of a relict landscape in the Black Hills of South Dakota has provided insight into the postuplift history (~30 Ma to present). A new watershed decomposition technique has been developed that utilizes high-resolution digital elevation models and traditional long valley profiles to identify the location and extent of the relict landscape as well as its geomorphic characteristics. Results identified an expansive relict landscape at elevation in the center of the Black Hills that contains fluvially mature features such as low-gradient channels and broad flood valleys. Moreover, the relict landscape and associated watersheds that drain it are undergoing active fluvial incision via knickzone migration. This observation suggested that the Black Hills were once much more subdued than today. At present, channel confinement, lack of potential floodwater storage, and steepened stream gradients exists in reaches of active incision downstream of the relict landscape. Large paleofloods influenced by these processes have been well documented and have been restricted to the narrow and steep canyons that drain the relict landscape. Therefore, active fluvial incision into the Black Hills influences the ability of more recent (Holocene) large floods to leave substantial evidence of their occurrence inside incised canyon walls and should also be considered when studying watershed factors influencing large flood events on the flanks of the Black Hills.

Impacts of small woody debris on slurrying, persistence, and propagation in a low-gradient channel of the Dongyuege debris flow in Nu River, Southwest China

Debris flows occurring in well-vegetated alpine areas usually contain a range of sizes of woody debris. Large woody debris (LWD), which has a retaining effect on further transportation of debris downstream, is mainly distributed in upstream reaches, and the amount of small woody debris (SWD) deriving from LWD increases dramatically midstream and downstream. The Dongyuege (DYG) bouldery debris flow with a sandy-matrix took place in a wildwood area, causing 96 deaths and its clay-sized fraction does not contain typical clay minerals. However, its total travel distance and runout distance in a low-gradient reach (between 2° and 5°) upstream of the depositional fan apex reached 11 km and 3.3 km, respectively. The abundant SWD in the DYG debris flow might have played a crucial role in slurrying, persistence, and the long runout over the low gradient. To understand why this debris flow extended so far, slurrying experiments, pore water escape experiments, and excess pore pressure experiments were performed. Crude debris (CD) collected from the DYG debris flow deposit was used throughout the experiments, the tested materials of which are separated into CD-containing SWD with a maximum grain size (MGS = 2 mm), purified debris (PD) without SWD with a MGS of 2 mm, and SWD < 2 mm in diameter. In the five slurrying experiments with PD-SWD-water mixtures, as the SWD content was elevated from 0.0 to 2.0 wt%, the current velocity of escaping pore water decreased uniformly from 17.2 to 0.9 mm/s. When the SWD content was 1.0 wt% or greater, the mixtures can be considered as one-phase flows of viscous fluids. The six pairs of pore water escape experiments based on the slurries remolded with CD and PD, respectively showed that the time needed for pore water to escape from the CD slurries was much greater than those from their PD counterparts. Also, measured was the dissipation rate of the relative excess pore pressure of CD and PD slurries of various densities and volumes, which showed that most of the rates of the PD-slurries were always greater than CD-slurries. Overall, the results show that SWD has a strong influence on the slurrying of the DYG debris without typical clay minerals found in other debris flows, and SWD helps to sustain the high excess pore pressure in the interior of the debris flow mass which resulted in the extended travel distance over such a low gradient. SWD favors the formation and stability of one-phase water-debris mixtures because of its large specific surface area and low density, which makes it able to absorb fine particles and able to be suspended in slurry flows over long timescales. In well-vegetated mountainous areas, SWD should be taken into account in the assessment of debris-flow hazards.

Effect of excess pore pressure on the long runout of debris flows over low gradient channels: A case study of the Dongyuege debris flow in Nu River, China

Debris flows with long reaches are one of the major natural hazards to human life and property on alluvial fans, as shown by the debris flow that occurred in the Dongyuege (DYG) Gully in August 18, 2010, and caused 96 deaths. The travel distance and the runout distance of the DYG large-scale tragic debris flow were 11km and 9km, respectively. In particular, the runout distance over the low gradient channel (channel slope<5°) upstream of the depositional fan apex reached up to 3.3km. The build-up and maintenance of excess pore pressure in the debris-flow mass might have played a crucial role in the persistence and long runout of the bouldery viscous debris flow.Experiments to measure pore pressure and pore water escape have been carried out by reconstituting the debris flow bodies with the DYG debris flow deposit. The slurrying of the debris is governed by solid volumetric concentration (SVC), and the difference between the lower SVC limit and the upper SVC limit can be defined as debris flow index (Id). Peak value (Kp) and rate of dissipation (R) of relative excess pore pressure are dependent on SVC. Further, the SVC that gives the lowest rate of dissipation is regarded as the optimum SVC (Cvo). The dissipation response of excess pore pressure can be characterized by the R value under Cvo at a given moment (i.e., 0.5h, 1h or 2h later after peak time). The results reveal that a relatively high level of excess pore pressure developed within the DYG debris-flow mass and had a strong persistence capability.Further research shows that the development, peak value and dissipation of excess pore pressure in a mixture of sediment and water are related to the maximum grain size (MGS), gradation and mineralogy of clay-size particles of the sediment. The layer-lattice silicates in clay particles can be the typical clay minerals, including kaolinite, montmorillonite and illite, and also the unrepresentative clay minerals such as muscovite and chlorite. Moreover, small woody debris can also contribute to the slurrying of sediments and maintenance of debris flows in well vegetated mountainous areas and the boulders suspended in debris flows can elevate excess pore pressure and extend debris-flow mobility.The parameters, including Id, Kp, R and etc., are affected by the intrinsic properties of debris. They, therefore, can reflect the slurrying susceptibility of sediments, and can also be applied to the research on the occurrence mechanisms and risk assessment of other debris flows.

Wood and sediment storage and dynamics in river corridors

AbstractLarge wood along rivers influences entrainment, transport, and storage of mineral sediment and particulate organic matter. We review how wood alters sediment dynamics and explore patterns among volumes of in‐stream wood, sediment storage, and residual pools for dispersed pieces of wood, logjams, and beaver dams. We hypothesized that: volume of sediment per unit area of channel stored in association with wood is inversely proportional to drainage area; the form of sediment storage changes downstream; sediment storage correlates with wood load; the residual volume of pools created in association with wood correlates inversely with drainage area; and volume of sediment stored behind beaver dams correlates with pond area. Lack of data from larger drainage areas limits tests of these hypotheses, but the analyses suggest that sediment volume correlates positively with drainage area and wood volume. The form of sediment storage in relation to wood appears to change downstream, with wedges of sediment upstream from jammed steps most prevalent in small, steep channels and more dispersed sediment storage in lower gradient channels. Pool volume correlates positively with wood volume and negatively with channel gradient. Sediment volume correlates well with beaver pond area.More abundant in‐stream wood and beaver populations present historically equated to greater sediment storage within river corridors and greater residual pool volume. One implication of these changes is that protecting and re‐introducing wood and beavers can be used to restore rivers. This review of the existing literature on wood and sediment dynamics highlights the lack of studies on larger rivers. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Beaver ponds' impact on fluvial processes (Beskid Niski Mts., SE Poland)

Beaver (Castor sp.) can change the riverine environment through dam-building and other activities. The European beaver (Castor fiber) was extirpated in Poland by the nineteenth century, but populations are again present as a result of reintroductions that began in 1974. The goal of this paper is to assess the impact of beaver activity on montane fluvial system development by identifying and analysing changes in channel and valley morphology following expansion of beaver into a 7.5km-long headwater reach of the upper Wisłoka River in southeast Poland. We document the distribution of beaver in the reach, the change in river profile, sedimentation type and storage in beaver ponds, and assess how beaver dams and ponds have altered channel and valley bottom morphology. The upper Wisłoka River fluvial system underwent a series of anthropogenic disturbances during the last few centuries. The rapid spread of C. fiber in the upper Wisłoka River valley was promoted by the valley's morphology, including a low-gradient channel and silty-sand deposits in the valley bottom. At the time of our survey (2011), beaver ponds occupied 17% of the length of the study reach channel. Two types of beaver dams were noted: in-channel dams and valley-wide dams. The primary effect of dams, investigated in an intensively studied 300-m long subreach (Radocyna Pond), was a change in the longitudinal profile from smooth to stepped, a local reduction of the water surface slope, and an increase in the variability of both the thalweg profile and surface water depths. We estimate the current rate of sedimentation in beaver ponds to be about 14cm per year. A three-stage scheme of fluvial processes in the longitudinal and transverse profile of the river channel is proposed. C. fiber reintroduction may be considered as another important stage of the upper Wisłoka fluvial system development.

The geomorphological context and impact of the linear emergent macrophyte, Sparganium erectum L.: a statistical analysis of observations from British rivers

ABSTRACTThis paper explores the geomorphological context and impact of the widely‐occurring, linear emergent macrophyte, Sparganium erectum.Forty‐seven sites across Britain were selected for field investigation, spanning the range of environmental conditions within which Sparganium erectum had been found to be present in previous analyses of national data sets. A combination of descriptive graphs and statistics, principal components analysis, and Kruskal–Wallis tests were used to explore the large multivariate data set collected at the 47 sites.The analyses showed that Sparganium erectum is present in significant quantities in relatively narrow and shallow (&lt; 18 m wide and &lt; 0.9 m deep to the limit of terrestrial vegetation), low gradient (maximum 0.004) channels of varying bed sediment calibre (cobble to silt). Within these environments, S. erectum stands (features) were associated with fine sediment retention, aggradation and submerged landform construction, leading to bench development and so, potentially, to adjustments in channel form and position. Sediment retention and landform construction within S. erectum features was most strongly apparent within reaches with a relatively high S. erectum cover and the presence of large area S. erectum features. It was also associated more weakly with S. erectum features that were comprised of relatively higher densities of plants with relatively smaller inter‐plant spacing and fewer leaves. The sediment retained in S. erectum features and associated bench and bank toe deposits showed larger numbers and species of viable seeds, indicating the potential for colonization and growth of other species on S. erectum features once they aggrade above the low flow water level and are no longer a suitable habitat for S. erectum. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Ecological function of constructed perennial stream channels on reclaimed surface coal mines

Mountaintop removal–valley fill mining results in the conversion of steep, forested headwater catchments to low gradient and open canopy channels. We compared the ecological functions of five reference stream channels to five constructed channels (age ranging from 3 to 20 years) on reclaimed mines in southern West Virginia. Variables included stream flow, habitat, water chemistry, riparian vegetation, organic matter (OM) processing, and invertebrate and amphibian communities. Although dissolved metal concentrations remained low, constructed channels produced significantly higher levels of conductivity and total dissolved solids as compared to reference streams. Macroinvertebrate and amphibian richness were comparable between constructed and reference channels; however, there was a distinct shift from sensitive lotic taxa in reference channels to tolerant lentic taxa in constructed channels. Constructed channels also had reduced OM decomposition rates. Nevertheless, constructed channels had significantly higher OM retention than reference channels, and consequently exhibited significantly higher overall OM processing and higher dissolved carbon concentrations. As the time since reclamation increased, we observed slight declines in conductivity and significant increases in total invertebrate richness. Our results provide measures of functional equivalencies between reference and constructed streams, which can serve as a basis for informed permitting and mitigation decisions in mined watersheds.

Erosion of a granite inselberg, Gross Spitzkoppe, Namib Desert

Namibia, with its passive margin setting, long-term tectonic stability, and long-lasting arid climate is a typical granitic landscape characterized by numerous dominating inselbergs. The Namib has been the focus of quantitative studies on the overall rates of bedrock exhumation, escarpment retreat, and sediment generation, transport, and deposition. Results from these studies indicate steady bedrock erosion rates ranging between 1 and 5mmkyr−1 over the last 105–106yr. This rate was determined from samples collected mostly from small inselbergs with low relief. How fast the large and “classic” inselbergs, which dominate the Namib Desert landscape, erode, generate sediment, and contribute to the overall sediment supply has yet to be systematically examined.We document erosion rates of the Gross Spitzkoppe (GS), one of the largest inselbergs in the Namib Desert, Namibia, based on measured concentrations of 10Be in samples collected from bedrock, boulders, and surface grus. The results suggest a slow lowering rate (1–2mmkyr−1), while the overall slope processes deliver sediment to the base of the cliff with 5–6×105atomsg−1 quartz. This concentration is equivalent to an average erosion rate of ~8mmkyr−1, 2–3 times faster than the bedrock lowering rate. Thus, the inselberg has been eroding by cliff retreat. The concentration of cosmogenic isotopes in slope sediment is controlled by exposure time on the slope, weathering of exposed bedrock, and weathering of subsurface bedrock.Sediment continues to accumulate cosmogenic isotopes while being transported on the flat pediments surrounding the inselberg. Within 1–2km from the base of the GS, the concentration doubles to ~10×105atomsg−1 quartz. The increase is achieved by dosing due to exposure and mixing with highly dosed sediment eroded from near subsurface bedrock. From this area, grus is transported through a network of low gradient channels to the Atlantic Ocean, ~100km away. Cosmogenic isotope concentration in sediment increases to ~30×105 atoms of 10Beg−1 quartz, which is 3–4 times higher than that measured in the grus close to the GS and is controlled by the erosion rate of stable bedrock throughout the entire drainage basin. Dosing due to exposure during transport obviously contributes to the overall measured concentration in the sediment as it approaches the ocean.

Impacts of precipitation on the cryologic regime of stream channels

AbstractThis paper examines the controlling influence of snow and rain on river ice processes in creeks and streams. Winter precipitation (in the form of rain and snow) has been observed to affect river ice processes and channel parameters of low and high gradient channels in unsuspected ways that can have significant impacts on channel hydraulics, hydrology and habitat. On a low gradient stream, a snowfall event initiated the development of an ice cover by creating unconsolidated snow slush bridges that eventually froze in place. Afterward, both snowfalls and rainfalls in alternation with cold spells dramatically increased the thickening rate of the ice cover well beyond that predicted by classic equations. In a smaller low‐gradient agricultural creek, wind‐blown snow impeded the formation of an ice cover by insulating the flow from cold atmospheric conditions. On steep channels (of different sizes and morphologies), anchor snow slush has been seen to accumulate on the bed substrate. As opposed to anchor ice, anchor snow slush is not believed to require supercooling water conditions to form nor to stay in place. Finally, in a steep headwater creek, a rain‐on‐snow event generated a snow slush flow and multiple snow slush jams. This phenomenon was seen to divert most of the water out of the channel into another watershed and concomitantly signalled a mid‐winter breakup in the greater watershed downstream. These observations suggest that the role of precipitation on small channel winter ice morphology and water flows, levels and currents has been severely underestimated and that any ecological winter studies, hydraulic structure designs and river modelling efforts need to include processes that are sometimes dominated by rain, slush and snow. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Identifying dissolved oxygen variability and stress in tidal freshwater streams of northern New Zealand

Tidal streams are ecologically important components of lotic network, and we identify dissolved oxygen (DO) depletion as a potentially important stressor in freshwater tidal streams of northern New Zealand. Other studies have examined temporal DO variability within rivers and we build on this by examining variability between streams as a basis for regional-scale predictors of risk for DO stress. Diel DO variability in these streams was driven by: (1) photosynthesis by aquatic plants and community respiration which produced DO maxima in the afternoon and minima early morning (range, 0.6-4.7 g/m(3)) as a product of the solar cycle and (2) tidal variability as a product of the lunar cycle, including saline intrusions with variable DO concentrations plus a small residual effect on freshwater DO for low-velocity streams. The lowest DO concentrations were observed during March (early autumn) when water temperatures and macrophyte biomass were high. Spatial comparisons indicated that low-gradient tidal streams were at greater risk of DO depletions harmful to aquatic life. Tidal influence was stronger in low-gradient streams, which typically drain more developed catchments, have lower reaeration potential and offer conditions more suitable for aquatic plant proliferation. Combined, these characteristics supported a simple method based on the extent of low-gradient channel for identifying coastal streams at risk of DO depletion. High-risk streams can then be targeted for riparian planting, nutrient limits and water allocation controls to reduce potential ecological stress.

Along-strike topographic variation of the Longmen Shan and its significance for landscape evolution along the eastern Tibetan Plateau

Regional topographic and geomorphic analyses reveal first-order topographic variations from high-elevation and low-relief interior plateau to the relatively low elevation, high-relief marginal plateau in eastern Tibet. Field investigation and slip distribution modeling after 2008 Ms. 8.0 Wenchuan earthquake indicate significant along-strike variability during the rupture that appears to correspond to different segments of a single fault system. This observation motivates a more careful examination of topographic features along the Longmen Shan to explore the connection between the seismic cycle and mountain building. Analyses of topographic relief, hillslope gradient, and channel gradient indices reveal significant differences in the character of topography along the Longmen Shan mountain front. The central portion of the range exhibits the highest slope, relief and steepness of river longitudinal profiles. Whereas the southern Longmen Shan exhibits only subtle differences associated with slightly lower hillslope and channel gradients, the northern Longmen Shan is characterized by topography of significantly lower relief, lessened hillslope gradients, and low-gradient channels. We consider two explanations for these topographic differences; first, that the differences in topographic development along the Longmen Shan reflect different stages of an evolutionary history. Alternatively, these may reflect differences in the rate of differential rock uplift relative to the stable Sichuan Basin.

Mobility of large woody debris (LWD) jams in a low gradient channel

Mobility of large woody debris (LWD) in low gradient channels is an important but often overlooked transport process. The majority of studies on LWD have focused on its role in geomorphic and ecologic river processes. When jams extend across the width of the channel, they have the potential to retain sediment and alter the channel profile. When jams obstruct only a portion of the channel, they can re-direct flow, altering patterns of scour and deposition. The boundary complexity created by LWD has a recognized role in riverine ecosystems which has led to programs of replacing LWD in-channel corridors where it was previously removed. Although LWD jams are common in rivers around the world, they have been studied most intensely in steep, forested channel reaches where they are often found to be stable channel features. It is not fully known how much of the information on LWD from steep forested channels will transfer to other channel types. Whereas it may be reasonable to assume that the ecological benefits of LWD are similar in low gradient channels, research has shown that a much higher rate of LWD transport occurs in low gradient channels, with jams mobilized on timescales of 10 0–10 2 years. This study evaluates the distribution and mobility of LWD over 72 km of the San Antonio River, a low gradient channel in southeast Texas. LWD jam locations were identified for 2003 and 2007 using a combination of aerial photography and field mapping. Each jam was cataloged according to its location in the channel cross-section and the amount of channel area blocked. During the four-year period, all the LWD jams were mobilized, including those jams extending across the channel width. Although easily mobilized, 34 jams re-form in the same locations, creating 34 channel locations with persistent LWD jams. Data from the San Antonio River are applied to two models developed to predict LWD mobility and transport distances to assess the applicability of each model to a low gradient channel. The locations of stable (or recurring) LWD jams were matched to model results where predicted LWD transport distances were equal to measured LWD jam spacing. Model results showed good agreement with the mean and median spacing of LWD jams when given input parameters specific to the channel and wood species. The ability to predict where LWD jams will persist over time in a low gradient channel has application in watershed management. Persistent LWD jams can exert a greater influence on channel morphology and may require active management.

Variability and uncertainty in reach bankfull hydraulic geometry

Observations of the hydraulic geometry of river channels are widely used in fluvial geomorphological research and river management. Variations in hydraulic geometry observed at sites through a region or through time are often assumed to be true variation, but measurement uncertainty will also contribute to these variations. Few studies have quantified the effect of error. This paper evaluates uncertainty in bankfull hydraulic geometry for 114 river sites in southeast Australia. These sites represent a broad range of river types, including lowland and upland rivers in temperate and semi-arid climates. Results will be applicable to hydraulic geometry investigations worldwide when consistent channel survey techniques are used. Uncertainty is quantified relative to observed variation in hydraulic geometry between sites. For bankfull discharge, width, depth, velocity and slope, respectively 4%, 3%, 7%, 23% and 42% of the site-to-site variation can be attributed to uncertainty. Uncertainty in the variability of bankfull metrics for each of the 114 sites (e.g. standard deviation in bankfull width) is higher, contributing between 40% and 50% of the observed between site variation. Mean width and depth are more sensitive to sample errors associated with cross-section surveys. Bankfull discharge, velocity and slope are more sensitive to errors in parameters for the gradually-varied flow model required to model flow conditions at bankfull discharge. We develop models to predict the error resulting from using hydraulic models to predict bankfull flow, which may be used to estimate the appropriate number and spacing of cross-section surveys required for accurate modelling. These models relate error in bankfull metrics to channel attributes and the number and spacing of cross-sections. To minimise errors, sites must be longer in low gradient channels and have more cross-sections in more variable channels.

Reach-scale channel geometry of mountain streams

The basic patterns and processes of steep channels remain poorly known relative to lower-gradient channels. In this analysis, characteristics of step-pool, plane-bed, and pool-riffle channels are examined using a data set of 335 channel reaches from the western United States, Nepal, New Zealand, and Panama. We analyzed differences among the three channel types with respect to hydraulics, channel geometry, boundary roughness, and bedforms. Step-pool channels have significantly steeper gradients, coarser substrate, higher values of shear stress and stream power for a given discharge, and larger ratios of bedform amplitude/wavelength ( H/ L). Pool-riffle channels have greater width/depth ratios and relative grain submergence ( R/ D 84) than the other channel types. Plane-bed channels tend to have intermediate values for most variables examined. Relative form submergence ( R/ H) is statistically similar for step-pool and pool-riffle channels. Despite the lesser relative grain submergence and greater bedform amplitude of step-pool channels, mean values of Darcy–Weisbach friction factor do not change in response to changes in relative grain submergence. These patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types.