Showing papers on "Stream power published in 2006"

Trees as riparian engineers: the Tagliamento river, Italy

Abstract: Pristine river corridors were characterized by island and floodplain development driven by the inter-play of flows, sediments and woody vegetation. Here we explore these relationships within topographically controlled settings within the upper, middle and lower reaches of a large, semi-natural alpine to mediterranean river. These reaches have expanding or contracting valley floors within which we show that there are more or less predictable patterns of stream power and rates of vegetation growth, reflecting water availability during dry periods and also the availability of sand and finer sediment. We relate these to the pattern of island distribution that is repeated within the three reaches and is indicative of the engineering role of riparian trees. Islands are shown to develop within thresholds defined by stream power, rates of woody vegetation growth and rates of sedimentation, and to develop most quickly where riparian species include those capable of sprouting from driftwood.

Rock-slope failure and the river long profile

Abstract: This study examines the geomorphic effects of large (>10 6 m 3 ) rock-slope failures on long profiles of rivers in the Swiss Alps and the New Zealand Southern Alps. Regression of channel slope versus drainage basin area objectively highlights knickpoints separating incised from aggraded reaches that often correspond to locations of large rock-slope failures. For a fixed concavity index, the highest values of the steepness index and erosion index along a given profile spatially coincide with breach channels cut into formerly river-damming rockslide debris as old as 10 k.y. Assuming that the knickpoints do not predate slope failure, data show that high profile steepness and inferred specific stream power are not always the cause, but often a result, of large river-blocking rock-slope failure in mountain basins. Omission of rockslide data from slope-area plots lowers the steepness index and increases the concavity index on average, yet only in few cases more than one standard deviation.

Influence of incision rate, rock strength, and bedload supply on bedrock river gradients and valley-flat widths: Field-based evidence and calibrations from western Alpine rivers (southeast France)

Abstract: Several process-based models of river incision have been proposed in recent years that attempt to describe fluvial landform development. Although some field tests have been performed, more data are required to test the ability of these models to predict the observed evolution of fluvial landforms. We have investigated several tens of rivers located in the French western Alps that flow across folded sedimentary rocks with strongly contrasting rock strengths. These rivers record significant variations in some of the parameters controlling river incision, notably bedrock lithology, stream power, incision rate, and sediment flux, potentially allowing discrimination between existing models. Variations in incision rates are driven by variations in the amount of disequilibrium introduced in the river profiles during the last glaciation. We use diagnostic indices to investigate transport- and detachment-limited conditions, which include the channel morphology, the occurrence of lithogenic knickpoints, the continuity of alluvial and bedrock reaches, and the slope-area scaling of the river long profile. We observe transitions from detachment-limited to transport-limited conditions with increasing discharge/drainage area and decreasing incision rate. Bedrock strength influences the location of the transition predictably. The formation of transport-limited rivers coincides with the development of a valley flat wider than the active channel, which accommodates variations in bedrock strength, stream power, and incision rate along the transport-limited reaches. We propose and calibrate a model for the development of valley flats along transport-limited rivers and explore some properties of landscape development in mountain ranges controlled by transport-limited rivers.

Sediment storage, sea level, and sediment delivery to the ocean by coastal plain rivers

Abstract: Coastal and marine sedimentary archives are sometimes used as indicators of changes in continental sediment production and fluvial sediment transport, but rivers crossing coastal plains may not be efficient conveyors of sediment to the coast. Where this is the case, changes in continental sediment dynamics are not evident at the river mouth. Stream power is typically low and accommodation space high in coastal plain river reaches, resulting in extensive alluvial storage upstream of estuaries and correspondingly low sediment loads at the river mouth. In some cases there is a net loss of sediment in lower coastal plain reaches, so that sediment input from upstream exceeds yield at the river mouth. The lowermost sediment sampling stations on many rivers are too far upstream of the coast to represent lower coastal plain sediment fluxes, and thus tend to overestimate sediment yields. Sediment which does reach the river mouth is often trapped in estuaries and deltas. Assessment of sediment flux from coastal plain rivers is also confounded by the deceptively simple question of the location of the mouth of the river. On low-gradient coastal plains and shelves, the location of the river mouth may have varied by hundreds of kilometers due to sea-level change. The mouth may also differ substantially according to whether it is defined based on channel morphology, network morphology, hydrographic or hydrochemical criteria, elevation of the channel relative to sea level, or the locus of deposition. Further, while direct continent-to-ocean flux may be very low at current sea-level stands, sediment stored in estuaries and lower coastal plain alluvium (including deltas) may eventually become part of the marine sedimentary package. The role of accommodation space in coastal plain alluvial sediment storage has been emphasized in previous work, but low transport capacity controlled largely by slope is also a crucial factor, as we illustrate with examples from Texas.

Evolution of channel morphology and hydrologic response in an urbanizing drainage basin

Abstract: The Dead Run catchment in Baltimore County, Maryland, has undergone intense urbanization since the late 1950s. Reconstruction of the channel planform from topographic maps dating back to the 1890s and aerial photographs dating back to the 1930s indicates that the channel has remained stable in planform since at least the 1930s. The relative stability of Dead Run contrasts with the alterations in channel morphology reported for other urbanizing streams in the Piedmont physiographic province of the eastern United States. Trend analyses of discharge records in Dead Run show that urban development and stormwater control measures have had significant impacts on the hydrologic response of the catchment. The flood hydraulics of the Dead Run catchment are examined for the event that occurred on 22 June 1972 in association with Hurricane Agnes. A two-dimensional hydraulic model, TELEMAC-2D, was used with a finite-element mesh constructed from a combination of high-resolution LiDAR topographic data and detailed field survey data to analyse the distribution of boundary shear stress and unit stream power along the channel and floodplain during flooding from Hurricane Agnes. The spatial and temporal distributions of these parameters, relative to channel gradient and channel/valley bottom geometry, provide valuable insights on the stability of the Dean Run channel. The stability of Dead Run's channel planform, in spite of extreme flooding and decades of urban development, is most likely linked to geological controls of channel and floodplain morphology. Copyright © 2006 John Wiley & Sons, Ltd.

Stratification effects by fine suspended sediment at low, medium, and very high concentrations

Abstract: This paper describes results of the second part of a study on stratification effects by cohesive and noncohesive sediment. Winterwerp (2001) applied classical stratified flow theory implemented in a one-dimensional vertical numerical model (the 1DV POINT MODEL), showing that sediment-induced stratification effects may occur at already fairly small suspended sediment concentrations (i.e., a few 100 mg/L). We also discussed a basic difference between the behavior of cohesive and noncohesive sediment, which emerges as a result of the large water content of mud flocs. In this paper we elaborate further on the hydrodynamic description of the transport of fine suspended sediment by analyzing field and laboratory observations over a very large range of concentrations. We propose a sediment stability diagram to explain some features of hyperconcentrated flows, such as those observed in the Yellow River. We show that the behavior of hyperconcentrated flows is affected largely by hindered settling effects reducing the energy required to keep the sediment in suspension. The hydrodynamic description of sediment transport is used to predict capacity conditions as a function of a dimensionless stream power, i.e., U3/hgWs. This prediction agrees favorably with observations reported in literature covering four orders of magnitude in suspended sediment concentration.

The effects of rock uplift and rock resistance on river morphology in a subduction zone forearc, Oregon, USA

Abstract: Relationships between riverbed morphology, concavity, rock type and rock uplift rate are examined to independently unravel the contribution of along-strike variations in lithology and rates of vertical deformation to the topographic relief of the Oregon coastal mountains. Lithologic control on river profile form is reflected by convexities and knickpoints in a number of longitudinal profiles and by general trends of concavity as a function of lithology. Volcanic and sedimentary rocks are the principal rock types underlying the northern Oregon Coast Ranges (between 46°30′ and 45°N) where mixed bedrock–alluvial channels dominate. Average concavity, θ, is 0·57 in this region. In the alluviated central Oregon Coast Ranges (between 45° and 44°N) values of concavity are, on average, the highest (θ = 0·82). South of 44°N, however, bedrock channels are common and θ = 0·73. Mixed bedrock–alluvial channels characterize rivers in the Klamath Mountains (from 43°N south; θ = 0·64). Rock uplift rates of ≥0·5 mm a−1, mixed bedrock–alluvial channels, and concavities of 0·53–0·70 occur within the northernmost Coast Ranges and Klamath Mountains. For rivers flowing over volcanic rocks θ = 0·53, and θ = 0·72 for reaches crossing sedimentary rocks. Whereas channel type and concavity generally co-vary with lithology along much of the range, rivers between 44·5° and 43°N do not follow these trends. Concavities are generally greater than 0·70, alluvial channels are common, and river profiles lack knickpoints between 44·5° and 44°N, despite the fact that lithology is arguably invariant. Moreover, rock uplift rates in this region vary from low, ≤0·5 mm a−1, to subsidence (<0 mm a−1). These observations are consistent with models of transient river response to a decrease in uplift rate. Conversely, the rivers between 44° and 43°N have similar concavities and flow on the same mapped bedrock unit as the central region, but have bedrock channels and irregular longitudinal profiles, suggesting the river profiles reflect a transient response to an increase in uplift rate. If changes in rock uplift rate explain the differences in river profile form and morphology, it is unlikely that rock uplift and erosion are in steady state in the Oregon coastal mountains. Copyright © 2006 John Wiley & Sons, Ltd.

Cumulative effective stream power and bank erosion on the sacramento river, california, usa

Abstract: Bank erosion along a river channel determines the pattern of channel migration. Lateral channel migration in large alluvial rivers creates new floodplain land that is essential for riparian vegetation to get established. Migration also erodes existing riparian, agricultural, and urban lands, sometimes damaging human infrastructure (e.g., scouring bridge foundations and endangering pumping facilities) in the process. Understanding what controls the rate of bank erosion and associated point bar deposition is necessary to manage large alluvial rivers effectively. In this study, bank erosion was proportionally related to the magnitude of stream power. Linear regressions were used to correlate the cumulative stream power, above a lower flow threshold, with rates of bank erosion at 13 sites on the middle Sacramento River in California. Two forms of data were used: aerial photography and field data. Each analysis showed that bank erosion and cumulative effective stream power were significantly correlated and that a lower flow threshold improves the statistical relationship in this system. These correlations demonstrate that land managers and others can relate rates of bank erosion to the daily flow rates of a river. Such relationships can provide information concerning ecological restoration of floodplains related to channel migration rates as well as planning that requires knowledge of the relationship between flow rates and bank erosion rates.

Channel-reach morphology dependence on energy, scale, and hydroclimatic processes with implications for prediction using geospatial data

Abstract: Received 28 April 2005; revised 6 January 2006; accepted 10 March 2006; published 20 June 2006. [1] Channel types found in mountain drainages occupy characteristic but intergrading ranges of bed slope that reflect a dynamic balance between erosive energy and channel boundary resistance. Using a classification and regression tree (CART) modeling approach, we demonstrate that drainage area scaling of channel slopes provides better discrimination of these forms than slope alone among supply- and capacity-limited sites. Analysis of 270 stream reaches in the western United States exhibiting four common mountain channel types reveals that these types exist within relatively discrete ranges of an index of specific stream power. We also demonstrate associations among regional interannual precipitation variability, discharge distribution skewness, and means of the specific stream power index of step-pool channels. Finally, we discuss a conceptual methodology for predicting ecologically relevant morphologic units from digital elevation models at the network scale based on the finding that channel types do not exhibit equal energy dissipation.

Modeling the effects of variable annual flow on river channel meander migration patterns, sacramento river, California, USA

Abstract: Flow regulation impacts the ecology of major rivers in various ways, including altering river channel migration patterns. Many current meander migration models employ a constant annual flow or dominant discharge value. To assess how flow regulation alters river function, variable annual flows -based on an empirical relationship between bank erosion rates and cumulative effective stream power - were added into an existing migration model. This enhanced model was used to evaluate the potential geomorphic and ecological consequences of four regulated flow scenarios (i.e., different hydrographs) currently being proposed on the Sacramento River in California. The observed rate of land reworked correlated significantly with observed cumulative effective stream power during seven time increments from 1956 to 1975 (r 2 = 0.74, p = 0.02). The river was observed to rework 3.0 ha/yr of land (a mean channel migration rate of 7.7 m/yr) with rates ranging from 0.8 ha/yr to 5.1 ha/yr (2.0 to 13.3 m/yr), during the analyzed time periods. Modeled rates of land reworked correlated significantly with observed rates of land reworked for the variable flow model (r 2 = 0.78, p = 0.009). The meander migration scenario modeling predicted a difference of 1 to 8 percent between the four flow management scenarios and the base scenario.

Spatial variability in erosion in the Brahmaputra basin : causes and impacts

Abstract: The rivers from the Himalaya supply large quantities of particulate and dissolved materials to the oceans Among the various rivers, the Brahmaputra ranks highest in contributing to the sediment budget of the Bay of Bengal The erosion rates among the sub-basins of the Brahmaputra vary over 1-2 orders of magnitude, the highest being in the Eastern Syntaxis basin which is eroding at an enormously high rate of ∼ 14 mm yr -1 , caused by the high stream power of the Siang river These contemporary erosion rates are consistent with the time-averaged erosion and exhumation rates derived for ∼ 100 ka based on cosmogenic isotopes and geophysical methods Both the Eastern and the Western syntaxes experience rapid erosion suggesting that the syntaxes, a characteristic feature of the collision belt, undergo rapid erosion under favourable conditions The rapid erosion of the Eastern Syntaxis has caused important tectonic and geomorphological changes, such as the rapid uplift of this region resulting in the great peaks of Namche Barwa and Gyala Peri, and the nickpoint in the Tsangpo river bed prior to its entrance to the gorge Further, the rapid erosion makes up about half of the particulate material transported by the Brahmaputra to the Bay of Bengal, an order of magnitude higher than its areal coverage The high sedimentation rate in the Bay of Bengal over the past ∼ 1 Ma can be due to the high erosion rate of the Eastern Syntaxis The mineralogical and isotopic composition of the sediments seems to suggest this inference

An evaluation of the geomorphically effective event for fluvial processes over long periods

Abstract: [1] Fluvial processes erode landscapes in response to a wide range of discharges. The importance of a given discharge to the erosion of a basin can be calculated by multiplying the discharge's frequency of occurrence and the erosion rate produced by the discharge. The discharge that contributes the most geomorphic work is called the geomorphically effective event (GEE). In this paper, the behavior of the GEE is examined when a generic stream power model with a threshold is used to describe either the detachment or transport of sediment by flowing water. The results suggest that the return period of the GEE depends primarily on the threshold value when the exponent on discharge is less than 2. Otherwise, it depends primarily on the exponent. The GEE usually cannot be substituted for the probability density function of discharge because it produces a different long-term erosion rate. Furthermore, the return period of the GEE can vary spatially in a basin. For example, the return period can be different between locations where the fluvial process is dominant and subdominant if the threshold is nonzero. For a detachment-limited model the return period of the GEE is different upstream and downstream of knickpoints, and for a transport-limited model the return period is different along channel profiles even at steady state. Spatial variation in streamflow generation also produces spatial variations in the return period of the GEE.

The relationship of lithology and watershed characteristics to fine sediment deposition in streams of the Oregon coast range.

Abstract: Lithology is one of many factors influencing the amount, grain size distribution, and location of fine sediment deposition on the bed of mountain stream channels. In the Oregon Coast Range, 18 pool-riffle stream reaches with similar slope and intact riparian area and relatively unaffected by logjams were surveyed for assessment of fine sediment deposition. Half of the streams were in watersheds underlain by relatively erodible sandstone. The other half were underlain by a more resistant basalt. Channel morphology, hydraulic variables, particle size, relative pool volume of fine sediment (V*), and wood characteristics were measured in the streams. A significantly higher amount of fine sediment was deposited in the sandstone channels than in the basalt channels, as indicated by V*. Grab samples of sediment from pools also were significantly finer grained in the sandstone channels. Geographic information systems (GIS) software was used to derive several variables that might correlate with fine sediment deposition. These variables were combined with those derived from field data to create multiple linear regression models to be used for further exploration of the type and relative influence of factors affecting fine sediment deposition. Lithology appeared to be significant in some of these models, but usually was not the primary driver. The results from these models indicate that V* at the reach scale is best explained by stream power per unit area and by the volume of wood perpendicular to the flow per channel area (R2 = 0.46). Findings show that V* is best explained using only watershed scale variables, including negative correlations with relief ratio and basin precipitation index, and positive correlations with maximum slope and circularity.

Grain-size distribution patterns of a point bar system in the Usri River, India

Abstract: Grain-size distribution patterns in a point bar system of the Usri River, India, were critically analysed in the light of log-normal, log-hyperbolic and log-skew-Laplace distribution models Sand samples were collected from the cross-bedding foreset of different sizes of bedform; the objectives were to (i) study whether bedform heights have any role in grain-size distribution patterns, (ii) offer a best-fit statistical model, (iii) study the downstream variation of size-sorting in a point bar system, and (iv) study the mechanism of grain sorting The results indicate that the bedform heights have no role in grain-size distribution patterns Quantitatively when the errors in three distribution models were analysed, it was observed that the log-normal distribution is the best-fit statistical model and the next one is the log-skew-Laplace However, in the upper reaches of the river, log-normal distribution is the best-fit model in the case of large bedforms, whereas in the lower reaches the log-normal model is the best-fit one in the case of small bed forms It is also observed that within a point bar, for large and small bedforms, there is a tendency for mean grain size to decrease downstream Between point bars for large bedforms there is no consistency in decreasing grain size downstream, whereas for small bed forms the decrease of grain size downstream is observed except near the confluence at Palkia With distance of transport, the coarser and finer fractions of sediments are gradually chopped off The coarser fractions are buried below the advancing bedforms on the lee sides and the finer ones are transported further downstream Thus the finer admixture giving rise to the fining-upward sequence overlies a carpet of coarser materials This mechanism provides a clue to the process of grain sorting in the fluvial environment An interpretation has been offered for the log-normality of the grain-size distribution pattern During prolonged transportation in a fluvial environment, the larger grain-size fractions are gradually chopped off and buried below the advancing bedforms on their lee sides On the other hand, the finer fractions are transported further downstream in suspension Thus the narrow, intermediate size fraction takes active part in the distribution patterns leading to the generation of unimodality and a symmetric distribution pattern downstream, which are the main criteria for log-normality Similarly, increase of bedform size is the effect of increase of stream power and Froude number leading to the selective segregation of bed materials Thus the intermediate size fractions take a more active part than the coarser and the finer size fractions in developing log-normality Besides the hydrodynamic parameters of the Usri, coarsening of grain size downstream has been attributed to (i) the aggrading nature of the Usri downstream, and (ii) the contribution of coarser materials to the Usri by its tributaries and bank erosion Copyright © 2006 John Wiley & Sons, Ltd

Sensitivity of channel profiles to precipitation properties in mountain ranges

Abstract: [1] The stream power erosion law, which describes the erosion rate as a function of channel discharge and gradient, has often been used for modeling landscape evolution in regions dominated by fluvial processes. However, most previous studies utilizing the stream power erosion law simply use drainage area as a surrogate for channel discharge. Despite its convenience this simplification has important shortcomings. Specifically, it ignores the effects of precipitation properties on channel discharge and hence erosion rate, and it ignores the interactions between mountain ranges and precipitation properties. By using the stream power erosion law together with the geomorphoclimatic instantaneous unit hydrograph we provide a method for linking the landscape evolution and precipitation properties directly. Our results demonstrate that the channel profile is sensitive not only to the total precipitation but also to precipitation properties like the rainfall frequency, intensity, duration, and their distribution in space. The channel profile is most sensitive to the variation of rainfall intensity and less sensitive to rainfall frequency and duration. Shorter and more intense rainfall could lead to significantly higher erosion rate and flatter channel profiles compared to longer and less intense rainfall. The spatial variation of precipitation can also influence the evolution of channel profile. Even if the total precipitation remains spatially homogeneous, different spatial behavior of rainfall intensity and rainfall duration may lead to different steady state river profiles. The channel profile tends to be flatter under the conditions of increasing rainfall intensity and decreasing rainfall duration with elevation and vice versa.

Spatial Variability of Controls on Downstream Patterns of Sediment Storage: a Case Study in the Lane Cove Catchment, New South Wales, Australia

Abstract: This study uses GIS techniques to examine the spatial distribution of stream power along major streamlines in the Lane Cove catchment in northern Sydney, Australia. Channel gradient estimates derived from a 5 m resolution digital elevation model (DEM) are combined with streamflow data to estimate stream power along river courses. Stream power and its constituent components are then related to a detailed field-based assessment of sediment storage along the trunk stream and primary tributaries. At the catchment scale, sediment storage per unit length decreases as channel gradient and gross stream power increase. However, local controls such as variability in valley width and occurrence of confluence zones exert a greater influence upon sediment storage, disrupting systematic catchment-wide relationships. The total volume of storage along each streamline has a strong linear relationship to the area of the subcatchment, but the distribution of sediment along streamlines varies between subcatchments. The GIS framework employed in this project allows generation of continuous, empirical data, thereby providing catchment-specific predictive capacity that can accompany theoretical approaches to stream power modelling.

Field Evaluation of a Multi-Class Sediment Net Deposition Model in Contour Channels

Abstract: Accumulation of sediment in contour channels or terraces often occurs when overland flow passes from a hillslope to a lesser channel slope and sediment load is reduced. This article describes a field evaluation of an approximate analytic solution to a multi-class net deposition equation in a channel. The approximate analytic solution requires input settling classes to be of equal mass, which can be interpolated from a measured settling velocity distribution. Lateral input of runoff and suspended sediment into the channel was approximated by representing the channel as a series of 20 m segments that received hillslope inputs at their uppermost end via ephemeral gullies. At the beginning of each channel segment, new settling velocity classes (of equal mass) were interpolated, following mixing of inputs from the contributing hillslope segment and the previous channel segment. This procedure was found to be stable over the whole length of the channel only if the number of interpolated settling velocity classes (I) was >60. Precise measurement of such a large I value is infeasible, and users of the interpolation procedure should consider any potential errors that might occur if interpolating from a small number of measured data points. Using the model, the input sediment load was partitioned into deposition in the channel and output suspended sediment, which compared well with field measurements. However, enrichment of slower settling velocity fractions, relative to the input sediment, was overpredicted when compared to the only available measurement of the output settling velocity distribution. This finding is consistent with other applications of the model and with some applications of simple settling theory. Outputs were shown to be relatively insensitive to: surface roughness, threshold stream power beyond which re-entrainment occurs, and the proportion of stream power available for re-entrainment. It is likely that the approximate analytic solution is not applicable where channel segments are 10 m or less, in which case the full solution should be used.

Morphological changes in Alluvial rivers

Abstract: The independent parameters governing the morphology of alluvial rivers and the dependent variables are identified and defined. The relationships are given which enable determination of the dependent variables uniquely when the independent parameters are known. Morphological adjustments are examined when the sediment concentration increases with the bankful discharge Q remaining constant. The river is found to accommodate the increased sediment load by retaining the same sediment transport rate per unit width but reducing the discharge per unit width. In this process depth d reduces but the channel slope Sc proportionately increases. The stream power ϒqs per unit width thus remains unchanged. The width w increases due to side erosion and the meander sinuosity MS achieves a lower value. The required increase in SC accordingly becomes available by reduction in MS. Bank-erosion is a long-time process spread over a number of years and hence called a long-term adjustment. On the other hand, seasonal adjustment occurring during the annual flood period is short-time adjustment when changes can occur in bed-form and hence in Manning roughness coefficient n but time is insufficient to effect change in w and MS. The response of the morphological parameters in seasonal adjustment is therefore different from that in the long-term adjustment. The implication of this differential behaviour in a river is illustrated considering a few specific field applications.

Rates and controls of streambank retreat and erosion in three tropical streams in North-eastern Queensland

Abstract: Streambank retreat is a natural fluvial process altered by a variety of direct and indirect human activities that is controlled by interactions between a range of hydrological, geomorphological and vegetative factors. These may include climate, discharge, bed slope, bank material and stratigraphy, bank height, bank angle, curvature and the various attributes of bank vegetation. There has been considerable progress in our understanding of these processes and their interactions in temperate regions, but our knowledge of bank retreat in tropical streams is relatively poor. Few quantitative studies of bank retreat or erosion are published for the tropics. In particular, there is a paucity of data on vegetation characteristics, their interaction with retreat-causing variables and their contribution to bank retreat or erosion. This thesis addresses these issues by investigating the response of 34 sites in three north-eastern Queensland streams (2 wet tropics, 1 wet-dry tropics) to the 2003/2004 wet season, observing rates and types of bank retreat and the suite of driving forces that were responsible for this retreat. Variations exist in streambank retreat rate between climatological regions. Banks of streams tropical environments tend to retreat at greater rates because they experience greater specific stream power, more frequent bankfull events and higher annual flows than streams in other regions. Global trends also exist between bank retreat and stream width and drainage area. However, no global trends appear to exist between bank retreat relative to channel size and stream width. Modelling retreat of the study banks against climatological regime showed that they retreated at equivalent rates to streams of similar size elsewhere but at lower rates than streams from similar climatological regimes. These comparisons are only valid as far as datasets of differing quality and quantity allow. Analysis of 2003/2004 wet season hydrology suggested that these low rates could be partly attributed to the high recurrence possibility of the wet season. Variations in streambank retreat rate also exist within climatological regimes. The largely heterogeneous nature of streams and associated variability of dominant erosion driving forces is responsible for this variation. This study did not identify any direct relationship between streambank retreat and any measured variable. However, thresholds existed with regard to specific stream power (> 130 W m-2), curvature ( 3.2 m) and bank angle (> 45o), which explained the variability of bank retreat rates. Bank retreat was low until these thresholds were passed. When these thresholds were exceeded, retreat rates were more variable, with the steep banks retreating faster than more gradually sloped banks. There was no direct relationship between root area ratio (RAR) at any point on the bank and bank retreat. However, an exponential decay relationship existed between RAR at depths of 3 m and maximum bank angle: banks occupied by dense basal root networks were less steep, indicating an indirect relationship between bank retreat and basal RAR. Variations in erosion at different depths down a bank can ultimately control overall bank retreat. Thus, variations in local factors and their control of erosion are as important to measure as retreat itself. Specifically, the variations in RARs and their interactions with other local factors, such as depth or sediment characteristics are a major control of scour rates. Erosion rate variability in the study streams decreased logarithmically with both increasing RAR and gravel content of the bank. Thus, those banks with denser root networks and greater coarse fragment content were less likely to erode. The absence of erosion of gravel-dominated strata in this study is anomalous, but may be partly attributed to the low magnitude and short duration of the flows of the 2003/2004 wet season. Riparian influence on bank erosion and retreat is largely attributed to its effect on bank sediment strength and cohesion, but its influence on flow redirection away from the bank is also important. Root densities play a major part in these processes – greater densities provide increased cohesion, improved armouring of the bank from primary and secondary flows and sediment aggregation due to the input of organic matter. Root densities generally vary according to above-ground vegetation characteristics, sediment characteristics (moisture, texture, gravel content) and depth. There were linear relationships between root density (using RAR as the measure) and tree density that declined in strength with increasing depth at the 34 study banks. RARs at shallow depths were shown to be highly variable where trees were tall. RAR also varied greatly with depth. Wet tropics banks showed marked drops in RAR at depths of 2.5-3.0 m. A similarly significant decline was evident in wet-dry tropics banks at 2.0 m. No significant relationship existed between sediment and RAR. This thesis has highlighted the multi-faceted and complex nature of bank erosion and retreat in tropical Queensland streams, as reported in the literature for many temperate systems and the few tropical systems that have been studied. It suggests that specific stream power, curvature, bank geometry, RAR and gravel content and the interaction between these variables are all important in understanding bank erosion and retreat. But despite the extremes of the climate in the study region, erosion responses to a flood of moderate magnitude were within the range expected from other studies, suggesting, albeit with a small dataset, an adaptation of these systems to regular flooding. A larger dataset, including data on their reaction to events of larger magnitude may alter this relationship. It is clear that knowledge of these fluvial processes and characteristics in association with an appreciation of other local and catchment-based processes is essential for the development of appropriate catchment-wide and reach-based management plans

Variable channel responses following land clearing of a dryland catchment, Dalyup River, southwestern Australia

Abstract: Southwestern Australia serves as an interesting partner to other dryland settings in investigating and understanding the impact of humans on hydrological processes. Massive land clearing in the last half century and replacement of over 95% of native, deeprooted vegetation with shallow-rooted seasonal crops has profoundly transformed hydrology in the Dalyup River catchment. Runoff as a proportion of rainfall has increased, and recent flood events have resulted in significant channel impacts. In order to understand how to best manage these changes there is a need to determine whether alterations to channel morphology are part of a shift under altered landscape conditions, or part of natural variability in response to extreme events. The transition from low-gradient headwaters on a semi-arid sandplain, to a Mediterranean-type climate with more perennial flow conditions and steeper sloped channels in the lower catchment, results in significant downstream variation in channel morphology and erosive potential. Additional to this inherent variability that existed prior to clearing is the impact of water quality changes that have accompanied land clearing (increased stream salinity), resulting in severe degradation of riparian vegetation and a consequent increase in erosive potential. In-field investigation, together with landscape modelling and analysis of aerial photography from the time of land clearing (1960s) and before and after a recent large flood event (March 2000), identifies variable channel responses. While responses included channel avulsion, widening and significant floodplain deposition, the response of individual channel reaches does not necessarily correspond to areas with the greatest change in erosive potential or absolute erosive potential (stream power). Rather, a more complex reach-by-reach change is observed that is also influenced by the erosive threshold and volume of available material that is directly coupled with the river channel (i.e. in-channel or on the floodplain). Reaches in the mid and lower catchment with floodplains constructed from easily-erodible, sandy material display major post-clearing changes in response to riparian vegetation degradation and increased discharge. Elsewhere, the high erosive thresholds of materials, lack of available material coupled directly to the channel, and very low gradient reaches have resulted in relatively minor, or indistinguishable changes to the altered hydrological regime.

Translation and Dispersion of Sediment Pulses Induced by an Extreme Rainfall in Mountain Rivers

Abstract: Understanding linkage between channel process along a river course and hill-slope process with lateral sediment sources can be significant for management of sedimentary systems. The linkage, which is characterized by distribution of storm-induced sediment in a river channel, is often expressed as sediment pulse (wave). This paper examines the propagation of sediment pulse formed by storm-induced sediment flow in two river channels of Southern Hokkaido, and classifies the patterns of sediment pulse based on dierence in the distribution of accumulated sediment volume. Longitudinal changes in sediment volume along a river channel were modified to a sediment mass curve. Auto-correlation and cross-correlation analyses were employed for examining sediment mass curves. The sediment pulses demonstrated with sediment mass curve was classified to four patterns, such as decreasing, increasing, intermediate and periodic types. Decreasing type along a transport-limited channel and increasing type along a supply-limited channel were dominated. Periodic type was not influenced by sedimentary link between channel and hill-slope but independently occurred by available sediment along a channel. Furthermore these types of sediment pulse were resulted to be influenced by channel width, channel slope and (quasi-)stream power. Although the highest peak of sediment pulse has demonstrated at the channel reach with wide section and/or gentle slope, it has shown dispersed and lower peak at the channel reach with larger stream power.