relative age dating of transported regolith and application to study of landform evolution in the appalachians abstract surficial deposits in the appalachians generally are thin discontinuous and difficult to date in the absence of numerical dates particularly for older deposits relative age indices based on degree of weathering and soil development have been used to distinguish and correlate deposits of different ages a favorite locale of relative age studies has been stream terraces where chronosequences have been developed to shed light on the relationship between time and weathering soil development the lack of applicable numerical dating techniques has led some researchers to establish relative chronosequences in which height of stream or fan terraces above modern river level arl serves as a proxy for time approximate incision rates can then be used to estimate numerical ages of the deposits relative age correlations between the weathering indices on the terraces and comparable indices on dated deposits on the coastal plain and in nearby glaciated regions suggest that ages so estimated are the right order of magnitude relative age indices show correlations with height arl that are as high as those with time perhaps because more data sets are available for relative chronosequences than for true chronosequences analogous relative age sequences can be established for alluvial fans and even for hillslope colluvium many weathering indices have been employed but weathering rind thickness on mafic to intermediate clasts has proved to be a particularly consistent and useful parameter in addition to showing how weathering and soil development progress over time relative age dating has shed light on the manner in which landforms evolve along some rivers for example the areal extent of old highly weathered alluvium far exceeds that of younger alluvium suggesting that formerly floodplains and low terraces were much more extensive than at present in addition the manner in which weathering indices on some river terrace sequences vary with height arl suggests that incision rate has changed through time relative age mapping of alluvial fans produces a map pattern of older and younger fan surfaces that allows the sequence of fan development to be inferred in one case suggesting a scenario different from those previously described for fans mapping also shows that the relative abundance of young intermediate and old fan surfaces varies greatly from one area to another several studies have demonstrated clustering of relative age values into a small number of groups suggesting temporal grouping and thus episodic deposition on fans regression of weathering rind thickness against height above modern drainageways of remnant fan surfaces in two areas suggests that fans are being incised more rapidly in one area than in another relative age of hillslope colluvium also aids geomorphic interpretation young brown colluvium is much more common than old red colluvium near the glacier border where red colluvium rarely occurs at the surface but typically is covered by brown colluvium probably mobilized during the late pleistocene distant from the ice margin however red colluvium is extensively exposed on interfluves relatively stable hillslopes can also be distinguished from active ones in this manner for example colluvium on hillslope noses is somewhat more weathered than that on hollow floors and sideslopes

the effects of rock uplift and rock resistance on river morphology in a subduction zone forearc oregon usa 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 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 0 82 south of 44 n however bedrock channels are common and 0 73 mixed bedrock alluvial channels characterize rivers in the klamath mountains from 43 n south 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 0 53 and 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

importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem 1 fluvial erosion of bedrock occurs during occasional flood events when boundary shear stress exceeds a critical threshold to initiate incision therefore efforts to model the evolution of topography over long timescales should include an erosion threshold and should be driven by a stochastic distribution of erosive events however most bedrock incision models ignore the threshold as a second order detail in addition climate is poorly represented in most landscape evolution models so the quantitative relationship between erosion rate and measurable climatic variables has been elusive here we show that the presence of an erosion threshold when combined with a well constrained probabilistic model of storm and flood occurrence has first order implications for the dynamics of river incision in tectonically active areas first we make a direct calculation of the critical shear stress required to pluck bedrock blocks for a field site in new york second we apply a recently proposed stochastic threshold bedrock incision model to a series of streams in california with known tectonic and climatic forcing previous work in the area has identified a weak relationship between channel gradient or relief and rock uplift rate that is not easily explained by simpler detachment limited models the results with the stochastic threshold model show that even low erosion thresholds which are exceeded in steep channels during high frequency flood events fundamentally affect the predicted relationship between gradient and uplift rate in steady state rivers in a manner consistent with the observed topography this correspondence between theory and data is however nonunique models in which a thin alluvial cover may act to inhibit channel incision in the low uplift rate zone also provide plausible explanations for the observed topography third we explore the broader implications of the stochastic threshold model to the development of fluvial topography in active tectonic settings we suggest that continued field applications of geomorphic models including physically meaningful thresholds and stochastic climate distributions are required to advance our knowledge of interactions among surficial climatic and crustal processes

long term change in regime of the new river indicated by vertical variation in extent and weathering intensity of alluvium investigations of stream history in appalachian valleys have for the most part been confined to deposits within a few meters of the valley floor where radiocarbon dating is possible detailed study of new river alluvium in the radford north and eggleston 7 5 minute quadrangles of southwest virginia shows that meaningful information can be extracted from high level unpaired terrace deposits even where solution of carbonate bedrock has obscured terrace form a statistical analysis of terrace elevations performed by superimposing a 100 m grid over terrace maps and then determining the elevation of each grid point above modern river level amrl shows that alluvial deposits are more abundant at 0 12 m 0 40 ft and 30 49 m 100 160 ft while the 12 30 m 40 100 ft interval has relatively few deposits munsell color percent sand silt and clay and point counts of the heavy mineral and light mineral sand fractions were made in the b horizon at 35 sites ranging in elevation from 1 1 to 85 4 m amrl th

the frequency and distribution of recent landslides in three montane tropical regions of puerto rico landslides are common in steep mountainous areas of puerto rico where mean annual rainfall and the frequency of intense storms are high each year landslides cause extensive damage to property and occasionally result in loss of life average population density is high 422 people km2 and is increasing this increase in population density is accompanied by growing stress on the natural environment and physical infrastructure as a result human populations are more vulnerable to landslide hazards the blanco cibuco and coamo study areas range in surface area from 276 to 350 km2 and represent the climatologic geographic and geologic conditions that typify puerto rico maps of recent landslides developed from 1 20 000 scale aerial photographs in combination with a computerized geographic information system were used to evaluate the frequency and distribution of shallow landslides in these areas several types of landslides were documented rainfall triggered debris flows shallow soil slips and slumps were most abundant hillslopes in the study area that have been anthropogenically modified exceed 12 in gradient are greater than 300 m in elevation and face the east northeast are most prone to landsliding a set of simplified matrices representing geographic conditions in the three study areas was developed and provides a basis for the estimation of the spatial controls on the frequency of landslides in puerto rico this approach is an example of an analysis of the frequency of landslides that is computationally simple and therefore may be easily transferable to other settings

morphological and sedimentological responses of small stream channels to extreme rainfall and land use in the darjeeling himalayas abstract three pairs of small fourth to sixth order catchments approximately 5 80 km2 draining the margin of the darjeeling himalayas into the piedmont were selected for a comparison of the land use impact on the morphology and sedimentology of their stream channels each pair experienced similar annual rainfall and comprised similar metamorphic bedrock steep topography and brown soils the members of each pair contrast with respect to their land use structure as they comprise large forest cover forest 90 and a significant contribution of agricultural land with tea plantations forest

quantifying differential rock uplift rates via stream profile analysis despite intensive research into the coupling between tectonics and surface processes our ability to obtain quantitative information on the rates of tectonic processes from topography remains limited due primarily to a dearth of data with which to test and calibrate process rate laws here we develop a simple theory for the impact of spatially variable rock uplift rate on the concavity of bedrock river profiles application of the analysis to the siwalik hills of central nepal demonstrates that systematic differences in the concavity of channels in this region match the predictions of a stream power incision model and depend on the position and direction of the channel relative to gradients in the vertical component of deformation rate across an active fault bend fold furthermore calibration of model parameters from channel profiles argued to be in steady state with the current climatic and tectonic regime indicates that 1 the ratio of exponents on channel drainage area and slope m n is 0 46 consistent with theoretical predictions 2 the slope exponent is consistent with incision either linearly proportional to shear stress or unit stream power n 0 66 or n 1 respectively and 3 the coefficient of erosion is within the range of previously published estimates mean k 4 3 10 4 m 0 2 yr application of these model parameters to other channels in the siwalik hills yields estimates of spatially variable erosion rates that mimic expected variations in rock uplift rate across a fault bend fold thus the sensitivity of channel gradient to rock uplift rate in this landscape allows us to derive quantitative estimates of spatial variations in erosion rate directly from topographic data

grain size sediment transport regime and channel slope in alluvial rivers the general relationship between channel morphology and the grain size of sediment in the channel bed is an important but poorly known aspect of alluvial rivers an analysis of an equation for total sediment flux in the limits of suspension bedload and mixed modes of transport indicates distinct steady state regimes of channel morphology such regimes are readily seen in published data for modern alluvial rivers by way of a conventional shields plot or a plot of channel slope as a function of relative grain size d h and the ratio ws u where d and ws are respectively mean diameter and fall speed of bed sediments and h and u are respectively mean depth and friction velocity of the flow with slope and mode of transport in an alluvial river constrained by grain size and channel depth alone estimates of discharge and sediment flux follow directly introduction of the sediment flux relationship into conventional diffusion models for the evolution of an alluvial system provides nominal estimates of

coarse sediment distribution as evidence of an elevation limit for flash flooding bear creek colorado bear creek is a tributary of the south platte river in central colorado the stream flows east from an elevation of 4348 m at the continental divide to the mountain front at 1670 m it thus encompasses the 2300 m elevation limit for substantial rainfall flooding in the colorado front range proposed by jarrett maximum paleoflood discharges estimated from flood deposits at four sites along bear creek demonstrate a consistent decrease in unit discharge with increasing elevation and support the hypothesis of an upper elevation limit for rainfall floods the unit discharge values were used to explain coarse sediment distribution along bear creek measurements of coarse grained channel sediment at 19 sites along the creek indicate a decrease in particle size in flood deposits with increasing elevation as well as a decrease in the size of clasts introduced to the main channel along tributaries these changes in grain size are hypothesized to reflect changes in the competence of channel transport as a result of snowmelt dominated versus rainfall dominated discharge regimes above and below 2100 m elevation calculations of flow competence versus entrainment thresholds for the deposits may support this interpretation one of the geomorphic implications of the elevation limit on flash flooding is a reversal of the usual downstream fining trend in coarse channel sediments

arkose subarkose quartz sand and associated muds derived from felsic plutonic rocks in glacial to tropical humid climates abstract in the range of climates from high latitude glacial to equatorial tropical the mineral and chemical compositions of over 600 modern nonmarine first cycle sands and muds produced from felsic crystalline basement provenance in various tectonic settings were studied under glacial conditions sediments are developed by mostly mechanical weathering and thus are mineralogically and chemically similar to their source rocks humid tropical climates yield quartz rich sands kaolinite rich muds along with si mg ca na and k in solution between these extremes a wide range of detrital sediment compositions are developed in arid to wet climates in arid climate and semi arid climate sands feldspar is commonly more abundant than quartz and plagioclase k feldspar except in the cases of granite source rocks the muds are feldspathic with smectite dominant in the clays with increasing temperature and moisture in temperate to subtropical climates mafic silicates and feldspars are destroyed so that the sands range from arkoses with plagioclase as the major feldspar to subarkoses with dominant k feldspar through this range of climate muds become more kaolinitic and thus more aluminous with increasingly severe weathering typically feldspathic sands are associated with feldspathic muds with decreasing feldspar in sands there is less feldspar in muds with increasingly intense chemical weathering rare earth elements ree are more fractionated into muds typically sands contain 150 ppm ree with much of the ree in heavy minerals abundance differences are greatest between humid tropical sands and muds in many samples the ree are mostly in monazite allanite sphene and apatite more intensely weathered samples contain more xenotime and zircon evidenced by yb enrichment

the role of catastrophic geomorphic events in central appalachian landscape evolution abstract catastrophic geomorphic events are taken as those that are large sudden and rare on human timescales in the nonglaciated low seismicity central appalachians these are dominantly floods and landslides evaluation of the role of catastrophic events in landscape evolution includes assessment of their contributions to denudation and formation of prominent landscape features and how they vary through space and time tropical storm paths and topographic barriers at the blue ridge and allegheny front create significant climatic variability across the appalachians for moderate floods the influence of basin geology is apparent in modifying severity of flooding but for the most extreme events flood discharges relate mainly to rainfall characteristics such as intensity duration storm size and location landslide susceptibility relates more directly to geologic controls that determine what intensity and duration of rainfall will trigger slope instability large floods and landslides are not necessarily effective in producing prominent geomorphic features large historic floods in the piedmont have been minimally effective in producing prominent and persistent geomorphic features in contrast smaller floods in the valley and ridge produced erosional and depositional features that probably will require thousands of years to efface scars and deposits of debris slide avalanches triggered on sandstone ridges recover slowly and persist much longer than scars and deposits of smaller landslides triggered on finer grained regolith even though the smaller landslides may have eroded greater aggregate volume the surficial stratigraphic record can be used to extend the spatial and temporal limits of our knowledge of catastrophic events many prominent alluvial and colluvial landforms in the central appalachians are composed of sediments that were deposited by processes similar to those observed in historic catastrophic events available stratigraphic evidence shows two scales of temporal variation one related to quaternary climate changes and a more recent higher frequency variation due to rare events during the holocene in much of the central appalachians landforms related to quaternary climate changes persist as the most prominent features despite the modifying effects of late holocene catastrophic events

erosion of steepland valleys by debris flows episodic debris flows scour the rock beds of many steepland valleys along recent debris flow runout paths in the western united states we have observed evidence for bedrock lowering primarily by the impact of large particles entrained in debris flows this evidence may persist to the point at which debris flow deposition occurs commonly at slopes of less than 0 03 0 10 we find that debris flow scoured valleys have a topographic signature that is fundamentally different from that predicted by bedrock river incision models much of this difference results from the fact that local valley slope shows a tendency to decrease abruptly downstream of tributaries that contribute throughgoing debris flows the degree of weathering of valley floor bedrock may also decrease abruptly downstream of such junctions on the basis of these observations we hypothesize that valley slope is adjusted to the long term frequency of debris flows and that valleys scoured by debris flows should not be modeled using conventional bedrock river incision laws we use field observations to justify one possible debris flow incision model whose lowering rate is proportional to the integral of solid inertial normal stresses from particle impacts along the flow and the number of upvalley debris flow sources the model predicts that increases in incision rate caused by increases in flow event frequency and length as flows gain material downvalley are balanced by rate reductions from reduced inertial normal stress at lower slopes and stronger less weathered bedrock these adjustments lead to a spatially uniform lowering rate although the proposed expression leads to equilibrium long profiles with the correct topographic signature the crudeness with which the debris flow dynamics are parameterized reveals that we are far from a validated debris flow incision law however the vast extent of steepland valley networks above slopes of 0 03 0 10 illustrates the need to understand debris flow incision if we hope to understand the evolution of steep topography around the world

slopewash surface runoff and fine litter transport in forest and landslide scars in humid tropical steeplands luquillo experimental forest puerto rico rainfall slopewash the erosion of soil particles surface runoff and fine litter transport at humid tropical steepland sites in the luquillo experimental forest puerto rico 188 20 n 658 45 w were measured from 1991 to 1995 hillslopes underlain by 1 cretaceous tuffaceous sandstone and siltstone in subtropical rain tabonuco forest with vegetation recovering from hurricane hugo 1989 and 2 tertiary quartz diorite in subtropical lower montane wet colorado and dwarf forest with undisturbed forest canopy were compared to recent landslide scars monthly surface runoff on these very steep hillslopes 248 to 438 was only 0 2 to 0 5 per cent of monthly rainfall slopewash was higher in sandy loam soils whose parent material is quartz diorite averaging 46 g m 2 a 1 than in silty clay loam soils derived from tuffaceous sandstone and siltstone where the average was 9gm 2 a 1 annual slopewash of 100 to 349 gm 2 on the surfaces of two recent small landslide scars was measured initially but slopewash decreased to only 3 to 4 g m 2 a 1 by the end of the study the mean annual mass of fine litter mainly leaves and twigs transported downslope at the forested sites ranged from 5 to 8 g m 2 and was lower at the tabonuco forest site where post hurricane hugo recovery is still in progress mean annual fine litter transport was 2 5gm 2 on the two landslide scars copyright 1999 john wiley sons ltd

trends of grain sizes on gravel bars in the rio chagres panama abstract we examined the trends of grain sizes along the upper 414 km 2 of the mountainous rio chagres drainage basin in panama gravel bars were sampled along 40 km of the rio chagres and five major tributary streams using a transect pebble count of median diameter lithology and clast rounding although previous investigators have found that downstream fining can be obscured by inputs of colluvial sediment and other local controls in mountain drainages we decided to examine the trends of grain sizes along a tropical mountain river where rapid weathering and high capability of transport might be capable of overriding the input effects of colluvium specifically we tested the hypotheses that downstream fining would be present as a result of selective sorting and that weak felsic particles would decrease in size preferentially to strong mafic particles because of abrasion statistical analyses reveal a weak downstream decrease of sediment size on gravel bars along the study reach of the rio chagres with a sternberg diminution coefficient for felsic and mafic grains of 0 013 and 0 017 respectively felsic clasts have thicker weathering rinds and become rounded downstream faster than mafic particles but tumbling mill tests of abrasion show no significant differences in rate of mass loss in relation to lithology and downstream decreases in grain size are similar between lithologies dividing the study reach into six sub reaches bounded by major tributary junctions we further tested the hypothesis that downstream trends in fining might be obscured at the basin scale by sediment input from tributaries but that trends in grain sizes might be more visible at the reach scale between tributaries we did not find any consistent trends in grain size between tributaries stream width appears to assert a local control on grain size coarse particles are associated with narrow channel reaches whereas smaller particles are associated with wide channel reaches

role of inherent geological and climatic characteristics on landscape variability in the tectonically passive western ghat india abstract how does lithological variation affect the landscape and its evolution is a fundamental issue in modern and post orogenic terrains in the absence of active tectonic uplift spatial variation of rock resistance and rainfall distribution are thought to primarily control the fluvial and hillslope processes however these lithologic and climatic controls are difficult to decouple owing to the fact that these post orogenic terrains endure long and complex geomorphic history to examine the relative importance of lithology and rainfall in a post orogenic setting we analyzed west flowing drainage basins in the western ghat wg with a multi proxy analysis we relied on digital topography sediment yield and hydrological data we evaluated topographic relief hypsometry river long profiles and sediment yield to elaborate the importance of spatial variability of rock resistance in the long term evolution of these drainage basins the dominant increase of relief 200 1500 m and decrease of sediment yield 3 32 1 91 ton month km2 from north to south of the escarpment is directly attributed to the rock erodibility and structural characteristics field verification suggests that despite high rainfall condition the channels are mostly detachment limited and therefore lack the abrasive tools we further showed that the harder lithologies sustain higher steepness on the contrary the spatial distribution of satellite derived rainfall showed a negligible effect on the long profile evolution our analysis is also supported by the existing knowledge of chemical denudation rate and implicates that rock hardness and weatheribility defines the topographic characteristics with little or no effect of rainfall in this passive margin setting

new constraints on the late cenozoic incision history of the new river virginia abstract the new river crosses three physiogeologic provinces of the ancient tectonically quiescent appalachian orogen and is ideally situated to record variability in fluvial erosion rates over the late cenozoic active erosion features on resistant bedrock that floors the river at prominent knickpoints demonstrate that the river is currently incising toward base level however thick sequences of alluvial fill and fluvial terraces cut into this fill record an incision history for the river that includes several periods of stalled downcutting and aggradation we used cosmogenic 10be exposure dating aided by mapping and sedimentological examination of terrace deposits to constrain the timing of events in this history 10be concentration depth profiles were used to help account for variables such as cosmogenic inheritance and terrace bioturbation fill cut and strath terraces at elevations 10 20 and 50 m above the modern river yield model cosmogenic exposure ages of 130 600 and 600 950 ka respectively but uncertainties on these ages are not well constrained these results provide the first direct constraint on the history of alluvial aggradation and incision events recorded by new river terrace deposits the exposure ages yield a long term average incision rate of 43 m my which is comparable to rates measured elsewhere in the appalachians during specific intervals over the last 1 ma however the new river s incision rate reached 100 m my modern erosion rates on bedrock at a prominent knickpoint are between 28 and 87 m my in good agreement with rates calculated between terrace abandonment events and significantly faster than 2 m my rates of surface erosion from ancient terrace remnants fluctuations between aggradation and rapid incision operate on timescales of 104 105 year similar to those of late cenozoic climate variations though uncertainties in model ages preclude direct correlation of these fluctuations to specific climate change events these second order fluctuations appear within a longer term signal of dominant aggradation until 2 ma followed by dominant incision a similar signal is observed on other appalachian rivers and may be the result of sediment supply fluctuations driven by the increased frequency of climate changes in the late cenozoic

morphology and sedimentation in caribbean montane streams examples from jamaica and puerto rico abstract this paper presents a summary description of the morphology sedimentation and behaviour of the montane streams of eastern jamaica and eastern puerto rico the area is located within a 200 km wide seismically active zone of neogene left lateral strike slip deformation which defines the plate boundary between the caribbean and north american plates tropical storms occasionally strengthening up to hurricane force affect the region periodically this is an area of steep mass movement scarred hillslopes which supply a large amount of coarse sediment to the rivers from the description presented we have constructed a model for the rivers of this region controlled by both neotectonics and periodic large floods the drainage density is low with a near rectangular stream network the gradients are steep with boulder accumulations in the channels their location at times related to the presence of large past landslides on hillslopes narrow steep and confined channels occur in the mountains but in wider sections and lower down near coastal plains flood depositional forms appear in coarse valley alluvium small scale deviations from the general pattern occur locally controlled by variations in lithology neotectonism seasonality in flow etc this model for caribbean montane streams differs considerably from the standard descriptions of alluvial rivers for which a number of detailed studies are available

modeling point velocity and depth statistical distributions in steep tropical and alpine stream reaches 1 statistical hydraulic models predict the frequency distributions of point hydraulic variables relative to their reach averaged values in a stream reach based on its average characteristics e g discharge depth width average particle size the models initially developed in europe have not been tested for steeper streams 4 with coarse grain size we recorded water velocities and depths in 44 reaches of steep streams in tropical islands and the alps during 69 surveys we fitted the observed distributions of velocities and depths using a mixture of two distributions one with low variance and the other with a high variance then we predicted the mixing parameter on the basis of the reach averaged characteristics we compared the observed and predicted frequencies for five classes of velocities including a class of negative velocities and four classes of water depths the predictions of class frequencies have a bias of 5 our statistical model of velocity distribution predicts the frequencies of velocity classes with an explained variance between 33 and 72 for four classes of velocity and null for a class of intermediate velocity the statistical model of depth distributions was less efficient with an explained variance between 25 and 38 for three classes of depth and null for large depths the average froude number the total height of large drops relative to the reach length and the average slope are the main explanatory variables of velocity and depth distributions

investigating the complex interface where bedrock transforms to regolith abstract the interface where bedrock transforms to regolith is not planar but rather has a roughness that varies with the scale of observation the complexity of this surface is manifested in both element depth and fragment size depth distributions and may sometimes be related to the longitudinal profiles of watershed streams the fractal nature of the bedrock regolith interface means that the interface has a thickness which is 20 m in two ridgetop examples from pennsylvania and puerto rico such weathering thicknesses modeled as a function of one dimensional fluid flow are affected by the balance between rates of weathering and erosion one dimensional models are consistent with weathering advance rates that vary with equilibrium solubility and porefluid velocities and not reaction kinetics however fluid flow is not strictly downward and one dimensional permeability of regolith changes as particle size and bulk density changes with depth thus both downward and lateral flow occurs especially at reaction fronts where reactions change permeability the rate of weathering advance is therefore affected by the 3 dimensional distribution of reaction zones that affect permeability across the watershed quantitative models of such phenomena over a range of spatial and temporal scales are needed

knickpoint initiation and distribution within fluvial networks 236 waterfalls in the waipaoa river north island new zealand if knickpoints transmit signals of base level fall in river networks then improvements in our understanding of their retreat rate and basin wide distribution helps constrain the transient response following perturbation many studies of knickpoint retreat focus on the response of trunk streams to base level fall here we examine the response of an entire fluvial network as recorded by 236 active knickpoints distributed within the waipaoa river on the north island of new zealand base level fall within the waipaoa catchment initiated 18 000 years ago in response to a climatically triggered and tectonically exacerbated pulse of incision using observations from field work aerial photo analysis and a digital elevation model dem we study the knickpoint positions within the network we find that 70 of the knickpoints are located at drainage areas between 1 10 5 m 2 and 1 10 6 m 2 and more than half are b1 km upstream of a large change in drainage area for the knickpoints b1 km upstream of large tributary junctions we find that the retreat distances were well correlated with the tributaries drainage areas in order to determine how a pulse of incision distributes itself throughout a fluvial network we develop two simple end member models and compare their behavior to the observed knickpoint distribution in the waipaoa in the first model we propose that a knickpoint initiated at the basin outlet retreats upstream and distributes the signal throughout the network at a rate that is a power law function of drainage area in the second model we propose that knickpoints form near a threshold drainage area below which channels cannot incise with the same efficiency as possible in downstream reaches though neither model addresses the along stream variability in substrate or knickpoint form the misfit between the modeled and the observed knickpoints along stream positions are surprisingly low b1 km 1 25 of the total stream length for knickpoints with drainage areas b1 10 6 m 2 large misfits up to 3 5 km are observed for knickpoints with present day drainage areas greater than 1 10 6 m 2 the large single step in channel elevation that characterizes knickpoints presently observed in tributaries of the waipaoa river may not characterize the base level fall signal that propagated through the trunk streams evidence for progressive rather than instantaneous incision in the trunk streams the knickpoints vicinities to tributary junctions and the equivalent success of the two end member models lead us to conclude that the present positions of the 236 observed knickpoints are largely a consequence of thresholds in channel incision at low drainage areas

controls on the channel width of rivers implications for modeling fluvial incision of bedrock on the basis of the manning equation and basic mass conser vation principles we derive an expression for scaling the steady state width w of river channels as a function of discharge q channel slope s roughness n and width to depth ratio a w 5 a a1 2 2 3 3 8 q 3 8 s 23 16 n 3 8 we propose that channel width to depth ratio in addition to roughness is a function of the material in which the channel is developed and that where a river is con fined to a given material width to depth ratio and roughness can be assumed constant given these simplifications the expression emulates traditional width discharge relationships for rivers incis ing bedrock with uniformly concave fluvial long profiles more sig nificantly this relationship describes river width trends in terrain with spatially nonuniform rock uplift rates where conventional discharge based width scaling laws are inadequate we suggest that much of observed channel width variability in river channels confined by bedrock is a simple consequence of the tendency for water to flow faster in steeper reaches and therefore occupy smaller channel cross sections we demonstrate that using conventional scaling relationships for channel width can result in underestima tion of stream power variability in channels incising bedrock and that our model improves estimates of spatial patterns of bedrock incision rates

relict landscape resistance to dissection by upstream migrating knickpoints expanses of subdued topographies are common at high elevation in mountain ranges they are often interpreted as relict landscapes and are expected to be replaced by steeper topography as erosion proceeds preservation of such relict fragments can merely reflect the fact that it takes time to remove any preexisting topography however relict fragments could also possess intrinsic characteristics that make them resilient to dissection we document here the propagation of a wave of dissection across an uplifted relict landscape in puerto rico using 10be 26al burial dating on cave sediments we show that uplift started 4 ma and that river knickpoints have since migrated very slowly across the landscape modern detrital 10be erosion rates are consistent with these long term rates of knickpoint retreat analysis of knickpoint distribution combined with visual observations along the streambeds indicates that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat the studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks earlier studies have shown that vegetation over the relict topography efficiently limits erosion allowing for the formation of a thick saprolite underneath such slow erosion reduces streambed load fluxes delivered to the knickpoints as well as bed load grain size both processes limit abrasion compounding the effect of slow abrasion wide joint spacing in the bedrock makes plucking infrequent thus the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream reducing the celerity at which the relict subdued landscape is dissected we conclude that similar top down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies

channel geometry median grain size and stream power in small mountain streams abstract small mountain streams are typically affected by the same variables as larger counterparts but small headwater channels are also coupled with hillslopes the added degree of system complexity limits extension of existing conceptual and quantitative models of stream response to small mountain streams our objective is to determine how the presence of multiple lithologies on channel beds and coarse sediments affect relationships between channel geometry and driving and resisting variables in small mountain streams when the variables are examined at the landscape scale we test for systematic scaling of channel geometry stream power indices and median bed grain size d 50 in headwater streams draining 2 and overlying multiple strata on a passive continental margin field data were collected from 157 reaches in 32 watersheds local strata can be divided into a thick sequence of resistant sandstones and a thick sequence of low resistance calcareous siltstones and shales with interbedded limestones both stratigraphic units are present in every watershed and all streams carry abundant coarse bed load derived from the resistant unit mean widths and gradients of the channels are correlated with upstream drainage area unit stream power is not statistically different as a function of rock unit but channel gradients are lower atop the low resistance unit d 50 is not correlated with drainage area but is positively correlated with unit stream power the correlation between unit stream power and d 50 is evidence for mutual adjustments among gradients widths and d 50 as a function of the primary driving force in the drainage area discharge the small effect of bed lithology on stream processes and strong relationship between channel geometry unit stream power and d 50 implies that available driving forces and grain size are the primary controls on stream processes in the study area we conclude that future research into system response in this and similar settings should focus on sediment characteristics production and flux

fractal analysis of shoreline patterns for crenulate bay beaches southern china shoreline patterns of 34 crenulate bay beaches in southern china were investigated the differentiated criteria for log spiral shoreline curves offered by silvester am soc civ eng proc 96 1970 275 coastal engineering elsevier amsterdam 1974 338 and hsu et al j waterw port coast eng 115 1989 285 were used to determine the equilibrium morphology of each beach results show that the discriminated principles of the equilibrium log spiral curve were not suitable for all the beaches based on the theory of the log spiral stable curve and beach morphology and the dynamic action setting in southern china fractal analysis was adopted to classify the patterns in the shorelines it shows that shorelines include three modes dynamic balance of accretion static equilibrium and the dynamic balance of erosion

downstream fining in a network of gravel bedded rivers a network based routing model is used to determine how spatial variations in sediment supply influence rates of downstream fining in a small watershed in central pennsylvania gravel of a specified size distribution and lithology is supplied to the network at zero order basins once in transport the total sample weight and moments of the grain size distribution are modified according to empirical functions of transport distance contributions from each zero order basin are summed to determine the total population of sizes and lithologies at any location in the study area resistant quartz sandstone ridges supply sediment an order of magnitude larger than other lithologies because these ridges occur primarily in the basin headwaters spatial variations in supply strongly influence rates of downstream fining model calculations indicate that spatial variations in supply alone account for over 80 of the decrease in mean grain size along the main channel

controls on bedrock channel incision along nahal paran israel nahal paran drains 3600 km2 of egypt s sinai peninsula and israel s negev desert much of the channel is alluvial but a canyon 10 5 km long has been incised into late cretaceous chert and dolomite in the lower portion of the basin slackwater deposits and paleostage indicators preserved within the canyon record approximately 10 floods of 200 to 2500 m3 s 1 over a period of at least 350 years step backwater simulations of flood flow hydraulics indicate extreme variations in stream power per unit area along the length of the canyon and associated variability in energy expenditure and sediment transport these variations reflect channel cross sectional morphology the greatest values of stream power occur along the lower half of the study reach in association with three pronounced knickpoints and an inner channel the locations of these features reflect the exposure of thick resistant chert layers along the channel the presence of several similar but buried and inactive knickpoints along the upper study reach indicates that the locus of most active channel incision has shifted with time probably in response to baselevel changes associated with tectonic activity along the dead sea rift thus the rate and manner of channel incision along the canyon of nahal paran are controlled by lithologic variability and tectonic uplift as they influence channel morphology and gradient which in turn influence hydraulics and sediment transport

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

reach scale channel geometry of a mountain river mountain rivers can be subject to strong constraints imposed by changes in gradient and grain size supplied by processes such as glaciation and rockfall nonetheless adjustments in the channel geometry and hydraulics of mountain rivers at the reach scale can produce discernible patterns analogous to those in fully alluvial rivers mountain rivers can differ in that imposed reach scale gradient is an especially important control on reach scale channel characteristics as indicated by examination of north st vrain creek in colorado north st vrain creek drains 250 km2 of the rocky mountains we used 25 study reaches within the basin to examine controls on reach scale channel geometry variables measured included channel geometry large woody debris grain size and mean velocity drainage area at the study reaches ranged from 2 2 to 245 km2 and gradient from 0 013 to 0 147 m m 1 we examined correlations among 1 potential reach scale response variables describing channel bankfull dimension and shape hydraulics bedform wavelength and amplitude grain size ow resistance standard deviation of hydraulic radius and volume of large woody debris and 2 potential control variables that change progressively downstream drainage area discharge or that are likely to re ect a reach speci c control bed gradient we tested the hypothesis that response variables correlate most strongly with local bed gradient because of the segmented nature of mountain channels results from simple linear regression analyses indicate that most response variables correlate best with gradient although channel width and width depth ratio correlate best with discharge multiple regression analyses using mallow s cp selection criterion and log transformation of all variables produced similar results in that most response variables correlate strongly with gradient these results suggest that the hypothesis is partially supported channel bed gradient is likely to be a good predictor for many reach scale response variables along mountain rivers but discharge is also an important predictor for some response variables copyright 2004 john wiley sons ltd

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

limits of downstream hydraulic geometry adjustments to flow width depth and velocity in response to changes in discharge are commonly characterized by using downstream hydraulic geometry relationships the spatial limits of these relationships within a drainage basin have not been systematically quantified where the erosional resistance of the channel substrate is sufficiently large hydraulic driving forces presumably will be unable to adjust channel form data sets from 10 mountain rivers in the united states panama nepal and new zealand are used in this study to explore the limits of downstream hydraulic geometry relationships where the ratio of stream power to sediment size d 84 exceeds 10 000 kg s3 downstream hydraulic geometry is well developed where the ratio falls below 10 000 kg s3 downstream hydraulic geometry relationships are poorly developed these limitations on downstream hydraulic geometry have important implications for channel engineering and simulations of landscape change

impact of hydrologically driven hillslope erosion and landslide occurrence on soil organic carbon dynamics in tropical watersheds the dynamics of soil organic carbon soc in tropical forests play an important role in the global carbon c cycle past attempts to quantify the net c exchange with the atmosphere in regional and global budgets do not systematically account for dynamic feedbacks among linked hydrological geomorphological and biogeochemical processes which control the fate of soc here we quantify effects of geomorphic perturbations on soc oxidation and accumulation in two adjacent wet tropical forest watersheds underlain by contrasting lithology volcaniclastic rock and quartz diorite in the luquillo critical zone observatory this study uses the spatially explicit and physically based model of soc dynamics tribs eco triangulated irregular network based real time integrated basin simulator erosion and carbon oxidation and measurements of soc profiles and oxidation rates our results suggest that hillslope erosion at the two watersheds may drive c sequestration or co2 release to the atmosphere depending on the forest type and land use the net erosion induced c exchange with the atmosphere was controlled by the spatial distribution of forest types the two watersheds were characterized by significant erosion and dynamic replacement of upland soc stocks results suggest that the landscape underlain by volcaniclastic rock has reached a state close to geomorphic equilibrium and the landscape underlain by quartz diorite is characterized by greater rates of denudation these findings highlight the importance of the spatially explicit and physical representation of c erosion driven by local variation in lithological and geomorphological characteristics and in forest cover this article is protected by copyright all rights reserved

effects of land use change on channel morphology in northeastern puerto rico between 1830 and 1950 much of northeastern puerto rico was cleared for agriculture runoff increased by 50 and sediment supply to the river channels increased by more than an order of magnitude much of the land clearance extended to steep valley slopes resulting in widespread gullying and landslides and a large load of coarse sediments delivered to the stream channels a shift from agriculture to industrial and residential land uses over the past 50 yr has maintained the elevated runoff while sediment supply has decreased allowing the rivers to begin removing coarse sediment stored within their channels the size abundance and stratigraphic elevation of in channel gravel bar deposits increases channel depth decreases and the frequency of overbank flooding increases downstream along these channels this is presumed to be a transient state and continued transport will lead to degradation of the bed in downstream sections as the channel adjusts to the modern supply of water and sediment a downstream decrease in channel size is contrary to the expected geometry of self adjusted channels but is consistent with the presence of partially evacuated sediment remaining from the earlier agricultural period reverse downstream decreasing channel morphology is not often cited in the literature although consistent observations are available from areas with similar land use history identification of reverse channel morphology along individual watercourses may be obscured in multiwatershed compilations in which other factors produce a consistent but scattered downstream trend identification of reverse channel morphology along individual streams in areas with similar land use history would be useful for identifying channel disequilibrium and anticipating future channel adjustments

morphological organization of a steep tropical headwater stream the aspect of channel bifurcation abstract channel characterization and instream wood surveys demonstrate the spatial complexity of an ungauged tropical headwater stream hws in the tilaran mountains of costa rica throughout the stream course we characterized 29 individual reaches based on their morphological organization and instream wood occurrences bifurcated reaches were the most numerous and consistently distributed they also had the highest values of wood abundance and loading and contained the majority of unattached instream wood we selected two bifurcated reaches for ultrahigh resolution terrestrial laser scanning tls surveys orthographic tls produced millimeter resolution point cloud data sets from which 2 cm planimetric resolution digital elevation models dems of the channel surface were derived for both reaches cross sectional surveys of these dems using the river bathymetry toolkit rbt produced hydraulic geometries at two flow levels high and low for every meter length of channel reach we input these geometries into a one dimensional analytical model to demonstrate the downstream evolution of mean stream power through varying morphology elements mean stream power variation displays a regular pattern through the vertical modulation of step pool sequences but this regular pattern is lost through channel widening and bifurcation reach geometric data indicates that increasing width to depth ratio is the most notable feature of channel bifurcation and corresponds to a reduction in slope and roughness variation this primary understanding of channel dynamics denotes the complex organization of a tropical hws and promotes further research into tropical hws systems

characterizing the 2d shape complexity dynamics of the islands of sundarbans bangladesh a fractal dimension approach fractal dimension index fdi and shape index si of the islands of bangladesh sundarbans situated at the estuarine part of the ganges brahmaputra meghna gbm delta were estimated with the aid of remote sensing techniques between the years 1989 and 2010 the main objective was to correlate the temporal change in these two indices within two decades with the possible geomorphic processes regulating these patterns the sundarban region is composed of almost 200 islands having self similar coastlines with fdi and si ranged from 1 002 to 1 202 and 1 016 to 7 546 respectively both fdi and si values along with their temporal change showed that the complexity and irregularity in the island shape increased in the northern and north eastern parts of the sundarbans while in the southern end it exhibited an opposite trend apart from a few exceptions the longer serrated coastlines of the north fdi 1 18 became more serrated and rough due to the predominant tidal effect particularly ebb tidal flow within the two decades the comparatively smoother coastlines of the southern islands fdi 1 18 became more compact and circular due to strong sea wave action during the same course of time the spatial variation in fdi was explained in terms of the relative elevation of the islands which was found to be lower in the northern part their differential response to rising sea level and the amplified tidal action in this funnel shaped estuary apart from changes in fractal dimension a net land loss of 60 km2 was observed within this island system during the study period these observations imply that with an increasing sea level rise the northern and southern parts of sundarbans island system will respond differentially resulting in inundation in northern area water encroachment in the low lying islands and land loss in the southern part erosion in the coastal boundary causing a severe loss of ecosystem services provided by the mangrove forest

periglacial geomorphology of the appalachian highlands and interior highlands south of the glacial border a review abstract many features reported in the appalachian region have been assigned a paleoperiglacial origin based on field relationships and their similarities with analogs active in present day actuoperiglacial environments these forms include but are not limited to sorted and nonsorted varieties of patterned ground grezes litees block fields block slopes and block streams cryoplanation terraces and hillslope and river terrace landscapes although very small scale features generally less than 1 m in plan or section dimension and some larger forms on steep hillslopes are known to be at least sporadically active today the large scale features discussed here are interpreted as either inactive or truly fossil periglacial phenomena thus they hav implications for paleoclimatic reconstruction and significance as indicators of relative landscape stability since the time s of their development this paper briefly reviews the historical dimension of early researchs work present selected examples of recent results and gives authors conclusions

major change in atlantic deep and bottom waters 700 000 yr ago benthonic foraminiferal evidence from the south atlantic abstract in the modern south atlantic the transition between deep water and bottom water is marked by a clear change in the associated benthonic foraminiferal fauna uvigerina and globocassidulina characterize oxygen poor circumpolar deep water which has long been isolated from the surface planulina and miliolids are found associated with the more newly formed oxygen rich north atlantic deep water antarctic bottom water is characterized by epistominella umbonifera analysis of the benthonic foraminiferal faunas in two sediment cores recovered from the vema and hunter channels in the western south atlantic shows that the level of the transition between deep and bottom waters shallowed sharply about 700 000 yr ago this rise indicates a sharp sustained increase in the volume of bottom water flowing through the south atlantic after this time prior to about 700 000 yr ago the amount of antarctic bottom water entering the western south atlantic was greatly reduced and circumpolar deep water apparently accounted for the bulk of northward flow production of southward flowing north atlantic deep water seems not to have been affected the timing of this change in circulation regime suggests a possible causal link to similar changes in records of terrestrial and sea surface climate

regional analysis of bedrock stream long profiles evaluation of hack s sl form and formulation and assessment of an alternative the ds form the equilibrium form of the fluvial long profile has been used to elucidate a wide range of aspects of landscape history including tectonic activity in tectonic collision zones and in continental margin and other intraplate settings as well as other base level changes such as due to sealevel fluctuations the hack sl form of the long profile which describes a straight line on a log normal plot of elevation normal versus distance logarithmic is the equilibrium long profile form that has been most widely used in such studies slope area analysis has also been used in recent years we show that the sl form is a special case of a more general form of the equilibrium long profile here called the ds form that can be derived from the power relationship between stream discharge and downstream distance and the dependence of stream incision on stream power the ds form provides a better fit than the sl form to river long profiles in an intraplate setting in southeastern australia experiencing low rates of denudation and mild surface uplift we conclude that if an a priori form of the long profile is to be used for investigations of regional landscape history the ds form is preferable in particular the ds form in principle enables equilibrium steepening due to an increase in channel substrate lithological resistance parallel shift in the ds plot to be distinguished from disequilibrium steepening due to long profile rejuvenation disordered outliers on the ds plot slope area analysis and the slope distance ds approach outlined here are complementary approaches reflecting the close relationship between downstream distance and downstream catchment area copyright 2006 john wiley sons ltd

numerical simulation of the impact of sediment supply and streamflow variations on channel grain sizes and chinook salmon habitat in mountain drainage networks climatically driven changes in streamflow and hillslope sediment supply could potentially alter stream surface grain size distribution patterns and thereby impact habitat for a number of threatened and endangered in stream fish species relatively little is known about hydrograph shape peak flow influence or the relative importance of chronic and episodic hillslope inputs on channel conditions to better understand these external drivers we calculated sediment routing through a gravel bedded river network using a one dimensional 1d bedload transport model we calculated changes in grain sizes and estimated chinook salmon habitat suitability caused by a dry year and an extreme flood hydrograph and chronic diffusive overland flow or pulse landslide debris flow hillslope sediment supplies to obtain accurate channel conditions a relatively high reference shields stress representative of steep mountain streams was needed an extreme event flood without any hillslope sediment inputs caused widespread bed coarsening and a decrease in aquatic habitat chronic sediment input combined with this hydrograph eliminated any changes in grain size and habitat although when combined with a dry year flow caused systematic bed fining the influence of a given hydrograph therefore highly depends on the hillslope sediment supply regardless of the flow hydrograph or sediment pulse timing grain size distribution or location pulse sediment inputs did not cause widespread grain size changes despite being 100 times the total chronic input volume widespread and continuous hillslope sediment inputs may influence channel grain sizes and aquatic habitat more than a single discrete sediment pulse depending on the magnitudes of flow hydrograph and sediment supply alterations climate change may induce no differences in grain sizes or very dramatic changes with significant consequences for long term sustainability copyright 2013 john wiley sons ltd

morpho dynamic evolution patterns of subcarpathian prahova river romania abstract the prahova river romania at the southern carpathian mountains was affected by a number of measureable changes in its planform pattern and longitudinal profile over the past 200 years the mountain river has been incising into bedrocks narrowing its valley abandoning its lateral channels and straightening its path while crossing the subcarpathian study section the purpose of this study is to analyze using gis the relationship between short term less than 200 years lateral and vertical morphological adjustments observed over the 18 km length subcarpathian prahova river and specific long term evolutionary patterns more than 200 years human disturbances and hydrodynamic characteristics during high intensity flood periods are assessed in the context of existing neotectonics and lithology controls on channel and valley morphology surveyed cross section profiles were used to construct the geometry of a 1d hydraulic model simulations were performed using the hec ras software usace under steady flow conditions for flow values between bankfull and the peak discharge value of typical flood events with a return period of 20 and 50 years three main areas with high values of computed shear stresses and velocities and corresponding low values of width to depth ratio w d were identified these areas were found to match the observed erosion areas where the river has incised into bedrock and eroded its banks other potentially aggradation areas of low shear stress values and high w d were confirmed by field observations the evolutional patterns of prahova river channel are tectonic structurally determined developed and maintained during the quaternary mainly by the principles of river morphodynamics the evolution over shorter timescale bears the mark of the man induced impact changes in sediment and hydrologic regimes in a long term perspective the overall channel evolution of this river is expected to be strongly influenced by human activity

a quantitative comparison of soil development in four climatic regimes abstract a new quantitative soil development index based on field data has been applied to chronosequences formed under different climatic regimes the four soil chronosequences developed primarily on sandy deposits have some numeric age control and are located in xeric inland merced calif xeric coastal ventura calif aridic las cruces n mex and udic susquehanna valley pa soil moisture regimes to quantify field properties points are assigned for developmental increases in soil properties in comparison to the parent material currently ten soil field properties are quantified and normalized for each horizon in a given chronosequence including two new properties for carbonate rich soils in addition to the eight properties previously defined when individual properties or the combined indexes are plotted as a function of numeric age rates of soil development can be compared in different climates the results demonstrate that 1 the soil development index can be applied to very different soil types 2 many field properties develop systematically in different climatic regimes 3 certain properties appear to have similar rates of development in different climates and 4 the profile index that combines different field properties increases significantly with age and appears to develop at similar rates in different climates the soil development index can serve as a preliminary guide to soil age where other age control is lacking and can be used to correlate deposits of different geographical and climatic regions

longitudinal profile development into bedrock an analysis of hawaiian channels analysis of topographic maps of rivers incised into dated hawaiian lava flows shows that the long term average bedrock erosion rate along certain reaches is linearly related to stream power field observations suggest that two processes may control hawaiian channel downcutting 1 stream power dependent erosion including abrasion of the channel bed by transported particles and 2 step wise lowering caused by knickpoint propagation modeling results indicate that a simple stream power dependent erosion law predicts the straight to weakly convex longitudinal profiles characteristic of kauai channels but is insufficient to predict two other characteristic features the upslope propagation of knickpoints and the straight 5 8 channel slopes below the knickpoints thus more than this single transport law is apparently required to model bedrock channel incision field surveys also indicate that significant portions of the channel lengths below the knickpoints are mantled with large boulders we propose that

quantifying the significance of abrasion and selective transport for downstream fluvial grain size evolution it is well known that pebble diameter systematically decreases downstream in rivers the contribution of abrasion is uncertain in part because 1 diameter is insufficient to characterize pebble mass loss due to abrasion and 2 abrasion rates measured in laboratory experiments cannot be easily extrapolated to the field a recent geometric theory describes abrasion as a curvature dependent process that produces a two phase evolution in phase i initially blocky pebbles round to smooth convex shapes with little reduction in axis dimensions then in phase ii smooth convex pebbles slowly reduce their axis dimensions here we provide strong evidence that two phase abrasion occurs in a natural setting by examining downstream evolution of shape and size of thousands of pebbles over 10 km in a tropical montane stream the geometric theory is verified in this river system using a variety of manual and image based shape parameters providing a generalizable method for quantifying the significance of abrasion phase i occurs over 1 km in upstream bedrock reaches where abrasion is dominant and sediment storage is limited in downstream alluvial reaches where phase ii occurs we observe the expected exponential decline in pebble diameter using a discretized abrasion model the so called box equations with deposition we deduce that abrasion removes more than one third of the mass of a pebble but that size selective sorting dominates downstream changes in pebble diameter overall abrasion is the dominant process in the downstream diminution of pebble mass but not diameter in the studied river with important implications for pebble mobility and the production of fine sediments

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

geomorphology disturbance and the soil and vegetation of two subtropical wet steepland watersheds of puerto rico abstract relationships between landforms soil nutrients forest structure and the relative importance of different disturbances were quantified in two subtropical wet steepland watersheds in puerto rico ridges had fewer landslides and treefall gaps more above ground biomass older aged stands and greater species richness than other landscape positions ridge soils had relatively low quantities of exchangeable bases but high soil organic matter acidity and exchangeable iron valley sites had higher frequencies of disturbance less biomass younger aged stands lower species richness and soils with more exchangeable bases soil n p and k were distributed relatively independently of geomorphic setting but were significantly related to the composition and age of vegetation on a watershed basis hurricanes were the dominant natural disturbance in the turnover of individuals biomass and forest canopy however turnover by the mortality of individuals that die without creating canopy openings was faster than the turnover by any natural disturbance only in riparian areas was forest turnover by treefall gaps faster than turnover by hurricanes the same downslope mass transfer that links soil forming processes across the landscape also influences the distribution of landslides treefall gaps and the structure and composition of the forest one consequence of these interactions is that the greatest above ground biomass occurs on ridges where the soil nutrient pools are the smallest geomorphic stability edaphic conditions and biotic adaptations apparently override the importance of spatial variations in soil nutrients in the accumulation of above ground biomass at this site

downstream coarsening in headwater channels 1 field data from four mountain drainage basins in western washington document systematic downstream coarsening of median bed surface grain size d50 and a subsequent shift to downstream fining at a drainage area of about 10 km2 analyses of network wide patterns of unit stream power derived from both channel surveys and digital elevation models reveal maximum unit stream power that in all four study areas roughly corresponds with both the grain size maxima and an inflection in the drainage area slope relation thought to represent the transition from debris flow dominated channels to fluvially dominated channels our results support the hypothesis that basin wide trends in d50 are hydraulically controlled by systematic variations in unit stream power in addition to lag deposits forced by mass wasting processes the similar relations found in our four study areas suggest that the tendency for downstream coarsening may be ubiquitous in headwater reaches of mountain drainage basins where debris flow processes set the channel gradient

downstream variation in stream power abstract stream power has a significant influence on many form and process attributes of the fluvial system using a similar approach to that suggested by d m lawler in 1992 and 1995 a model is developed to explore how stream power might vary in the downstream direction based on the assumption that the longitudinal profile of the river has an exponential form no particular discharge is necessarily specified but near bankfull flows are regarded as significant morphologically and one such discharge is the focus here total stream power per unit channel length qs is predicted to peak at an intermediate location the position of which depends on the ratio b where b and are the downstream rates of change of discharge and slope respectively if b is a relatively conservative quantity then the form of the longitudinal profile and in particular its degree of concavity will have the major influence on the position of maximum stream power specific stream power w is even more sensitive to and is expected to peak closer to the headwaters about half way between source and the location of the stream power maximum although this will depend on how channel width w varies with distance from source the model is tested along three connected streams of different size in the catchment of the river trent taking advantage of a relationship between mean annual flood and link magnitude which enables this near bankfull discharge to be estimated at any point within the network the model is verified in so far as stream power does attain a mid basin maximum but the observed and predicted locations of the maxima do not always correspond largely because of the distinctive variation in discharge and especially slope along individual pathways for example maximum power lies far downstream along the trent because of the large increase in mid basin discharge associated with a series of large closely spaced tributaries the importance of slope is borne out by the near headwater locations for specific power maxima which tend to be further upstream than predicted along all three rivers as a result of the steepness of upper reaches the variety of geomorphic settings produces deviations from expectation which has implications for the movement and storage of material in the fluvial system

lithological and fluvial controls on the geomorphology of tropical montane stream channels in puerto rico an extensive survey and topographic analysis of five watersheds draining the luquillo mountains in north eastern puerto rico was conducted to decouple the relative influences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels the luquillo mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic influence on the stream channels however the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment gis based topographic analysis was used to examine channel profiles and survey data were used to analyze downstream changes in channel geometry grain sizes stream power and shear stresses results indicate that the longitudinal profiles are generally well graded but have concavities that reflect the influence of multiple rock types and colluvial alluvial transitions non fluvial processes such as landslides deliver coarse boulder sized sediment to the channels and may locally determine channel gradient and geometry median grain size is strongly related to drainage area and slope and coarsens in the headwaters before fining in the downstream reaches a pattern associated with a mid basin transition between colluvial and fluvial processes downstream hydraulic geometry relationships between discharge width and velocity although not depth are well developed for all watersheds stream power displays a mid basin maximum in all basins although the ratio of stream power to coarse grain size indicative of hydraulic forcing increases downstream excess dimensionless shear stress at bankfull flow wavers around the threshold for sediment mobility of the median grain size and does not vary systematically with bankfull discharge a common characteristic in self forming threshold alluvial channels the results suggest that although there is apparent bedrock and lithologic control on local reach scale channel morphology strong fluvial forces acting over time have been sufficient to override boundary resistance and give rise to systematic basin scale patterns copyright 2010 john wiley and sons ltd

stable isotope and solute chemistry approaches to flow characterization in a forested tropical watershed luquillo mountains puerto rico abstract the prospect of changing climate has led to uncertainty about the resilience of forested mountain watersheds in the tropics in watersheds where frequent high rainfall provides ample runoff we often lack understanding of how the system will respond under conditions of decreased rainfall or drought factors that govern water supply such as recharge rates and groundwater storage capacity may be poorly quantified this paper describes 8 year data sets of water stable isotope composition 2 h and 18 o of precipitation 4 sites and a stream 1 site and four contemporaneous stream sample sets of solute chemistry and isotopes used to investigate watershed response to precipitation inputs in the 1780 ha rio mameyes basin in the luquillo mountains of northeastern puerto rico extreme 2 h and 18 o values from low pressure storm systems and the deuterium excess d excess were useful tracers of watershed response in this tropical system a hydrograph separation experiment performed in june 2011 yielded different but complementary information from stable isotope and solute chemistry data the hydrograph separation results indicated that 36 of the storm rain that reached the soil surface left the watershed in a very short time as runoff weathering derived solutes indicated near stream groundwater was displaced into the stream at the beginning of the event followed by significant dilution the more biologically active solutes exhibited a net flushing behavior the d excess analysis suggested that streamflow typically has a recent rainfall component 25 with transit time less than the sampling resolution of 7 days and a more well mixed groundwater component 75 the contemporaneous stream sample sets showed an overall increase in dissolved solute concentrations with decreasing elevation that may be related to groundwater inputs different geology and slope position a considerable amount of water from rain events runs off as quickflow and bypasses subsurface watershed flowpaths and better understanding of shallow hillslope and deeper groundwater processes in the watershed will require sub weekly data and detailed transit time modeling a combined isotopic and solute chemistry approach can guide further studies to a more comprehensive model of the hydrology and inform decisions for managing water supply with future changes in climate and land use

denudation rates determined from the accumulation of in situ produced 10be in the luquillo experimental forest puerto rico we present a simple method for estimation of long term mean denudation rates using in situ produced cosmogenic 10be in fluvial sediments procedures are discussed to account for the effects of soil bioturbation mass wasting and attenuation of cosmic rays by biomass and by local topography our analyses of 10be in quartz from bedrock outcrops soils mass wasting sites and riverine sediment from the icacos river basin in the luquillo experimental forest puerto rico are used to characterize denudation for major landform elements in that basin the 10be concentration of a discharge weighted average of size classes of river sediment corresponds to a long term average denudation of 43 m ma 1 consistent with mass balance results

fractal geometry in the san andreas fault system it has been noted that the spatial distribution of earthquakes and the mode of strain release in the san andreas fault system is related to the complexity of fault geometry because of their rough appearance over many length scales faults can be regarded as fractal surfaces direct estimates of fractal dimension d of portions of the san andreas fault system between the northern gabilan range and the salton sea including the postulated extent of the great 1857 fort tejon earthquake are obtained from measured fault lengths analogous to the lengths of coastlines as discussed by mandelbrot regions characterized by complicated fault geometry are associated with larger values of d based on fault traces mapped at a scale of 1 750 000 d is 1 3 for this reach of the fault defined as a 30 km wide band about a main fault trace for that part near parkfield which could be associated with the nucleation of the 1857 earthquake d is 1 1 at this same scale d is 1 4 for the san andreas and related faults near san bernardino where the 1857 rupture stopped compared to 1 2 for the san andreas san juan fault segments near the point of arrest of the 1966 parkfield earthquake at finer map scales 1 24 000 and 1 62 500 critical lengths of 500 m and 1 km are identified which might relate to the extent of off san andreas fault offsets the critical lengths also suggest that fault geometry is not self similar if this fractal geometry persists through the seismic cycle it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault

relationships between hydraulic variables and bedload transport in a subalpine channel colorado rocky mountains u s a abstract this study used three cross sections along a subalpine channel to examine associations between flow and sand and gravel bedload transport in a cobble bed channel at each section velocity was measured at 0 2 0 4 0 6 and 0 8 of flow depth for ten increments across the section at each increment velocity measurements were immediately followed by 10 min bedload samples with a 7 6 cm helley smith sampler linear regressions between bedload and flow variables suggest that grain shear stress stream power near bed velocity average velocity and the froude number are the most relevant indicators of bedload transport particularly when regressed against the d max or d 84 of the bedload grain size distribution no significant relationships however were constant at all three cross sections inconsistency in relationships between bedload discharge and hydraulic variables is at least in part the result of the extreme temporal and spatial variability present in east st louis creek photographs and observations made throughout the field season suggest the importance of random events such as tree fall and log jam formation in regulating bedload discharge the data from east st louis creek suggest equal mobility for sand and gravel transport

effects of a tectonically triggered wave of incision on riverine exports and soil mineralogy in the luquillo mountains of puerto rico we document the long term response of a tropical mountain stream to tectonics and show how this response exerts a first order spatial control on the delivery of weatherable minerals to forest soils in its catchment these minerals in turn affect nutrient availability and biogeochemical cycles the luquillo mountains is a rainforest covered isolated massif on puerto rico island instead of displaying typical concave up long profiles of most mountain streams the rivers draining the southern flank of these mountains display a systematic downstream increase in gradient using concentrations of in situ produced cosmogenic 10be in quartz from river sediments we find that the downstream steepening is due to the propagation of an erosion wave along the rivers from the coastal plain upstream toward the headwaters decreased 10be concentration along the steepened reaches knickpoint faces or knickzones results from faster denudation of the valley sides around the steepened reaches the upper portions of the steepened reaches knickpoint lips cluster in elevation around the altitude of a flat lying dissected surface traceable around the luquillo mountains inspection of similar platforms over other parts of puerto rico and in the surrounding seas suggests that the dissected surface is an uplifted shore platform formed in the early pliocene 4 my ago upstream of the knickpoints rivers possess alluvial reaches still graded to this uplifted platform they represent relict profiles of the rivers that once drained an island that was surrounded by the now uplifted platform we conclude that the knickpoints initiated when the platform started to rise above the caribbean sea the knickpoints then started propagating upstream defining the front of a slow moving wave of erosion 10be derived catchment scale denudation rates measured above and below the knickpoints indicate a 30 210 increase in denudation associated to the passage of the knickpoints mineralogical analysis of the soils show that soils upstream of the knickpoints overlie deeply depleted saprolite and only contain recalcitrant minerals whereas soils downstream of the knickpoints experience an input of fresh weatherable minerals this influx considerably increases nutrient availability porder et al 2015 in a forest where atmospheric inputs are otherwise the main source of nutrients mcdowell et al 1990 zarin and johnson 1995 pett ridge 2009

chemical weathering in a tropical watershed luquillo mountains puerto rico i long term versus short term weathering fluxes abstract the pristine rio icacos watershed in the luquillo mountains in eastern puerto rico has the fastest documented weathering rate of silicate rocks on the earth s surface a regolith propagation rate of 58 m ma 1 calculated from iso volumetric saprolite formation from quartz diorite is comparable to the estimated denudation rate 25 50 ma 1 but is an order of magnitude faster than the global average weathering rate 6 ma 1 weathering occurs in two distinct environments plagioclase and hornblende react at the saprock interface and biotite and quartz weather in the overlying thick saprolitic regolith these environments produce distinctly different water chemistries with k mg and si increasing linearly with depth in saprolite porewaters and with stream waters dominated by ca na and si such differences are atypical of less intense weathering in temperate watersheds porewater chemistry in the shallow regolith is controlled by closed system recycling of inorganic nutrients such as k long term elemental fluxes through the regolith e g si 1 7 10 8 moles m 2 s 1 are calculated from mass losses based on changes in porosity and chemistry between the regolith and bedrock and from the age of the regolith surface 200 ma mass losses attributed to solute fluxes are determined using a step wise infiltration model which calculates mineral inputs to the shallow and deep saprolite porewaters and to stream water pressure heads decrease with depth in the shallow regolith 2 03 m h2o m 1 indicating that both increasing capillary tension and graviometric potential control porewater infiltration interpolation of experimental hydraulic conductivities produces an infiltration rate of 1 m yr 1 at average field moisture saturation which is comparable with libr tracer tests and with base discharge from the watershed short term weathering fluxes calculated from solute chemistries and infiltration rates e g si 1 4 10 8 moles m 2 s 1 are compared to watershed flux rates e g si 2 7 10 8 moles m 2 s 1 consistency between three independently determined sets of weathering fluxes imply that possible changes in precipitation temperature and vegetation over the last several hundred thousand years have not significantly impacted weathering rates in the luquillo mountains of puerto rico this has important ramifications for tropical environments and global climate change

the nature and controls on downstream fining within sedimentary links abstract at scales of 100 to 102 km grain size change in gravelly alluvial systems is characterized by punctuated downstream fining significant lateral sediment inputs define a sequence of sedimentary links within which fluvial processes produce fining trends using a new set of surface grain size data eighteen such links along two canadian gravel bed rivers are examined negative exponential functions are found to be most appropriate for describing within link grain size changes development of a pragmatic means of predicting grain size is then pursued by searching for a means to relate model diminution coefficients to link attributes consideration of lithologically differentiated fining trends indicates that lithology plays an insignificant role in controlling fining rate examination of within link longitudinal profiles indicates a reasonable relation between fining rate and the rate of change of channel slope the predictive value of this relation is limited by the regional nature of the data upon which it is based and the relative difficulty of obtaining accurate gradient information however as previously noted for alluvial fans although for different reasons channel gradient is correlated with link length and a reasonable predictive tool is apparent in a relation between link length and diminution rate the validity of the derived relation cannot be tested for want of a suitable data set several lines of evidence indicate that abrasion is unimportant as a fining mechanism on these rivers and it is argued that this may be typical of sedimentary links fining rate must then be a function of the efficiency with which sorting processes operate and it is suggested that this efficiency is dependent on aggradation rates associated with the lateral sediment influxes at the head and terminus of individual links

downstream variations in the width of bedrock channels field surveys of channel width w and drainage area a in bedrock channel reaches reveal relationships where w 5 ca b similar to the classic hydraulic geometry of alluvial channels data from five mountain channel networks support the assumption used in many landscape evolution models that an alluvial hydraulic geometry relationship where b 5 0 3 0 5 holds for bedrock channel systems although there is substantial local variability in channel width in bedrock channel systems there is no systematic difference in width versus drainage area relations for the surveyed bedrock and alluvial reaches in sedimentary lithologies in coastal oregon and washington in contrast bedrock channels were narrower and therefore had deeper flow than alluvial channels with equal drainage areas in the granite and limestone terrain of the yuba river california in addition data from the mokelumne river show that bedrock channel width decreases substantially downstream at the contact between relatively weak limestone and more erosion resistant granite but that channel slope does not change appreciably across contacts between these two lithologies data from coastal oregon drainage basins further show systematic channel widening after flood flows and debris flow impacts we conclude that downstream variations in the width of bedrock channels generally follow traditional hydraulic geometry relations but also reflect the local influence of longitudinal patterns of bedrock erosivity and disturbance history

multi scalar controls on channel geometry of headwater streams in new zealand hill country abstract catchment reach and local scale controls upon channel geometry are examined for small headwater streams in adjacent sub catchments with variable land use histories in a typical hill country area of the north island of new zealand although systematic downstream increase in channel cross sectional area is evident in one sub catchment a result of catchment scale controls on geometry including discharge and slope marked local variability in channel geometry in the two other sub catchments reflects non systematic patterns of reach scale controls including the distribution of floodplain pockets and associated bank materials and local scale controls such as bedrock outcrops and wood land use change exerts a primary control upon these relationships for a given catchment area streams in native forest are wider and shallower than streams that flow within pasture or pinus plantations increased hillslope erosion following land clearance has instigated the accumulation of post settlement alluvium psa which has altered the boundary conditions and bank materials within which streams flow significant psa accumulation following native forest clearance in one of the sub catchments has allowed the stream to flow entirely within its own alluvial deposits in this instance catchment scale controls on channel geometry are dominant in contrast local scale controls on channel geometry are more important in the other sub catchments where lesser accumulation of psa reflects differences in land clearance these findings indicate that place based understandings which consider the spatial and historical contexts of landscape settings are required to interpret patterns of channel geometry and to inform river rehabilitation initiatives

variation in the reference shields stress for bed load transport in gravel bed streams and rivers 1 the present study examines variations in the reference shear stress for bed load transport r using coupled measurements of flow and bed load transport in 45 gravel bed streams and rivers the study streams encompass a wide range in bank full discharge 1 2600 m3 s average channel gradient 0 0003 0 05 and median surface grain size 0 027 0 21 m a bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate w versus the reach average dimensionless shear stress the reference dimensionless shear stress r was then estimated by selecting the value of corresponding to a reference transport rate of w 0 002 the results indicate that the discharge corresponding to r averages 67 of the bank full discharge with the variation independent of reach scale morphologic and sediment properties however values of r increase systematically with average channel gradient ranging from 0 025 0 035 at sites with slopes of 0 001 0 006 to values greater than 0 10 at sites with slopes greater than 0 02 a corresponding relation for the bank full dimensionless shear stress bf formulated with data from 159 sites in north america and england mirrors the relation between r and channel gradient suggesting that the bank full channel geometry of gravel and cobble bedded streams is adjusted to a relatively constant excess shear stress bf r across a wide range of slopes

downstream fining selective transport and hillslope influence on channel bed sediment in mountain streams colorado front range usa abstract channel bed sediment is composed of a combination of upstream and local hillslope sources factors that influence downstream changes in size sorting and lithological characteristics of bed material are complex in order to better understand this complexity we measured grain sizes and channel geometry on five streams draining the colorado front range the studied streams flow from steeply incised canyons in the mountains onto the gently sloping colorado high plains downstream fining occurs at the scale of the entire study reach 30 km but not in each individual stream differences between streams are likely related to watershed size and discharge characteristics at smaller spatial scales within each stream the fining pattern is less clear and is disrupted by lateral inputs from locally steep hillslopes in the incised canyons of the front range however the exclusion of local hillslope influence by distinguishing among individual lithologies in the channel bed material that are not present in the local hillslopes shows that fining also occurs over these smaller spatial scales in the absence of steep hillslopes adjacent to the channels bed sediment becomes a better mix of lithologies from the whole watershed rather than dominated by local hillslope sediment influx we propose four dominant factors that control the channel bed characteristics in the study area inheritance hydrodynamics lithologically controlled sediment abrasion rates and differences in denudation rates throughout the watershed with associated differences in hillslope steepness combining these factors leads to a conceptual model that explains our observations and illustrates the complexity of the behaviour of bed material in steep mountainous regions and their bounding basins distinguishing among the different lithologies in the bed material was instrumental in decomposing and understanding the complex trends in this system

grain size sediment transport and alluvial channel pattern abstract the shields parameter a dimensionless bed shear stress which can be expressed as a ratio of the slope depth and characteristic properties of bed material in an alluvial channel has been observed in natural rivers to take on modal values for the regimes of bedload mixed load and suspension transport dade w b friend p f 1998 grain size sediment transport regime and channel slope in alluvial rivers j geology 106 661 675 such conditions correspond to channel geometries associated with conserved sediment flux and thus to a state of approximate equilibrium in which the channels are neither aggrading nor degrading to a significant degree these findings are extended here to accommodate the effects of channel width in the context of a comprehensive data set on stable perennial rivers the new analysis reconciles existing empirical and theoretical approaches to predict alluvial channel geometry

evidence of transient topographic disequilibrium in a landward passive margin river system knickpoints and paleo landscapes of the new river basin southern appalachians the upper new river basin of the southern appalachian mountains a major tributary of the modern ohio river represents the unglaciated headwaters of the tertiary teays river system of eastern north america dating of relict fluvial gravels have suggested that new river incision may be outpacing lowering of the surrounding uplands but physical evidence of transient topographic disequilibrium has yet to be identified we use focused topographic analysis of the upper new river basin to delineate a perched low relief paleo landscape that is experiencing transgressive dissection due to incision by the new river and its tributaries accelerated incision has decoupled hillslopes from the drainage network generating knickpoints which represent the boundary between remnants of the paleo landscape and actively adjusting topography downstream steepening of hillslopes downstream of knickpoints suggests dynamic headward migration which along with knickpoint occurrence throughout the drainage network is inconsistent with the development of fixed stream profile convexities atop strike extensive geologic contacts in the absence of tectonic forcing we favor a climatically forced drop in external base level as driver of the incision pattern we observe plio pleistocene glacial damming and diversion of the teays river to form the modern ohio river lowered regional base level for the study area potentially forcing the paleo landscape developed during the teays era to adjust to the modern drainage pattern the upper new river may therefore represent the potential for glacially driven drainage rearrangement to drive transient topographic evolution hundreds of kilometers away from the ice margin long after the disappearance of ice sheets copyright 2013 john wiley sons ltd

channel geometry of mountain streams in new zealand abstract a dataset of 21 study reaches in the porter and kowai rivers eastern side of the south island and 13 study reaches in camp creek and adjacent catchments western side of the south island was used to examine downstream hydraulic geometry of mountain streams in new zealand streams in the eastern and western regions both exhibit well developed downstream hydraulic geometry as indicated by strong correlations between channel top width bankfull depth mean velocity and bankfull discharge exponents for the hydraulic geometry relations are similar to average values for rivers worldwide factors such as colluvial sediment input to the channels colluvial processes along the channels tectonic uplift and discontinuous bedrock exposure along the channels might be expected to complicate adjustment of channel geometry to downstream increases in discharge the presence of well developed downstream hydraulic geometry relations despite these complicating factors is interpreted to indicate that the ratio of hydraulic driving forces to substrate resisting forces is sufficiently large to permit channel adjustment to relatively frequent discharges