channel bed steps along nahal yael negev desert israel abstract the height and spacing of channel bed steps formed in boulder and bedrock in the ephermeral channels of the nahal yael watershed show a strong correlation with the channel slope the distance between steps is inversely proportional to slope at slopes of 0 to 20 but then remains fairly constant at higher slopes step height increases proportionally with channel slope these bed steps are similar to those described on perennial channels elsewhere where step formation has been attributed to the effects of high flows which submerge the clasts forming the steps however the ephemeral flows in the tributary channels of the nahal yael system are not sufficient to submerge the clasts forming the boulder steps using rouse s criteria channel bed steps are creating maximum flow resistance at slopes of 12 to 50 along the channels of nahal yael using davies criteria bed steps are creating maximum flow resistance at slopes of 3 to 10 the bed steps also relate to sediment transport nahal yael has a low sediment yield by world standards and the increase in flow resistance due to step formation may serve to increase sediment movement both the boulder and bedrock steps at nahal yael are hypothesized to reflect the scales of flow turbulence along the channels we were unable to evaluate the role of flow volume or boundary roughness in controlling these scales but channel slope appears to exert some control on the scale of turbulence based on the correlation between slope and bed steps the hypotheses developed from field evidence at nahal yael should be testable in flume simulations

seismic constraints on a large mafic intrusion with implications for the subsidence history of the danish basin 1 seismic refraction data from the estrid 1 profile are used for seismic velocity modeling along the strike of a large mafic intrusion in the norwegian danish basin central denmark the p wave velocity structure identifies a 8 km thick sedimentary succession with velocities between 1 8 and 5 7 km s the top basement is defined by a step to vp 6 2 km s in the middle to lower crust a high velocity body vp 6 7 km s with its top located at about 10 12 km depth is interpreted as a high velocity and high density gabbroic intrusion of permian age this high velocity body explains the large 50 mgal positive gravity anomaly known as silkeborg gravity high the intrusion has a minimum volume of 40 000 km3 which implies that the magma influx and the consequent cooling of the lithosphere from high temperature could have had profound effects on the subsidence of the danish basin in particular because the magma probably intruded during only a few events and other similar structures cover much of the basin an anomalously high velocity gradient from 7 0 km s in the middle crust to 7 7 km s at 30 32 km depth in the central part of the intrusion coincides with an interval without moho reflections indicative of a gradual transition zone between the crust and the mantle this feature may show the location of the feeder dykes of the intrusion

hydraulics morphology and energy dissipation in an alpine step pool channel 1 to investigate the relationship between hydraulics and channel morphology in step pool channels we combined three dimensional velocity measurements with an acoustic doppler velocimeter and topographic surveys in a steep step pool channel the rio cordon italy measurements were organized around step pool and tread units and occurred within a range of 36 57 of bankfull discharges as flow moved from steps to their downstream pools in our study reach an average of approximately two thirds of the total energy was dissipated as measured by relative head loss through step pool sequences much of this head loss was achieved by elevation potential energy loss rather than velocity reductions although an overall expected pattern of flow acceleration toward step crests and deceleration in pools was present pool velocities were high especially where upstream step crests were irregular and where residual pool depths were low many steps were porous or leaky with irregular cross channel bed and water surface topography producing high velocity jets and less flow resistance than channel spanning dammed steps longitudinal variations in hydraulics are thus often overshadowed by lateral variations arising from morphologic complexities velocity and turbulence characteristics in the rio cordon show marked differences from data we have collected in a more stable and wood rich channel in the colorado rockies in which ponded steps are more prevalent and pools are slower and more turbulent comparison of these channels illustrates that step pool structure and hydraulics are strongly influenced by flow regime sediment supply lithology time since the last step forming flood and availability of in stream wood

analysis of changes in the step pool morphology and channel profile of a steep mountain stream following a large flood abstract the step pool morphology of steep mountain streams is reorganized during large floods by the removal of steps scouring of the channel bed and the formation of a new step pool sequence by the re deposition of step forming grains in this paper we document these changes caused by the largest recorded flood return period 50 years in the steep mountain stream erlenbach in central switzerland in 2007 by comparing pre flood and post flood surveys of a 650 m long study reach we show that the flood led to statistically significant changes in the distributions of step height and spacing most notably an increase in the variance of both variables organization at the river reach scale is not apparent and the correlations between step properties both pre and post flood are generally weak the flood led to widespread erosion of the river bed locally up to 2 3 m of vertical channel change was observed at least 60 of the steps moved even the largest ones however they remained constrained to the parts of the study reach where landslides on the adjacent hillslopes are most active detailed changes in bed structure were analyzed from the pre and post flood profiles with a statistical scaling approach which identifies the complexity of the spatial organization of sediment in the longitudinal profile multiscaling found in both step and pool sequences reflects the complex spatial structure which is the result of the superposition of steps formed by the history of flood events

the geomorphic significance of step pools in mountain streams abstract this paper develops a conceptual model to define the changing role and significance of step pools as energy dissipators in steep mountain channels although energy dissipation is a significant function of step pools this role changes over time with variations in discharge steps are effective in reducing stream energy at low flows but their effectiveness diminishes with increasing stage accordingly the manner in which energy dissipation occurs also varies with increasing flows spill resistance gives way to a dominance in form and grain resistance the conceptual model for the changing role of step pools is illustrated with data from hydraulic analysis and modeling of step pools in the santa monica mountains of california the model points to the importance of the size of steps in determining their role in energy dissipation and in their interactions with channel hydraulics the model offers a new articulation for the geomorphic significance of step pools in mountain streams and it serves as a useful template for a more complete understanding of step pools over a longer time scale

the periodic nature of step pool mountain streams rhythmic sequences of steps and pools in steep mountain streams have captured the attention of numerous workers but whether periodicities exist in the occurrence of step pools has not been investigated in a comprehensive way and the implications for such periodicities have not been explored spectral analysis was applied to bed elevation data from streams in the santa monica mountains of southern california to investigate periodicity in the occurrence of step pools results indicate significant step pool periodicities confirmed by independent field measurements that vary from 0 43 channel widths to 2 40 channel widths spectral analysis of channels affected by external influences tends to indicate multiple significant peaks that reflect higher variance in step pool spacing underlying periodicities are nevertheless de tected suggesting that external factors obscure but do not destroy the effects of more general internal mechanisms periodicity in the occurrence of step pools suggests that step pools are bedforms that represent a fundamental mutual adjustment between flow channel morphology and energy expenditure a rhythmic channel thalweg supports the notion that step pools are analogous to meandering in the vertical dimension and implies that because step pool channels are not able to adjust energy expenditure in the plane dimension they instead adjust boundary roughness in the vertical dimension a continuum of rhythmic gravel bedforms that displays the results of such adjustments and that shows how step pools merge into pools and riffles in the stream channel system is discussed and illustrated with empirical data from the santa monica mountains although many differences exist between steep mountain streams and broad alluvial rivers the findings of this study suggest that generalized principles of uniform energy expendi ture may also apply to rhythmic step pool streams these findings are potentially valuable in the management and design of steep channels

magma compensated crustal thinning in continental rift zones continental rifts are elongated depressions in the earth s surface often more than 1 000 km long 20 km wide and 10 km deep filled with sediments and volcanic rocks conventional models of rift zones include three characteristic features surface manifestation as an elongated topographic trough moho shallowing due to crustal thinning and reduced seismic velocity in the uppermost mantle hans thybo and christoffer nielsen present a new seismic profile that demonstrates that the baikal rift zone in siberia displays only one of these characteristic features the elongated surface trough there is a localized zone in the lower crust with exceptionally high seismic velocity and high reflectivity the expected moho uplift appears to have been compensated by magmatic intrusion into the lower crust producing the observed high velocity zone this finding points to a previously unknown role for magmatism in rifting this study presents a seismic profile across southern lake baikal in siberia demonstrating that only a surface manifestation is observed there these findings may point toward a new role for magmatism in the rifting process continental rift zones are long narrow tectonic depressions in the earth s surface where the entire lithosphere has been modified in extension1 rifting can eventually lead to rupture of the continental lithosphere and creation of new oceanic lithosphere or alternatively lead to formation of wide sedimentary basins around failed rift zones conventional models of rift zones include three characteristic features surface manifestation as an elongated topographic trough moho shallowing due to crustal thinning and reduced seismic velocity in the uppermost mantle due to decompression melting or heating from the earth s interior2 3 4 here we demonstrate that only the surface manifestation is observed at the baikal rift zone whereas the crustal and mantle characteristics can be ruled out by a new seismic profile across southern lake baikal in siberia instead we observe a localized zone in the lower crust which has exceptionally high seismic velocity and is highly reflective we suggest that the expected moho uplift was compensated by magmatic intrusion into the lower crust producing the observed high velocity zone this finding demonstrates a previously unknown role for magmatism in rifting processes with significant implications for estimation of stretching factors and modelling of sedimentary basins around failed rift structures

a new geodynamical thermal model of rift evolution with application to the dnieper donets basin ukraine abstract a model of the lithosphere incorporating both dynamic and thermal processes has been developed by solving a coupled system of differential equations governing stress and temperature in a 2 d block structured geophysical medium designed to study the roles of tectonic and geothermal factors in continental rift formation and evolution the model incorporates syn sedimentary and or erosional faulting of an upper crustal layer and allows the thermal regime of the lithosphere to be calculated through time the method has been applied to the formation and evolution of the northwest dnieper donets basin ddb along one regional profile controlled by seismic and other geophysical and subsurface data the results are compared with those published earlier for the same profile using different methods of modelling the rift and early post rift development of the region the final basement geometry at the end of the rifting stage predicted by the new model satisfactorily corresponds with geological data and is qualitatively similar to that predicted by the previously published models however the new results imply an important role for an active mechanism during rifting that generates greater mantle thinning than crustal thinning and elevated temperatures in the upper mantle beneath the rift

pool spacing in forest channels field surveys of stream channels in forested mountain drainage basins in southeast alaska and washington reveal that pool spacing depends on large woody debris lwd loading and channel type slope and width mean pool spacing in pool riffle plane bed and forced pool riffle channels systematically decreases from greater than 13 channel widths per pool to less than 1 channel width with increasing lwd loading whereas pool spacing in generally steeper step pool channels is independent of lwd loading although plane bed and pool riffle channels occur at similar low lwd loading they exhibit typical pool spacings of greater than 9 and 2 4 channel widths respectively forced pool riffle channels have high lwd loading typical pool spacing of 2 channel widths and slopes that overlap the ranges of free formed pool riffle and plane bed channel types while a forced pool riffle morphology may mask either of these low lwd loading morphologies channel slope provides an indicator of probable morphologic response to wood loss in forced pool riffle reaches at all study sites less than 40 of the lwd pieces force the formation of a pool we also find that channel width strongly influences pool spacing in forest streams with similar debris loading and that reaches flowing through previously clear cut forests have lower lwd loading and hence fewer pools than reaches in pristine forests

substrate influences on step pool sequences in the christopher creek drainage arizona abstract step pool sequences formed in alluvial and bedrock channel segments of the christopher creek drainage in central arizona suggest that bedform geometries may reflect subtle differences in relative substrate resistance and in the ratio of driving forces to resisting forces three alluvial channel segments and two bedrock channel segments along a 72 km2 drainage and a 4 km2 drainage facilitated comparison across different substrates and hydraulic environments as substrate resistance increased or the ratio of hydraulic driving forces and boundary resisting forces decreased channel gradient increased the ratio of step height to length normalized by gradient decreased and pool dimensions normalized by gradient decreased these changes in bedform geometry suggest that as the ratio of driving to resisting forces increases proportionally more flow energy is dissipated within the flow as turbulence and shearing as in the alluvial reaches rather than being applied to erosion of the channel boundaries

characteristics of log and clast bed steps in step pool streams of northwestern montana usa the role of log and clast steps was studied in 53 step pool reaches in 32 streams in northwestern montana in each reach 20 consecutive steps were sampled drainage areas ranged from 0 5 to 11 5 km2 mean step spacing was found to be proportional to channel gradient width of the bankfull channel and drainage area mean step height was proportional to the width of the channel no significant difference exists between clast and log steps with regard to step spacing and step height this suggested either that 1 woody debris is mobile in even the smallest channels in the study area or 2 channel flows arrange clast steps around immobile woody debris calculations of theoretical entrainment and transport of sediment suggest that the coarse clasts forming steps are capable of being entrained at bankfull discharge in most channels the presence of immobile log steps composed of living roots in the upstream reaches an increase in the percentage of log steps with gradient and a decrease in this percentage with the width of the bankfull channel and drainage area imply that woody debris is mobile primarily in the downstream reaches calculations of flow resistance suggest that most steps serve to maximize flow resistance these results support the second hypothesis that the height and spacing of clast steps are adjusted around immobile log steps to maintain consistent step characteristics

velocity prediction in high gradient channels summary in 15 mountain stream reaches containing instream wood we characterized velocity and flow resistance at bankfull through low flows these data were 1 used to assess the accuracy of previously published velocity prediction techniques for high gradient channels and 2 were combined with field data from other studies to develop general methodologies for predicting velocity and flow resistance in alluvial and mixed alluvial bedrock channels both with and without step forming instream wood velocity and flow resistance were poorly predicted by variables characterizing grain size and relative grain submergence conversely methods based on detrended standard deviation of bed elevations rz and relative bedform submergence h rz explained up to 84 of the variance in the measured flow resistance coefficients and 97 of the variance in dimensionless velocity with an average velocity of 0 44 m s for the collected measurements velocity was predicted with rms root mean square error as low as 0 071 m s 16 of average when discharge and bedform geometry is known and 0 10 m s 23 when only bedform geometry is known additionally an empirical relationship indicates v u h r z supporting previouslypublished laboratory findings using a field based dataset in complex high gradient channels interactions between instream wood and clasts result in substantially enhanced step heights and flow resistance this compound effect defies description by grain size and relative grain submergence however rz and h rz quantify variability due to both clasts in combination with wood and clasts alone providing relatively accurate predictions for the tested dataset and indicating substantial predictive capabilities in channels where bedforms are the primary source of flow resistance published by elsevier b v

development of channel organization and roughness following sediment pulses in single thread gravel bed rivers large episodic inputs of coarse sediment sediment pulses in forested mountain streams may result in changes in the size and arrangement of bed forms and in channel roughness a conceptual model of channel organization delineates trajectories of response to sediment pulses for many types of gravel bed channels channels exhibited self organizing behavior to various degrees based on channel gradient presence of large in channel wood or other forcing elements the size of the sediment pulse and the number of bed mobilizing flows since disturbance typical channel changes following a sediment pulse were initial decreases in water depth in variability of bed elevations and in the regularity of bed form spacing trajectories of change subsequently showed increased average water depth more variable and complex bed topography and increased uniformity of bed form spacing bed form spacing in streams with abundant forcing elements developed at a shorter spatial scale two to five channel widths than in streams without such forcing mechanisms five to 10 channel widths channel roughness increased as bed forms developed

influence of large woody debris on channel morphology and dynamics in steep boulder rich mountain streams western cascades oregon this study used 20 year records of stream channel change and wood to test hypotheses about the long term influence of large woody debris lwd on channel morphology channel stability and sediment dynamics in a steep boulder rich mountain stream we compared two nearly adjacent reaches of third order mack creek over the period 1978 1997 after virtually all wood was removed from the channel of the lower reach in 1964 we assessed the long term legacy of wood removal using repeated crosssection surveys streamflow data lwd inventory data and detailed mapping and longitudinal profile surveys at each of 11 cross sections in the upper reach and 19 in the lower reach we calculated areas of scour and fill in response to the two largest floods in the record we used quasi likelihood logistic regression models to test the proportion of each reach that experienced change between consecutive surveys over the entire record 1978 1997 as a function of flood return periods the longitudinal profile of the site without lwd was more variable than the reach with lwd at the finest scale f1 m due to a greater frequency of boulder steps but the reach with lwd was more variable at the channel unit scale lwd created steps 1 to 2 5 m high in the wood rich reach accounted for nearly 30 of the total channel fall and created low gradient upstream channel segments one to three channel widths long as a result both reaches have the same average slope about 9 but nearly three times as much of the channel in the wood rich reach had a slope of v5 as in the reach without wood 20 4 of total channel length vs 7 5 of channel length the reach with abundant lwd was less responsive to moderate streamflow events return period f5 years but it responded similarly to peak flows with a return period of about 10 to 25 years although the average magnitude of crosssection changes was the same during the largest flood in the record 25 year return period the reach without lwd experienced scour and coarsening of the bed surface whereas the reach with lwd experienced aggradation upstream of lwd features mack creek may be representative of many steep mountain streams in which channel structure is strongly influenced by nonfluvial processes a legacy of large boulders from glacial or mass movement processes and a legacy of dead wood from ecological processes sediment limited mountain streams with large boulders when deprived of lwd appear to exhibit less morphological variation at the channel unit scale to store less sediment and to release it more readily than those with lwd d 2002 elsevier science b v all rights reserved

large woody debris and flow resistance in step pool channels cascade range washington abstract total flow resistance measured as darcy weisbach f in 20 step pool channels with large woody debris lwd in washington ranged from 5 to 380 during summer low flows step risers in the study streams consist of either 1 large and relatively immobile woody debris bedrock or roots that form fixed or forced steps or 2 smaller and relatively mobile wood or clasts or a mixture of both arranged across the channel by the stream flow resistance in step pool channels may be partitioned into grain form and spill resistance grain resistance is calculated as a function of particle size and form resistance is calculated as large woody debris drag combined grain and form resistance account for less than 10 of the total flow resistance we initially assumed that the substantial remaining portion is spill resistance attributable to steps however measured step characteristics could not explain between reach variations in flow resistance this suggests that other factors may be significant the coefficient of variation of the hydraulic radius explained 43 of the variation in friction factors between streams for example large woody debris generates form resistance on step treads and spill resistance at step risers because the form resistance of step pool channels is relatively minor compared to spill resistance and because wood in steps accentuates spill resistance by increasing step height we suggest that wood in step risers influences channel hydraulics more than wood elsewhere in the channel hence the distribution and function not just abundance of large woody debris is critical in steep step pool channels

dniepr donets rift deep structure and evolution from dss profiling abstract forward modelling has been applied to refraction and wide angle reflection traveltime data along two profiles crossing the intracratonic dniepr donets rift ddr a devonian sedimentary basin located in the southwestern part of the eastern european platform between the ukrainian shield and the voronezh massif the 2 d p wave velocity models show that the crystalline crust beneath the rift basin is thinner and characterised by higher velocity values of almost 1 0 km s than beneath the shield differences along the strike of the ddr are also indicated the two profiles cross the dniepr and donets segments of the ddr and reveal respectively sedimentary infill thickness of 7 and 20 km and crystalline crustal thicknesses of 31 and 17 km in both cases the crystalline crust comprises two distinct velocity layers beneath the dniepr basin the moho lies at 38 km and the upper mantle velocity is 8 4 km s in the donets region the crustal layers are underlain by velocity boundaries at depths of 37 and 42 km with velocities of 7 6 and 8 0 km s respectively the velocity models are interpreted to indicate that a key process in the formation and evolution of the ddr was the exchange of material between the crust and uppermost mantle it is hypothesised that the extended and faulted pre rift shield crust was intruded by mantle melts during rifting and that crust mantle material exchange may also have occurred during the subsequent cooling and subsidence stage of development rifting was less intense in the dniepr segment of the ddr compared to the donets to explain other differences in uppermost mantle structure between the two segments of the ddr it is postulated that density reduction of some portion of the donets subcrustal mantle occurred in response to compressional deformation and inversion during the permian uralian orogeny

wave images of the crustal structure from refraction and wide angle reflection migrations along the dobre profile dnieper donets paleorift abstract the dobre profile crosses the donbas foldbelt that is an uplifted part of the dnieper donets paleorift the depth to the basement reaches 20 km the refraction and wide angle reflection survey was carried out along this 360 km profile with 245 recording stations and 11 shot points dobrefraction 99 working group et al 2003 the velocity modeling traced three continual boundaries the basement surface the intra crustal boundary and the moho the refraction and wide angle reflection migrations applied to the data enable to reveal some new structural features of the boundaries it determines a system of faults which divide the basement into several flat segments with stepwise increasing of their depths in the central part of the profile the intra crustal boundary is imaged which corresponds to a strong refraction boundary at a depth of 15 25 km the moho was traced as a weak boundary at a depth of 35 40 km beneath the donbas and the crust mantle transition zone is characterized by thick heterogeneous zone of lens form

dobrefraction 99 velocity model of the crust and upper mantle beneath the donbas foldbelt east ukraine abstract the donbas foldbelt df is the uplifted and deformed part of the up to 20 km thick dniepr donets basin ddb that formed as the result of rifting of the east european craton eec in the late devonian uplift especially of the southern margin of the basin occurred in early permian times in a trans tensional tectonic stress regime while folding and reverse faulting mainly occurred later mainly during the late cretaceous a seismic refraction wide angle reflection survey was carried out in 1999 dobrefraction 99 to complement existing deep seismic sounding dss data from the area that did not record significant pn phase arrivals because of insufficient maximum offset dobrefraction 99 comprised some 245 recording stations along a line of 360 km length with 11 in line shot points as well as a 100 km away parallel 190 km long subsidiary fan profile the main profile runs between the shores of the azov sea in the south to the ukraine russia border in the north across the azov massif ukrainian shield the foldbelt and the voronezh massif particular scientific targets include the nature of the crust mantle transition and the geometry of crustal upper mantle structures related to rifting and subsequent basin inversion tomographic inversion as well as ray trace based velocity modelling has been carried out the velocity signature of the sedimentary basin itself is well resolved indicating an asymmetric form with a steeper basement surface in the south than in the north and a total thickness of about 20 km a thick 10 km high velocity 6 9 km s lower crustal body lies beneath the rift basin itself offset slightly to the north compared to the main basin depocentre likely related to the rifting processes velocities in the crust below the ukrainian shield south of the foldbelt are in general higher than beneath the voronezh massif to the north the moho displays only slight topography around a depth of 40 km along the profile

deep seismic investigation of crustal extensional structures in the danish basin along the estrid 2 profile summary the crust and uppermost mantle in the danish basin are investigated by modelling the p wave velocity distribution along the north south trending seismic profile estrid 2 seismic tomography and ray inversion modelling demonstrate a variable depth to the top of the crystalline crust from 10 km in the northern part of the profile to 2 km depth in the southernmost part the crystalline crust shows very high p wave velocity in the central part of the profile with 6 7 km s 1 at depths as shallow as 12 km and 7 3 7 5 km s 1 in the lowermost crust these values confirm previous results obtained along the orthogonal estrid 1 profile and the eugeno s profile 2 this high velocity zone in the middle to lower crust is interpreted as a mafic intrusion which explains a positive gravity anomaly of 50 mgal silkeborg gravity high the total length of the intrusion is at least 80 km in the east west direction and 25 35 km in the north south direction the estimated thickness of the intrusion from its top to the moho level is 18 20 km which gives a total minimum volume of 40 50 000 km3 the reflectivity properties of the moho discontinuity are variable along the profile below the intrusion the pmp signal is very weak due to the small velocity contrast between the lowermost crust 7 4 km s 1 and uppermost mantle 7 6 7 7 km s 1 the main moho reflection has a reverberative character to the south of the intrusion this feature is interpreted by layering at the moho level possibly due to magmatic underplating the occurrence of a large crustal mafic intrusion associated with magmatic underplating may be related to extensional transtensional tectonism in the tornquist fan area in the late palaeozoic the extensional event probably caused the opening of a plumbing system for intrusion of mantle derived magma into the crust the ascending magma may have been injected at upper middle crustal levels and during the late phases of the development squeezed laterally along the moho

step pool and cascade morphology mosquito creek british columbia a test of four analytical techniques the identification and geometric definition of individual cascade and step pool bedforms are investigated in a steep coarse grained mountain stream mosquito creek by testing four analytical techniques visual identification zero crossing bedform differencing and power spectral analysis the test is the first use of these techniques in a headwater stream and the analysis of two bed profiles showed that visual identification was able to i identify ii determine the geometry of and iii classify the type of individual bedforms better than the other methods the other techniques were not able to differentiate step pools from cascades and the large range of grain sizes and bedform heights hampered their ability to consistently identify stepped bedforms the step pool pronounced channel spanning steps that alternate with channel spanning pools and cascade multi tiered partially channel spanning structures morphology in mosquito creek has formed in the last 20 years as fluvial action has restructured its previously engineered revetment lined planar bed the channel bed exhibits a morphologic regularity that power spectral analysis captured as periodic fluctuations in the bed profiles with mean wavelengths slightly greater than those identified by the other methods further the active reorganization of revetment has formed stepped structures with geometries similar i e height to wavelength ratios to stepped features found in natural mountain streams channel slope partially controlled bedform geometry wavelength and height and bedform height weakly controlled individual step spacing but there was no relation between wavelength and grain size d 90

structural features and evolution of the dniepr donets basin ukraine from regional seismic reflection profiles abstract a set of regional multichannel seismic reflection profiles crossing the dniepr donets dd basin in ukraine 100 200 km long and with a spacing of 10 15 km along strike illuminates the structure of the whole sedimentary cover the rift filled with pre and syn rift devonian sequences and overlain with post rift carboniferous permian mesozoic and palaeogene fill is clearly seen on all sections a basement layer previously identified as a riphean rift succession by refraction seismic studies constitutes in the main strata of the devonian and early carboniferous successions the structure of the rift is characterised by large scale rotational fault blocks and half grabens carried by basin parallel to normal marginal faults or by major horst blocks the total thickness of the devonian and younger succession is up to 19 km in the study area post rift carboniferous and mesozoic sediments cover the rift flanks and increase in thickness towards the rift axis and to the southeast the cumulative thickness of the carboniferous succession alone reaches 11 km with the maximum depth of their base at about 15 km major post rift tectonic events took place at the end of early visean in the middle of serpukhovian in permian times and between the mesozoic and cenozoic tectonic reactivations during carboniferous and early permian times were most pronounced in the southeastern part of the dd basin while they were least pronounced in the northwestern part of the basin merging with the pripyat trough where they are barely observed the most active development of salt structures and stocks corresponds with the periods of tectonic reactivation

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

step pool evolution in the rio cordon northeastern italy the principle that formative events punctuated by periods of evolution recovery or temporary periods of steady state conditions control the development of the step pool morphology has been applied to the evolution of the rio cordon stream bed the rio cordon is a small catchment 5 km2 within the dolomites wherein hydraulic parameters of floods and the coarse bedload are recorded detailed field surveys of the step pool structures carried out before and after the september 1994 and october 1998 floods have served to illustrate the control on step pool changes by these floods floods were grouped into two categories the first includes ordinary events which are characterized by peak discharges with a return time of one to five years 1 8 5 15 m3 s 1 and by an hourly bedload rate not exceeding 20 m3 h 1 the second refers to exceptional events with a return time of 30 50 years a flood of this latter type occurred on 14 september 1994 with a peak discharge of 10 4 m3 s 1 and average hourly bedload rate of 324 m3 h 1 step pool features were characterized primarily by a steepness parameter c h ls s the evolution of the steepness parameter was measured in the field from 1992 to 1998 the results indicate that maximum resistance conditions are gradually reached at the end of a series of ordinary flood events during this period bed armouring dominate the sediment transport response however following an extraordinary flood and unlimited sediment supply conditions the steepness factor can suddenly decrease as a result of sediment trapped in the pools and a lengthening of step spacing the analogy of step spacing with antidune wavelength and the main destruction and transformation mechanism of the steps are also discussed copyright 2001 john wiley sons ltd

frequency and magnitude of bedload transport in a mountain river magnitude and frequency of bedload transport was examined in the lainbach river bavaria using magnetically tagged particles during the study 18 small to moderate events and one major event occurred about 1 per cent of the flow period exceeded the entrainment threshold and at least once every year about 50 per cent of the tagged particles were mobile themajor event which occurred during the study period resulted in the deposition of a thick layer of sediment in parts of the channel and passive burial of most of the recovered particles the step pool pattern which characterized the study site prior to the large event was obliterated however the channel recovered quickly and has returned to a new step pool pattern the event changed the boundary conditions by increasing the availability of loose sediment and creating higher river bed slopes in reaches between breached check dams as a result movement of individual particles measured for events of both the same magnitude and duration occurring before and after the large event yielded different values for events which occurred after the large event the range and the mean distance of movement were about ten times higher

channel morphology gradient profiles and bed stresses during flood in a step pool channel abstract the stability of a step pool channel is investigated on the basis of measurements during a median annual flood the study reaches range in gradient from 5 to 10 and the flow was 3 m3 s 1 in an 8 m wide channel the estimated shear stress based on the channel gradient was 102 pa which implies that the channel ought to move half meter class boulders calculations of the shear stress using flow resistance equations yield values of about 30 pa these figures imply that the largest boulders in the channel could be moved in fact the channel is stable the effective stress remains well below the threshold to move the large structure forming boulders the steps are stable structures that result from interlocking of the larger stones they are semi randomly located depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks the high variation in step pool wavelength step height residual pool depth and pool length is indicative of the random nature of step location and structure along a reach there is no convincing evidence that special conditions govern their formation

bed morphology and generation of step pool channels flume experiments have been carried out to study the formation processes and the bed morphology of step pool channels from the experiments different step types and step configurations could be distinguished depending on the stream power these step types can be seen as an image of the generation mechanisms of step pool systems these results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution in contrast to a variety of other studies antidunes did not contribute to the generation of the step structures however the data of the presented study fits well into the region of antidune formation proposed by kennedy for sand bed rivers this observation points out that step pool field data located in the kennedy region do not inevitably prove that antidunes played a role in step development it is rather proposed that in kennedy s region of antidune formation there exist hydraulic conditions where the flow resistance is maximized it is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height independently of whether these are antidunes in sand and gravel bed rivers or step pool units in boulder bed streams the considerations of the kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self stabilization by modifying the step pool structure towards a geometry that provides maximum flow resistance and maximum bed stability copyright 2008 john wiley sons ltd

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

dissipative analogies of step pool features from rills to mountain streams abstract in this paper the dissipative similarity of step pool units at different spatial scales ranging from rills to streams is analyzed this investigation benefits from the latest theoretical advances in open channel flow resistance high resolution topography from close range photogrammetry applied to rill erosion and the availability of published data from literature on step pool streams at first the integration of a power velocity distribution allowed to obtain a theoretically based expression of darcy weisbach friction factor in which function and exponent of the velocity profile are included then this theoretically deduced flow resistance relationship is calibrated and tested by flow measurements carried out in rill reaches where the step pool units occurred in particular the proposed function is calibrated using the measurements corresponding to 88 rill reaches shaped on a plot having a slope equal to 14 22 and 24 and the calibrated equation is also positively tested with measurements carried out in 48 rill reaches shaped on a 26 sloping plot for the rill flow the developed analysis stated that the friction factor is characterized by estimate errors which are less than or equal to 15 for 86 of cases and less than or equal to 10 for 70 of cases using measurements of flow velocity water depth width and bed slope measurements carried out in 109 reaches of step pool streams this investigation demonstrates that the theoretical flow resistance equation can be applicable to step pool streams carrying out a specific calibration of the function the comparison between the darcy weisbach friction factor values measured in streams and step pool rills demonstrates that in the stream features the friction factor values are on average higher than those related to rills with step pool sequences in conclusion the comparison between rills and streams with step pool units highlights that the same theoretical flow resistance equation can be applied even if a scale effect between the two features is detected

velocity and flow resistance in step pool streams flow resistance in steep streams with step pool morphology consists of form drag as well as skin friction it therefore alters rapidly as discharge increases and is not expected to show constant manning s n or simple log law behaviour but there is no widely accepted alternative representation we report measurements of at a station hydraulic geometry n and 1 f 0 5 for a range of discharges at each of six field sites and in flume experiments loosely froude scaled to the prototypes velocity and resistance vary strongly with discharge in all cases the flume data extend to formative flows and when rescaled extend the range of the field data but plot in line with them this suggests a single resistance law can describe the full range from trickle to formative flow equations based on the log law with ks step d84 perform unexpectedly well especially when allowance is made for flow blocking and or the ks d84 multiplier is optimised separately for each site the latter suggests step d84 is not a good summary representation of effective roughness but none of a variety of alternative grain size and microtopographic measures performed much better

origin of step pool systems in mountain streams mountain streams very often present a step pool aspect antidune formation alone dispersion and sorting theory or velocity reversal as advocated for the origin of riffle pool sequences do not explain how steps and pools form experiments have been conducted to clarify their origin it was found that the step pool aspect appears after high flows had deformed the bed during the presented tests a self armoring process occurred which resulted in a stabilization of the bed when coarsening of the top layer was little classic antidunes resulted an important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance the substantial increase in resistance was an essential factor in bed stabilization

fluvial geomorphology of a boulder bed debris flow dominated channel in an active tectonic environment abstract numerous streams with a drainage area of a few square kilometers draining the south flank of the santa ynez range in southern california are characterized by steep boulder bed debris flow dominated channels rattlesnake creek is a good example approximately 90 of the stored sediment in the stream valley is stable debris flow deposits found in debris flow terraces and levees adjacent to the channel the remaining 10 of stored sediment resides in the active channel where the channel morphology is dominated by large boulders 1 to 4 m in diameter derived from debris flows or rock fall that along with bedrock outcrops form rock or boulder transverse ribs with frequent step pools the dominant channel pool morphology is the step pool which accounts for most of the pools and approximately 30 of relief drop in elevation through the study reaches less common pool types identified in the study reaches include forced pools that result from convergence of flow with scour between lbres large boulder roughness elements and bedrock outcrop along the channel at near bankfull discharge the lbres and boulders in step pools armor the channel and are primary controls on channel morphology and gradient rock strength is a secondary control on channel morphology location and abundance of pools we hypothesize that step pools when inundated with sand and fine gravel from a disturbance such as wildfire during a flow event fill near bankfull stage and scour on falling stage below bankfull when unit stream power is a maximum over the step thus as with forced pools step pool maintenance depends on a specific pattern of discharge and process the process of scour during falling stage reestablishes step pools during moderate flows following disturbance

the self organization of step pools in mountain streams abstract spontaneous autogenic self organization has been described in numerous geomorphic systems but it has not been investigated in detail with respect to coarse bedforms in general or step pools in particular in this paper we review the spatial organization of step pool systems and present example evidence of step pool development as an autogenic self organization process we then outline the mathematical language for defining spatially divergent self organization and test these ideas using two unique field examples from oregon andrews experimental forest and california baxter creek where step pools developed from planar beds in artificially manipulated channels results show that step pool development is consistent with a spatially divergent self organization phenomenon entropy increases as initially undifferentiated planar channels diverge into steps and pools then declines when a series of steps and pools of consistent size and spacing is established signifying stability in the system the self organization process is accompanied by increasing flow resistance and decreasing slope through increasing the vertical sinuosity of the step pool profile and creation of low or negative gradient pool areas suggesting a minimization of stream power the self adjustment of the step pool bed profile over time represents another manifestation of a general process that results in rhythmic patterns on the surface of earth

the morphologic structure of step pools in mountain streams abstract step pool sequences in streams in the santa monica mountains of southern california impart a characteristic morphologic structure that varies with slope but despite the strong association with slope that suggests a direct process control analysis of 464 step pools in 13 study reaches reveals the relationship to reflect correlation but not necessarily causality the direct control on step height is particle size whereas discharge is the probable causal process determining step wavelength because both particle size and discharge generally change with channel slope downstream a slope variation with step wavelength and height is the apparent indirect geometric result a conceptual model of process form linkages is proposed based on these results whereby the step pool morphology is produced by positive relationships between wavelength and discharge and between height and particle size the model offers new insights for explaining the step pool morphology and it is potentially useful in channeling future efforts toward the appropriate scales and variables

characteristic dimensions of the step pool bed configuration an experimental study 1 the dominant dimension of the step pool bed form is its spacing which is determined by the locations at which large step forming grains are deposited we conducted laboratory experiments involving active transport of widely sorted sediment including large grains whose deposition readily formed steps direct observation of step development indicates that deposition of step forming grains was predominantly associated with bed roughness impeding transport or with earlier flow and transport conditions not contemporaneous with step development neither mechanism is likely to occur at regular locations on the bed measured step spacing was found to follow a poisson distribution supporting the conclusion that steps do not develop in preferred or regular locations this contrasts with previous arguments that regular step spacing develops in response to deposition forced at regular locations beneath water surface waves we observe neither the regular distribution nor the associated depositional mechanism although the observed step distribution is well defined with a strong mode that could be interpreted as indicating regular spacing

inversion of gps measurements for a layer of negative dislocation distribution in north china 1 in north china most earthquakes occur at depths of 10 25 km and are considered to be the direct result of deformation or rupture of the brittle upper crustal layer to describe this mechanism a planar horizontal negative dislocation plane is used to represent the force of action of the lower crustal layer on an overlying brittle upper crust layer an area around beijing in north china has been chosen for applying this negative dislocation layer assumption a gps network capital circle gps network ccgn has been set up for monitoring crust deformations since 1992 in this paper observations from 1992 1995 and 1996 gps surveying campaigns were used to determine model parameters of a negative dislocation layer using a bayesian inversion procedure more than 95 of data residuals are found to be 2 mm yr indicating that the negative dislocation layer model can fit gps data well the inversion results show that the local tectonic movement is 2 1 mm yr in the north and 7 1 mm yr in the east and the high negative dislocation rates occur mainly in the south part of the north taihang mountain zone with a magnitude of 4 1 mm yr and the east part of the yan mountain zone with a magnitude of 3 1 mm yr by applying this negative dislocation layer model the continuous gps surveying data can be inverted to determine the negative dislocation rate distributions in the middle or upper seismogenic crust layer so as to predict the locations of potential earthquake sources

alluvial architecture in headwater streams with special emphasis on step pool topography alluvial mountain streams exhibit a range of channel forms pool riffle plane bed step pool and cascades previous work suggested that these forms exist within discrete and progressively steeper slope classes measurements conducted at over 100 sites in west central and central idaho confirm that slope steepens progressively as one moves from pool riffle to plane bed to step pool and finally to cascades median slope for pool riffle topography is 0 0060 for plane beds 0 013 for step pools 0 044 and for cascades 0 068 there is substantial overlap in the slopes associated with these channel forms pool riffle topography was found at slopes between 0 0010 and 0 015 plane beds between 0 0010 and 0 035 step pools between 0 015 and 0 134 and cascades between 0 050 and 0 12 step pools are particularly striking features in headwater streams they are characterized by alternating steep and gentle channel segments the steep segments step risers are transverse accumulations of boulder and cobbles while the gentle segments pools contain finer material step wavelength is best correlated to step height which is in turn best correlated to the median particle size found on step risers this result differs from past studies that have reported channel slope to be the dominant control on step wavelength the presumed geometry and froude number associated with the features under formative conditions are consistent with the existence field for antidunes and by extension with the hypothesis that step pools are formed by antidunes copyright 2000 john wiley sons ltd

flow regimes bed morphology and flow resistance in self formed step pool channels received 3 july 2008 revised 28 december 2008 accepted 13 january 2009 published 25 april 2009 1 we used a mobile bed flume with scaled grain size distribution channel geometry and flow to examine morphology and hydraulics of stepped channels we hypothesized that 1 step geometry and flow resistance differs significantly as a function of the range of grain sizes present 2 a transition from nappe to skimming flow occurs in stepped channels with mobile beds for conditions similar to stepped spillways and 3 the partitioning of flow resistance changes significantly when flow passes from nappe to skimming conditions results support each of these hypotheses and help to illuminate the complexity of v q relationships in stepped channels in which a dramatic decrease in flow resistance and increase in velocity accompany the transition from nappe to skimming flow near step forming events therefore a single flow resistance equation applicable to both ordinary and large floods may not be ideal in stepped channels nonetheless models based on dimensionless velocity and unit discharge appear more robust compared to those based on the darcy weisbach friction factor

formation of riffle pool sequences field evidence for an autogenetic process abstract riffle pool sequences are the characteristic reach scale bedforms of mixed and gravel bedded rivers while most research has been directed to the quantitative description of these sequences and to their maintenance comparatively little attention has been given to the processes responsible for their formation in this paper field results from quasi continuous velocity records obtained with an array of electro magnetic current meters are analysed with respect to a model of macro scale structure in the turbulent velocity field of natural channels this model has been suggested as the most plausible basis for riffle pool formation the results demonstrate the occurrence of a variety of coherent flow structures some of which scale on channel dimensions as predicted by the model but which also suggest that some revision of the association between flow structure and riffle pool formation is necessary it is suggested that riffle pool units are initiated with the generation of roller eddies upstream and downstream from a major flow obstacle corresponding to engineering criteria for scour around bridge piers the obstacle persists long enough to fix the flow pattern and hence account for significant modification of channel form but is ultimately removed as part of the process of bed modification via the extension of scour three distinct stages in the process are involved local scour of a single pool creates deposition downstream which then generates the next downstream flow irregularity a riffle pool sequence is thus created autogenetically as the summation of a sequence of irregularities each unit of which is formed and maintained only by flow dynamics operating at the local scale the process is essentially both deterministic and statistical in character it is further suggested that spatial differences in the near bed turbulence field arising from incipient riffle pool topography themselves create differences in surface sediment entrainment which enhance and maintain the sequence in a form process feedback mechanism similar reasoning might possibly be extended to the entire range of bedform sizes