En masse debris transport in a mountain stream

Accumulations of stored sediments in a sub-alpine environment of a front range of New Zealand's Southern Alps closely resemble sieve deposits previously described from southeastern California. The locations of such deposits, within active river channels, has important implications for long term sediment yields from catchments where such deposits occur. The catastrophic failure of a sieve deposit in the Torlesse Stream catchment, the site of an earlier investigation of mountain stream sediments, resulted in a sediment yield equivalent to a third of the total yield recorded over an eight-year period. Derived bed-load transport rates represent, at a minimum, a four-fold increase over calculated average transport rates. Such deposits are only likely to fail during extreme low frequency events since their highly permeable gravels allow for the continuation of stream flow underneath the bulk of the deposit and restricts significant increases in moisture content within the deposit. The loss of water is responsible for the inhibition of flowage-type mass movement transport mechanisms wherein available water contributes to the maintenance of positive pore pressures. For the example studied here the percentage of silt and clay is less than 3 per cent of the total sample while sorting values (σ₁) are generally less than 2.0?. These values are approximately one-third and one-half respectively, of typical values obtained for these parameters from debris flow deposits. The feature described here is believed to have originated through failure of a higher altitude perched scree-field. This mode of origin would account for the distinctive sediment size parameters.     

Suspended sediment and total dissolved solid yield patterns at the headwaters of Urumqi River,northwestern China: a comparison between glacial and non‐glacial catchments

In order to understand the differences in the suspended sediment and total dissolved solid (TDS) yield patterns between the glacial and non‐glacial catchments at the headwaters of Urumqi River, northwestern China, water samples were collected from a glacier catchment and an empty cirque catchment within the region, during three melting seasons from 2006 to 2008. These samples were analyzed to estimate suspended sediment and TDS concentrations, fluxes and erosion rates in the two adjoining catchments. There were remarked differences in suspended sediment and TDS yield patterns between the two catchments. Suspended sediment concentrations were controlled mainly by the sediment source, whereas TDS concentrations were primarily related to the hydrologic interaction with soil minerals. Generally, the glacial catchment had much higher suspended sediment and TDS yields, together with higher denudation rates, than the non‐glacial catchment. Overall, glacial catchment was mainly dominated by physical denudation process, whereas the non‐glacial catchment was jointly influenced by physical and chemical denudation processes. The observed differences in material delivery patterns were mainly controlled by the runoff source and the glacial processes. The melting periods of glacier and snow were typically the most important time for the suspended sediment and TDS yields. Meanwhile, episodic precipitation events could generate disproportionately large yields. Subglacial hydrology dynamics, glaciers pluck and grind processes could affect erodibility, and the large quantities of dust stored on the glacier surface provided additional sources for suspended sediment transport in the glacial catchment. These mechanisms imply that, in response to climate change, the catchment behaviour will be modified significantly in this region, in terms of material flux. Copyright © 2013 John Wiley & Sons, Ltd.     

Sediment dynamics in the Castleton karst,Derbyshire, U.K.

Chemical and mineralogical data are presented for a suite of 59 sediment samples collected from the P8 and Speedwell cave systems (and mineralogical data are presented for a further six surface sediments) in the Castleton karst catchment, Derbyshire, England. Sediments are grouped according to age and depositional environment and these groups show distinctive mineralogical and chemical characteristics. Clay mineralogy is effective at distinguishing different sediment sources for ancient and modern sediments which are derived from pre-Devensian tills and Devensian solifluction deposits, respectively. Major element chemistry reflects these differences in clay mineral content, while trace elements demonstrate the importance of abrasion of ore minerals in conduits in the aquifer which follow mineral veins, even under natural, pre-mining conditions. Clay minerals are confirmed as the host for uranium in uranium-rich ancient sediments. Consideration of organic carbon and hydrogen concentrations shows that plant material is the dominant source of organic material in the modern sediments. Copyright © 1999 John Wiley & Sons, Ltd.     

Sediments,porewaters and diagenesis in an urban water body,Salford, UK: impacts of remediation

Contaminated sediments deposited within urban water bodies commonly exert a significant negative effect on overlying water quality. However, our understanding of the processes operating within such anthropogenic sediments is currently poor. This paper describes the nature of the sediment and early diagenetic reactions in a highly polluted major urban water body (the Salford Quays of the Manchester Ship Canal) that has undergone remediation focused on the water column. The style of sedimentation within Salford Quays has been significantly changed as a result of remediation of the water column. Pre‐remediation sediments are composed of a range of natural detrital grains, predominantly quartz and clay, and anthropogenic detrital material dominated by industrial furnace‐derived metal‐rich slag grains. Post‐remediation sediments are composed of predominantly autochthonous material, including siliceous algal remains and clays. At the top of the pre‐remediation sediments and immediately beneath the post‐remediation sediments is a layer significantly enriched in furnace‐derived slag grains, input into the basin as a result of site clearance prior to water‐column remediation. These grains contain a high level of metals, resulting in a significantly enhanced metal concentration in the sediments at this depth. Porewater analysis reveals the importance of both bacterial organic matter oxidation reactions and the dissolution of industrial grains upon the mobility of nutrient and chemical species within Salford Quays. Minor release of iron and manganese at shallow depths is likely to be taking place as a result of bacterial Fe(III) and Mn(IV) reduction. Petrographic analysis reveals that the abundant authigenic mineral within the sediment is manganese‐rich vivianite, and thus Fe(II) and Mn(II) released by bacterial reactions may be being taken up through the precipitation of this mineral. Significant porewater peaks in iron, manganese and silicon deeper in the sediment column are most probably the result of dissolution of furnace‐derived grains in the sediments. These species have subsequently diffused into porewater above and below the metal‐enriched layer. This study illustrates that the remediation of water quality in anthropogenic water bodies can significantly impact upon the physical and chemical nature of sedimentation. Additionally, it also highlights how diagenetic processes in sediments derived from anthropogenic grains can be markedly different from those in sediments derived from natural detrital material. Copyright © 2003 John Wiley & Sons, Ltd.     

Some fine-grained sedimentation phenomena in caves

Fine-grained sedimentation in caves often occurs by the parallel accretion of laminations upon the underlying bedforms. These laminations show remarkable persistence in unit thickness and grain size in a down-passage direction. A translatory flow mechanism ofdeposition is postulated which results in the rhythmic pulsing of sediment-laden water into subterranean lakes in response to variations in surface climate. The sediments are pulsed into standing water via multiple inputs, often resulting in coarser sediment being deposited on steeper slopes than on more gentle slopes. The final extension of this sediment input is the complete filling of the cave passage to the roof. The problem of sediment stability on steep slopes remains an unanswered anomaly.     

Speleoseismology: A critical perspective

Speleoseismology is the investigation of earthquake records in caves. Traces can be seen in broken speleothems, growth anomalies in speleothems, cave sediment deformation structures, displacements along fractures and bedding plane slip, incasion (rock fall) and co-seismic fault displacements. Where earthquake origins can be proven, these traces constitute important archives of local and even regional earthquake activity. However, other processes that can generate the same or very similar deformation features have to be excluded before cave damage can be interpreted as earthquake induced. Most sensitive and therefore most valuable for the tracing of strong earthquake shocks in caves are long and slender speleothems, such as soda straws, and deposits of well-bedded, water-saturated silty sand infillings, particularly in caves close to the earth's surface. Less easily proven is a co-seismic origin of an incasion and other forms of cave damage. The loads and creep movements of sediment and ice fillings in caves can cause severe damage to speleothems which have been frequently misinterpreted as evidence of earthquakes. For the dating of events in geological archives, it is important to demonstrate that such events happened at approximately the same time, i.e. within the error bars of the dating methods. A robust earthquake explanation for cave damage can only be achieved by the adoption of appropriate methods of direct dating of deformation events in cave archives combined with correlation of events in other geological archives outside caves, such as the deformation of lake and flood-plain deposits, locations of rock falls and active fault displacements.     

The influence of tectonic subsidence on volcaniclastic sedimentation: The Cretaceous upper Daeri Member,Wido Island,Korea

The Daeri Member, a Cretaceous volcanic–sedimentary succession, can be divided into lower, middle, and upper parts based on vertical changes in its lithologic characters. The lower Daeri Member is composed of siliciclastic deposits formed in a semi‐arid floodplain environment, which is overlain by the middle Daeri Member consisting mainly of andesite lava flow. After the emplacement of the andesite, activities of intrabasinal normal faults created accommodation on hanging wall blocks together with the development of intrabasinal topographic relief. The upper Daeri Member occurs only in hanging wall blocks and is composed of rhyolitic volcaniclastic sediments formed during an explosive volcanic eruption. Following the eruption, owing to semi‐arid climatic conditions and the destruction of vegetation, the eruptive materials were easily remobilized and deposited by episodic sediment gravity flows, resulting in deposition of the resedimented volcaniclastic deposits with sheet‐like geometry. Away from the intrabasinal normal faults, the resedimented volcaniclastic deposits show a decrease in grain size together with changes in inferred depositional processes from debris flows to hyperconcentrated flows and supercritical sheetfloods. This suggests that the resedimented volcaniclastic deposits were stacked on alluvial fan environments induced by intrabasinal topographic relief associated with normal fault activities. In addition, episodic movement of the faults gave rise to periodic fluctuation of the accommodation and an increase in gradient of the alluvial fan surface, resulting in the development of coarsening‐upward trends in the resedimented volcaniclastic deposits. The development of the alluvial fan and the coarsening‐upward trends indicate that dynamic tectonic subsidence and concomitant changes in the intrabasinal physiographic relief influenced the depositional processes and sizes of the transported volcaniclastic sediments of the upper Daeri Member. Thus, it is necessary to carefully observe tectonic signatures in volcaniclastic successions, particularly the syneruptive lithofacies, in order to reconstruct the tectonic and volcanic histories of receiving basins.     

Calibration and identifiability analysis of a water quality model to evaluate the contribution of different processes to the short-term dynamics of suspended sediment and dissolved nutrients in the surface water of a rural catchment

A method that combines calibration and identifiability analysis of a dynamic water quality model to evaluate the relative importance of various processes affecting the dynamic aspects of water composition is illustrated by a study of the response of suspended sediment and dissolved nutrients to a flood hydrograph in a rural catchment area in the Netherlands. Since the water quality model simulates the observed concentrations of suspended sediment and dissolved nutrients reasonably well, the most important processes during the observed flood hydrograph could be determined. These were erosion, exchange between dissolved phase and bed sediments and denitrification. It is concluded that the method is very useful for identifying the most significant model parameters and processes that are essential for water quality modelling. © 1998 John Wiley & Sons, Ltd.     

The structure and sedimentology of relict talus,Trotternish, northern Skye,Scotland

Sections up to 3·5 m deep cut through the upper rectilinear segment of relict, vegetated talus slopes at the foot of the Trotternish escarpment reveal stacked debris-flow deposits intercalated with occasional slopewash horizons and buried organic soils. Radiocarbon dating of buried soil horizons indicates that reworking of sediment by debris flows predates 5·9–5·6 Cal ka BP , and has been intermittently active throughout the late Holocene. Particle size analyses of 18 bulk samples from these units indicates that c. 27–30 per cent of the talus deposit is composed of fine (<2 mm) sediment. Sedimentological comparison with tills excludes a glacigenic origin for the talus debris, and the angularity of constituent clasts suggests that in situ weathering has been insignificant in generating fine material. We conclude that the fine sediment within the talus is derived primarily by granular weathering of the rockwall, with syndepositional accumulation of both fine and coarse debris, implying that c. 27–30 per cent of rockwall retreat since deglaciation reflects granular weathering rather than rockfall. The abundance of fines within the talus deposits is inferred to have been of critical importance in facilitating build-up of porewater pressures during rainstorms, leading to episodic failure and flow of debris on the upper parts of the slope. A wider implication of these findings is that the mechanical properties of talus slopes cannot be regarded as those of free-draining accumulations of coarse clasts, and that models that treat talus slopes as such have limited value in explaining their form and evolution. Our findings lend support to models that envisage the upper straight slope on talus accumulations as the product of mass-transport as well as rockfall, and indicate that episodic debris flow has been the primary agent of mass-transport at this site. © 1998 John Wiley & Sons, Ltd.     

High-frequency monitoring of hydrological and biogeochemical fluxes in forested catchments of southern Chile

The variability of rainfall-dependent streamflow at catchment scale modulates many ecosystem processes in wet temperate forests. Runoff in small mountain catchments is characterized by a quick response to rainfall pulses which affects biogeochemical fluxes to all downstream systems. In wet-temperate climates, water erosion is the most important natural factor driving downstream soil and nutrient losses from upland ecosystems. Most hydrochemical studies have focused on water flux measurements at hourly scales, along with weekly or monthly samples for water chemistry. Here, we assessed how water and element flows from broad-leaved, evergreen forested catchments in southwestern South America, are influenced by different successional stages, quantifying runoff, sediment transport and nutrient fluxes during hourly rainfall events of different intensities. Hydrograph comparisons among different successional stages indicated that forested catchments differed in their responses to high intensity rainfall, with greater runoff in areas covered by secondary forests (SF), compared to old-growth forest cover (OG) and dense scrub vegetation (CH). Further, throughfall water was greatly nutrient enriched for all forest types. Suspended sediment loads varied between successional stages. SF catchments exported 455 kg of sediments per ha, followed by OG with 91 kg/ha and CH with 14 kg/ha, corresponding to 11 rainfall events measured from December 2013 to April 2014. Total nitrogen (TN) and phosphorus (TP) concentrations in stream water also varied with rainfall intensity. In seven rainfall events sampled during the study period, CH catchments exported less nutrients (46 kg/ha TN and 7 kg/ha TP) than SF catchments (718 kg/ha TN and 107 kg/ha TP), while OG catchments exported intermediate sediment loads (201 kg/ha TN and 23 kg/ha TP). Further, we found significant effects of successional stage attributes (vegetation structure and soil physical properties) and catchment morphometry on runoff and sediment concentrations, and greater nutrients retention in OG and CH catchments. We conclude that in these southern hemisphere, broad-leaved evergreen temperate forests, hydrological processes are driven by multiple interacting phenomena, including climate, vegetation, soils, topography, and disturbance history.     

Assessment of sediment yield in a small karst catchment by using 137Cs tracer technique

There is very thin soil layer in karst rocky desertification areas in Southwest China,sediment deposition and sediment yield in the karst area affects the development of vegetation greatly.In the present study,the ¹³⁷Cs technique was used to assess the rate of sediment deposition and sediment yield in a small karst catchment.The ¹³⁷Cs inventory within the depression varied between 800 m^-2 and 2,200 Bq m^-2,with the mean value of 1,500.1 Bq m^-2.The ¹³⁷Cs reference inventory at a nearby reference site was 805.9 Bq m^-2.It could be inferred to that sediment deposition had occurred in the catchment.The mean depth of sediments deposition in the depression was 6 cm and the deposition rate was approximately 0.13 cm yr^-1.The analysis of the topographic characteristics of the catchment revealed that the sediment deposition occurred mainly at the lower part of the small catchment.Although,there was a sinkhole in the depression,little sediment had drained out with runoff through the sinkhole,because the local people built ridges around the sinkholes for storing water.According to this,sediment yield rate in the small catchment was estimated to be approximately 19.25 to 27.5 t km^-2 yr^-1,and the extremely low sediment yield was maybe the main obstacle to vegetation restoration in karst rocky desertification areas.     

柴达木盆地苏干湖表层沉积与尘暴事件——元素示踪的初步结果

通过分析苏干湖地区10个地表表层沉积物、9个大气降尘(含2个尘暴降尘)、16个湖泊表层沉积物等样品的8种常量-微量元素和11种稀土元素的含量,以及同类沉积不同元素含量和不同沉积同一元素含量的相关性,结果表明,在湖泊沉积体系中Al、Fe、Sc含量的变化与强风-尘暴过程有关,指示了湖泊沉积碎屑颗粒风成输入的可能性．元素Fe、Sc含量在3种不同的沉积物中差异不大,与尘暴事件的强度和频次有关．稀土元素结果显示3类沉积具有同源性．苏干湖沉积可以用来重建研究区尘暴演化历史．     

Rates and mechanisms of discontinuous gully erosion in a red-brown earth catchment,new south wales,australia

Rainfall, runoff, sediment discharge, and gully expansion were measured for five years in a small (45 ha) catchment in central N.S.W. Sixty per cent of the sediment produced from this catchment originated from gully erosion. Gully head erosion was episodic depending on pipe development, cracking, and soil detachment during small runoff events which prepared the heads for rapid soil movement by the infrequent large event.     

The sediment delivery ratio in a small catchment in the black soil region of Northeast China

The black soil region of northeast China,which covers the Provinces of Heilongjiang,Jilin and the Inner Mongolia autonomous region with black soil,chernozem and meadow soil,has experienced soil erosion since intense agricultural reclamation began approximately 100 years ago.However,the sediment delivery ratio,defined as the fraction of gross erosion that is transported from a given area in a given time interval,is still unclear.In this study,we calculated the delivery ratio and analysed changes in erosive processes within Hebei catchment from 1977 to 2007 based on an analysis of sediments of the Liudui reservoir.The original vegetation layer clearly identified the bottom of the reservoir when it was constructed in 1977;thus,the reservoir sediments could be precisely dated.The delivery ratio,calculated by comparing the sediment deposition in the reservoir with the total soil erosion in the upstream catchment,was found to be exponentially correlated（r² = 0.95,P < 0.01） with decreasing grain size,except for the fraction <0.002 mm.The delivery ratio for the clays（<0.002 mm） was low,averaging 0.10 during the study period, which indicated partial removal of clays from the reservoir.The changes in the reservoir deposition rate reflected the temporal changes in the erosion processes.The exceptionally high rainfall in 1998 was confirmed by the distributions of ¹³⁷Cs,²¹⁰Pb,and the grain-size of the sediments.Beginning from the position of the original grass layer,we defined three periods from 1977 to 2007 based on deposition rates:2.40 cm year^-1 from 1977 to 1997,5.60 cm year^-1 in 1998 due to unusually high rainfall,and 1.55 cm year^-1 from 1999 to 2007.The overall average deposition rate for the entire period was 2.26 cm year^-1.Precipitation was found to be the main factor affecting the soil erosion of the study area.     

Miocene hydrovolcanism in NW Colorado,USA, fuelled by explosive mixing of basic magma and wet unconsolidated sediment

The Yampa and Elkhead Mountains volcanic fields were erupted into sediment-filled fault basins during Miocene crustal extension in NW Colorado. Post-Miocene uplift and erosion has exposed alkali basalt lavas, pyroclastic deposits, volcanic necks and dykes which record hydrovolcanic and strombolian phenomena at different erosion depths. The occurrence of these different phenomena was related to the degree of lithification of the rocks through which the magmas rose. Hydrovolcanic interactions only occurred where rising basaltic magma encountered wet, porous, non-lithified sediments of the 600 m thick Miocene Brown's Park Formation. The interactions were fuelled by groundwater in these sediments: there was probably no standing surface water. Dykes intruded into the sediments have pillowed sides, and local swirled inclusions of sediment that were injected while fluidized in steam from heated pore water. Volcanic necks in the sediments consist of basaltic tuff, sediment blocks and separated grains derived from the sediments, lithic blocks (mostly derived from a conglomerate forming the local base of the Brown's Park Formation), and dykes composed of disaggregated sediment. The necks are cut by contemporaneous basalt dykes. Hydrovolcanic pyroclastic deposits formed tuff cones up to 100 m thick consisting of bedded air-fall, pyroclastic surge, and massive, poorly sorted deposits (MPSDs). All these contain sub-equal volumes of basaltic tuff and disaggregated sediment grains from the Brown's Park Formation. Possible explosive and effusive modes of formation for the MPSDs are discussed. Contemporaneous strombolian scoria deposits overlie lithified Cretaceous sedimentary rocks or thick basalt lavas. Volcanic necks intruded into the Cretaceous rocks consist of basalt clasts (some with spindle-shape), lithic clasts, and megacrysts derived from the magma, and are cut by basalt dykes. Rarely, strombolian deposits are interbedded with hydrovolcanic pyroclastic deposits, recording changes in eruption behaviour during one eruption. The hydrovolcanic eruptions occurred by interaction of magma with groundwater in the Brown's Park sediments. The explosive interactions disaggregated the sediment. Such direct digestion of sediment by the magma in the vents would probably not have released enough water to maintain a water/magma mass ratio sufficient for hydrovolcanic explosions to produce the tuff cones. Probably, additional water (perhaps 76% of the total) was derived by flow through the permeable sediments (especially the basal conglomerate to the formation), and into the vents.     

Soil cracking effects on hydrological and erosive processes: a study case in Mediterranean cultivated vertisols

Shrink–swell soils, such as those in a Mediterranean climate regime, can cause changes in terms of hydrological and erosive responses because of the changing soil water storage conditions. Only a limited number of long‐term studies have focused on the impacts on both hydrological and erosive responses and their interactions in an agricultural environment. In this context, this study aims to document the dynamics of cracks, runoff and soil erosion within a small Mediterranean cultivated catchment and to quantify the influence of crack processes on the water and sediment supplied to a reservoir located at the catchment outlet using water and sediment measurements at a cultivated field outlet as baseline. Detailed monitoring of the presence of topsoil cracks was conducted within the Kamech catchment (ORE OMERE, Tunisia), and runoff and suspended sediment loads were continuously measured over a long period of time (2005–2012) at the outlets of a field (1.32 ha) and a catchment (263 ha). Analysis of the data showed that topsoil cracks were open approximately half of the year and that the rainfall regime and water table level conditions locally control the seasonal cracking dynamics. Topsoil cracks appeared to seriously affect the generation of runoff and sediment concentrations and, consequently, sediment yields, with similar dynamics observed at the field and catchment outlets. A similar time lag in the seasonality between water and sediment delivery was observed at these two scales: although the runoff rates were globally low during the presence of topsoil cracks, most sediment transport occurred during this period associated with very high sediment concentrations. This study underlines the importance of a good prediction of runoff during the presence of cracks for reservoir siltation considerations. In this context, the prediction of cracking effects on runoff and soil erosion is a key factor for the development of effective soil and water management strategies and downstream reservoir preservation. Copyright © 2016 John Wiley & Sons, Ltd.     

Suspended sediment yield modeling in Mahanadi River,India by multi-objective optimization hybridizing artificial intelligence algorithms

River sediment produced through weathering is one of the principal landscape modification processes on earth.Rivers are an integral part of the hydrologic cycle and are the major geologic agents that erode the continents and transport water and sediments to the oceans.Estimation of suspended sediment yield is always a key parameter for planning and management of any river system.It is always challenging to model sediment yield using traditional mathematical models because they are incapable of handling the complex non-linearity and non-stationarity.The suspended sediment modeling of the river depends on the number of factors such as rock type,relief,rainfall,temperature,water discharge and catchment area.In this study,we proposed a hybrid genetic algorithm-based multi-objective optimization with artificial neural network(GA-MOO-ANN)with automated parameter tuning model using these factors to estimate the suspended sediment yield in the entire Mahanadi River basin.The model was validated by comparing statistically with other models,and it appeared that the GA-MOO-ANN model has the lowest root mean squared error(0.009)and highest coefficient of correlation(0.885)values among all comparative models(traditional neural network,multiple linear regression,and sediment rating curve)for all stations.It was also observed that the proposed model is the least biased(0.001)model.Thus,the proposed GA-MOOANN is the most capable model,compared to other studied models,for estimating the suspended sediment yield in the entire Mahanadi river basin,India.The results also suggested that the proposed GA-MOO-ANN model is unable to estimate suspended sediment yield satisfactorily at gauge stations having very small catchment areas whereas performing satisfactorily on locations having moderate to the large catchment area.The models provide the best result at Tikarapara,the gauge station location in the extreme downstream,having the largest catchment area.     

Sediment trapping by a tree belt: processes and consequences for sediment delivery

Restoring belts of perennial vegetation in landscapes is widely recognized as a measure of improving landscape function. While there have been many studies of the transport of pollutants through grass filter strips, few have addressed sediment related processes through restored tree belts. In order to identify these processes and quantify their relative contribution to sediment trapping, a series of rainfall simulations was conducted on a 600 m² hillslope comprising a pasture upslope of a 15 year old tree belt. Although the simulated events were extreme (average recurrence intervals ～10 and 50 yr), the trapping efficiency of the tree belt was very high: at least 94% of the total mass of sediments was captured. All the size fractions were trapped with a minimum Sediment Trapping Ratio (STR) of 91% for the medium‐sized fragments. Fractions < 1·3 µm and > 182 µm were totally captured (STR = 100%). Through the joint analysis of sediment budgets and soil surface conditions, we identified different trapping processes. The main trapping process is the sedimentation (at least 62% of trapped sediment mass) with deposits in the backwater and as micro‐terraces within the tree belt. Modelling results show that the coarsest size fractions above 75 µm are preferentially deposited. Joint infiltration of water and sediments has also been noticed, however, this process alone cannot explain the selective trapping of the finest fractions. We suggest that the finest fractions transported by the overland flow may be trapped by adsorption on the abundant litter present within the tree belt. Copyright © 2008 John Wiley & Sons, Ltd.     

The role of landscape setting in minimizing hydrogeomorphic impacts of flow regulation

The Tongariro Power Development Scheme (TPDS) is used to regulate flow in the headwaters of the largest catchment on the North Island of New Zealand (the Waikato). Two small dams, the Rangipo Dam and the Poutu Intake Dam, were constructed in 1973 and 1983. The flow regime of the river is managed to divert freshes into the power scheme, but allows flows larger than 100 m³ s^?1 to be released, to rework and transport sediment through the catchment. Analysis of aerial photos and maps spanning 1928 to 2007, alongside field measurements, show that there have been few hydrogeomorphic adjustments since dam construction. This includes limited changes to channel geometry, channel planform and bed material organization immediately downstream of the dams. In addition, offsite effects are minimal, both 500 m downstream of each dam, and in the more sensitive, less confined reaches in the lower catchment (11 km downstream of the Poutu Intake dam). The limited changes can be attributed to the locations of the dams within reaches characterised by bedrock gorges and confined within terraces. These locations act to flush sediments and impose margins that allow minimal adjustment of the channel. Bed material within this reach is characterised by the presence of a boulder lag. This is sourced from long-term incision into lahar deposits, and acts to limit the rate of incision, creating a steep and stable base upon which active fractions are transported. Just as importantly, significant storage in the low-relief volcanic plateau located in the upper catchment acts to disconnect and store the high sediment yields generated by active volcanic cones in the western sub-catchment upstream of the dams. This limits the rate of sediment supply to regulated reaches. Findings from this study show that analysis of reach-scale controls is essential in framing dam site locations in relation to the distribution of reaches and landscape units across the catchment. In this instance, tributary inputs downstream of the dams do not replenish the sediment and flow removed at the dam locations, as has been observed in other regulated systems. Rather, the river itself is resilient to change and flow variability is well managed allowing geomorphically effective floods to occur. Landscape setting is a key consideration in determining the hydrogeomorphic impact of flow regulation.