Thalweg Profile Research Articles

Characteristics of thalweg profile and cross-sectional geometry along alluvial river bends

ABSTRACT River bends have always drawn attention for their complicated hydraulic, geometric and migration nature. Laboratory as well as field experiments have been carried out to study the geometric behaviour of channel bends. This study aims to analyse the nature of the thalweg profile, longitudinal and transverse slope and distribution of depth in the cross-sectional geometry of both single and consecutive double bends of the channel. In the single bend, the thalweg approaches to the outer bank and becomes closest after the apex of the bend whereas in the successive bends of a double bend, the thalweg is closest before the apex of the bend. These results indicate that the thalweg of a bend depends on the previous consecutive bend. The longitudinal slopes vary periodically along the river bend creating the deepest pool before the apex of the bend. The centre line transverse slope varies periodically; however, the outer transverse slope with respect to the thalweg is maximum where the thalweg is closest to the outer bank. The thalweg and outer quarter line to centre line depth ratio is higher before the apex and reduces after the apex of the bend. The inner quarter line to centre line depth ratio varies periodically with a relatively smaller amplitude.

Deep‐water sedimentary systems and tectono–sedimentary interactions on the oblique convergent margin in Antarctica

ABSTRACTActive continental margins are tectonic sites with complex sedimentary processes. Tectono–sedimentary interactions occur over geological time and determine the modern morphology of the margin. However, such interactions have been poorly documented in high‐latitude regions. This study focuses on deep‐water sedimentary systems on the South Shetland convergent margin, which is the only remaining active continental margin in Antarctica. Numerous gullies, 20 canyons and three contourite drifts are identified by interpreting bathymetry, oceanography and reflection seismic data. Heavy mineral contents of a gravity core, located at a downslope elongated mounded drift, show the asynchronous interplay of downslope and alongslope processes. The rest of the margin is dominated by turbidity currents. Variations in the slope gradient control thalweg profiles and sinuosity of canyons. Tectonic‐related morphology restricts the distribution of contourite drifts. Earthquakes triggered by underplating and tectonic compression may induce slope instability at the lower flank of the plastered drift. Tectonic uplift influences the amount of sediments transported by ice sheets and controls the shelf stacking pattern. The mixed turbidite–contourite system transitions to separated canyon and drift systems from passive to active continental margins under the influence of the tectonic‐related morphology. A systematic comparison has been made among several convergent margins with similar tectonic settings. Results suggest that submarine canyons on these shelf‐slope systems have similar morphological features and are comparable in size. Coarse sediment input and steep continental slopes (mean slope 7 to 10°) control the canyon morphology on these margins. Therefore, this research has important implications for furthering the current understanding of sedimentary systems on high‐latitude convergent margins and tectono–sedimentary interactions.

Assessing geomorphic floodplain models for large scale coarse resolution 2D flood modelling in data scarce regions

River valleys are dynamic living ecosystems of utmost importance for flood attenuation that are shaped by inundation dynamics. Topographic and bathymetric surveys represent pivotal information for accurate and up-to-date floodplain studies. Both floodplain morphology as well as fluvial river cross section and thalweg profiles are required for inundation modelling and mapping. However, economic and technical limitations hinder their availability in some regions, resulting in challenges to build two dimensional (2D) flood wave routing simulations at proper accuracy and resolution. In this study, we assess the effectiveness of characterizing the fluvial morphology by means of geomorphic methods (GMs). Different GMs, used as surrogates of fluvial bathymetry, are tested and compared to a dataset of surveyed natural cross sections available for the Tiber River basin (Italy). Quantitative flood modelling performances are developed using a validated 150 m flood model, providing inundation extent and floodplain flow depths for the 200 years return period event. The ability (or inefficiency) of surrogating the lack of surveyed fluvial bathymetric data with GMs for supporting large scale hydraulic inundation modelling studies is assessed by testing the following 5 floodplain modelling configurations: (1) rectangular shaped cross section; (2) floodplain-based; (3) global river database; (4) linear regression bathymetric relationship; and (5) and a very coarse 700 grid resolution. In addition, two Global Flood Hazard Mapping (GFHM) products (hydrological, and hydrogeomorphic) were used as part of the large scale floodplain modelling evaluation framework. Results demonstrate that, once the fluvial channel flow area is preserved, all tested GM models produce consistent simulation of inundation depths and extents (Fit index ≥ 0.90). This work provides a quantitative assessment of the validity of the hypothesis stressing that the floodplain conveyance capacity is the driving principle of flood inundation dynamics under extreme flooding scenarios. Understanding the role of geomorphology during extreme magnitude floods may support the idea that under specific conditions, high resolution models and detailed topography/bathymetry are surplus to requirements. This work supports the application of geomorphic approaches over large riparian domains as a parsimonious solution for flood hazard mapping in data scarce regions for applications beyond flood mitigation and forecasting.

Responses of channel morphology to flow-sediment variations after dam construction: a case study of the Shashi Reach, middle Yangtze River

AbstractUpstream damming has profoundly impacted downstream channel morphology by altering inflowing water and sediment conditions, which can mostly be ascribed to variations in the flow hydrograph and sediment supply regime. In this paper, channel erosion and deposition during different flow-sediment processes are quantified using a 2D hydro-morphodynamic model. Our results revealed that the net erosion mainly occurred during the flood season when the flow discharges were above 15,000 m3/s. Together, larger peak discharges and less sediment supply could produce greater net erosion. Flow hydrograph variations could alter the inundation extent, thus creating a more widespread redistribution of channel deposition and erosion and possibly causing a shift in the active channel adjustment area, where more channel scouring and siltation occurred. The channel adjustments caused by the sediment supply regime variations underwent a gradual downward propagation process, and most of the riverbed thalweg profile variations could first be observed at a very short distance from the studied reach entrance. A larger cross-sectional area and channel depth as well as a lower width-depth ratio could result from larger floods and less incoming upstream sediment load. We found that a comprehensive flow-sediment combination coefficient with a value of m ranging from 2 to 4 most appropriately reflected the post-dam flow-sediment imbalance regime at the studied reach, which implied the leading role of flow hydrograph variations in shaping channel morphology. In summary, the combined results presented herein for the Shashi Reach of the Yangtze River can provide a better understanding of the downstream morphological impacts of different flow-sediment processes caused by dam operation.

Decadal-Scale Variations of Thalweg Morphology and Riffle–Pool Sequences in Response to Flow Regulation in the Lowermost Mississippi River

The lowermost Mississippi River (LMR) is one of the largest deltaic systems in North America and one of the heavily human-manipulated fluvial river systems. Historic hydrographic surveys from the mid-1900s to the early 2010s were used to document the thalweg morphology adjustments, as well as the riffle–pool sequences. Extensive aggradation was observed during 1950s to 1960s, as the Atchafalaya River was enlarging before the completion of the Old River Control Structure (ORCS). Following the completion of the ORCS, reductions in sediment input to the LMR resulted in net degradation of the thalweg profile patterns since the mid-1960s except for the 1992–2004 period. Different flood events that supplied sediment might be the cause of upstream aggradation from 1963–1975 and net aggradation along the entire reach from 1992–2004. Furthermore, the change pattern of thalweg profiles appear to be controlled by backwater effects, as well as the Bonnet Carré spillway opening. Results from riffle–pool sequences reveal that the averaging Ws ratios (length to channel width) are 6–7, similar to numerous previous studies. Temporal variations of the same riffles and pools reveal that aggradation and degradation might be heavily controlled by similar factors to the thalweg variations (i.e., sediment supply, backwater effects). In sum, this study examines decadal-scale geomorphic responses in a low-lying large river system subject to different human interventions, as well as natural flood events. Future management strategies of this and similar river systems should consider recent riverbed changes in dredging, sediment management, and river engineering.

Floodplain Terrain Analysis for Coarse Resolution 2D Flood Modeling

Hydraulic modeling is a fundamental tool for managing and mitigating flood risk. Developing low resolution hydraulic models, providing consistent inundation simulations with shorter running time, as compared to high-resolution modeling, has a variety of potential applications. Rapid coarse resolution flood models can support emergency management operations as well as the coupling of hydrodynamic modeling with climate, landscape and environmental models running at the continental scale. This work sought to investigate the uncertainties of input parameters and bidimensional (2D) flood wave routing simulation results when simplifying the terrain mesh size. A procedure for fluvial channel bathymetry interpolation and floodplain terrain data resampling was investigated for developing upscaled 2D inundation models. The proposed terrain processing methodology was tested on the Tiber River basin evaluating coarse (150 m) to very coarse (up to 700 m) flood hazard modeling results. The use of synthetic rectangular cross sections, replacing surveyed fluvial channel sections, was also tested with the goal of evaluating the potential use of geomorphic laws providing channel depth, top width and flow area when surveyed data are not available. Findings from this research demonstrate that fluvial bathymetry simplification and DTM resampling is feasible when the terrain data resampling and fluvial cross section interpolation are constrained to provide consistent representation of floodplain morphology, river thalweg profile and channel flow area. Results show the performances of low-resolution inundation simulations running in seconds while maintaining a consistent representation of inundation extents and depths.

Submarine canyons of NW Madagascar: A first geomorphological insight

The continental slope of Madagascar is underrepresented in the literature in comparison to other continental slopes worldwide. In particular, the submarine geomorphology of this zone has not been discussed in detail. During research cruise SO230 approximately 1900 km2 of high resolution multibeam bathymetric data were collected along the northwest continental slope of Madagascar. These data show four, previously unrecognised, submarine canyons extending to the toe of the continental slope in the eastern Mozambique Channel. Measured canyon thalweg lengths vary from 40 to 51 km and exhibit straight to sinuous paths. The thalweg profiles include concave, linear and slightly convex characteristics. Canyon relief decreases with depth downslope, while canyon width increases. The interfluves of the upper canyon reaches are dominated by large, square to rectangular, block-like features of 100–200 m vertical relief. These features decrease in prominence down slope. Canyons mark the coast-perpendicular flank of the blocks, while the coast-parallel block flanks are delineated by elongate valleys. The geomorphology of the canyons is best explained by varied stages (youthful to mature) of canyon evolution. The discovery of these canyons highlights the complexity of the Madagascan Continental slope, and the future potential for multidisciplinary research in this region.

The Application of a Multi-Beam Echo-Sounder in the Analysis of the Sedimentation Situation of a Large Reservoir after an Earthquake

The Wenchuan Earthquake took place in the upper reach catchment of the Min River. It resulted in large amounts of loose materials gathering in the river channel, leading to changes in the sediment transport system in this area. The Zipingpu Reservoir is the last and the largest reservoir located in the upper reach of the Min River. It is near the epicenter and receives sediment from upstream. This paper puts forward a study on the reservoir sedimentation and storage capacity of the Zipingpu Reservoir, employing a multi-beam echo-sounder system in December 2012. Then, the data were merged with digital line graphics and shuttle radar topography mission data in ArcGIS to build a digital elevation model and triangulate the irregular network of Zipingpu Reservoir. Via the analysis of the bathymetric data, the results show the following: (1) The main channels of the reservoir gradually aggrade to a flat bottom from the deep-cutting valley. Sedimentation forms a reach with a W-shaped longitudinal thalweg profile and an almost zero slope reach in the upstream section of the reservoir due to the natural barrier induced by a landslide; (2) The loss ratios of the wetted cross-section surface are higher than 10% in the upstream section of the reservoir and higher than 40% in the natural barrier area; (3) Comparing the surveyed area storage capacity of December 2012 with March 2008, the Zipingpu Reservoir has lost 15.28% of its capacity at the dead storage water level and 10.49% of its capacity at the flood limit water level.

STUDY OF WATER AND BED LEVEL VARIATION IN THE GANGES-PADMA RIVER, BANGLADESH

This study investigated the water and bed levels together with thalweg profile variations in graphical and numerical analysis using MS Excel for the Ganges-Padma River between 1996 and 2014 at three different selected stations. It was found that there was no consistency in cross section profiles over the years. The cross section profiles varied rapidly at Mawa and Harding Bridge stations, whereas the changes at Baruria Transit were more consistent comparatively. The mean bed levels at Harding Bridge and Mawa stations gradually rose while at Baruria Transit station, it showed a dynamic behavior. Recently in most cases, the water and bed levels lowered or rose simultaneously, but in the years 1996 to 2009 they showed dissimilar patterns. The maximum water level variation was found at Harding Bridge station at 0.75m (rise). The shifting of thalweg points also varied significantly. The most frequent movement of thalweg vertically was found at Baruria Transit station and horizontally at Mawa station.

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

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

On kinematics and flow velocity prediction in step‐pool channels

AbstractThis paper verifies methods for the prediction of mean flow velocity at the reach scale in mountain streams, investigating the kinematics of a series of two small‐scale artificial step‐pool sequences and a transitional reach between plane‐bed and step‐pool under well‐controlled hydraulic conditions, and improving the estimation of the energy expenditure between the step crest and the downstream pool. Experimental data were collected using three fish ladder reaches with slopes between 2.6 and 10%. Four types of field measurements were conducted: topographical surveys to extract the thalweg profiles and cross‐sectional geometry of reference cross sections; grain size analyses of the bed surface; steady state runs with a given flow rate (0.005–0.234 m3/s), and surveying of the water profile in the most significant cross sections. The following main conclusions were reached: (i) the dominance of spill resistance at the lowest discharge (pool water depth‐step height ratios of 0.4) causes primary dimensionless head losses of up to 80%, and these losses progressively decrease to approximately 40% when the water discharge and related pool water depth submerge the upstream step height. A specific predictive equation for the head loss was calibrated and then verified via data from the Rio Cordon. (ii) The verification of literature‐sourced equations to predict the reach‐averaged flow velocity provided suitable results for several of these equations indicating that the use of a specific step‐pool equation does not appear to be crucial to achieving accurate predictions.

Semi-two dimensional numerical prediction of non-equilibrium sediment transport in reservoir using stream tubes and theory of minimum stream power

This paper presents numerical modeling methods to predict long term reservoir sedimentation. The Generalized Stream Tube computer models for Alluvial River Simulation version 3.0 (GSTARS3) model which has capability of semi-two dimensional hydraulic computation and sediment routing by using stream tube concept combined with the theory of minimum stream power was implemented. In GSTARS3, non-equilibrium sediment transport equation is incorporated in accounting for the reservoir sedimentation processes. Capability of numerical simulation with and without applying stream tube concept, theory of minimum stream power, and non-equilibrium sediment transport equation was evaluated with nine years of sedimentation in the Kardeh Reservoir, Iran, and the best combinations of theories were investigated in this study. Simulated thalweg profile and channel geometry were compared to the measured results. From the comparisons, non-equilibrium sediment transport equation is preferably recommended method to predict reservoir sedimentation. The application of theory of minimum stream power improved the performance of model predictions significantly. In addition, simulations with three stream tubes were slightly better than the one with five stream tubes. Numerical simulations indicated that a semi-two dimensional modeling with non-equilibrium sediment transport equation, three stream tubes, and theory of minimum stream power is applicable to a long term prediction of reservoir sedimentation.

Geomorphic characterization of four shelf-sourced submarine canyons along the U.S. Mid-Atlantic continental margin

Shelf-sourced submarine canyons are common features of continental margins and are fundamental to deep-sea sedimentary systems. Despite their geomorphic and geologic significance, relatively few passive margin shelf-breaching canyons worldwide have been mapped using modern geophysical methods. Between 2007 and 2012 a series of geophysical surveys was conducted across four major canyons of the US Mid-Atlantic margin: Wilmington, Baltimore, Washington, and Norfolk canyons. More than 5700km2 of high-resolution multibeam bathymetry and 890 line-km of sub-bottom CHIRP profiles were collected along the outer shelf and uppermost slope (depths of 80-1200m). The data allowed us to compare and contrast the fine-scale morphology of each canyon system. The canyons have marked differences in the morphology and orientation of canyon heads, steepness and density of sidewall gullies, and the character of the continental shelf surrounding canyon rims. Down-canyon axial profiles for Washington, Baltimore and Wilmington canyons have linear shapes, and each canyon thalweg exhibits morphological evidence for recent, relatively small-scale sediment transport. For example, Washington Canyon displays extremely steep wall gradients and contains ~100m wide, 5–10m deep, v-shaped incisions down the canyon axis, suggesting modern or recent sediment transport. In contrast, the convex axial thalweg profile, the absence of thalweg incision, and evidence for sediment infilling at the canyon head, suggest that depositional processes strongly influence Norfolk Canyon during the current sea-level high-stand. The north walls of Wilmington, Washington and Norfolk canyons are steeper than the south walls due to differential erosion, though the underlying cause for this asymmetry is not clear. Furthermore, we speculate that most of the geomorphic features observed within the canyons (e.g., terraces, tributary canyons, gullies, and hanging valleys) were formed during the Pleistocene, and show only subtle modification by Holocene processes active during the present sea-level high-stand.

A critical test of the concept of submarine equilibrium profile

The existence of a slope equilibrium profile has been widely used to account for erosional and depositional processes on submarine slopes and turbidite systems. Profiles out-of-equilibrium are commonly observed in actively deforming areas where channels seem to be deflected or diverted by seafloor structures. In this study the concept of the submarine equilibrium profile is tested in an area of extensive surface faulting to examine whether channels adopt an equilibrium-type profile through time. The study area is on the slope of the Nile Delta, which is disrupted by a number of surface-rupturing normal faults. Prior to fault linkage, several submarine channels flowed down the slope and either utilised relay ramps or flowed through fault scarps of the fault array. Where a relay ramp had been utilised, post fault linkage, the channels of the area either avulsed or converged into one major channel in response to a change in the deformed slope profile to a more concave shape. The thalweg of the post fault linkage channel and two slope profiles either side of it are measured in the area of the fault array, to understand how the channel evolved in response to the active faulting. When fault displacement is relatively small the combination of channel erosion and aggradation results in a channel thalweg profile near-equilibrium with predictable modifications of channel dimensions (depth and width) even if sediment supply was infrequent and episodic. It is concluded that turbidite channels can conform to the concept of equilibrium and submarine base level if it is the most energy efficient route for submarine gravity flows downslope. The most energy efficient route will be one where flows bypass the slope without eroding or depositing and move in a direct downslope course towards base level.

Simple measures of channel habitat complexity predict transient hydraulic storage in streams

Stream thalweg depth profiles (along path of greatest channel depth) and woody debris tallies have recently become components of routine field procedures for quantifying physical habitat in national stream monitoring efforts. Mean residual depth, standard deviation of thalweg depth, and large woody debris (LWD) volumes are potential metrics of habitat complexity calculated from these survey data. We used 42 intensive dye-transit studies to demonstrate the relevance of these easily measured channel habitat complexity metrics to transient hydraulic (“dead zone”) storage, a channel process important for biotic habitat as well as retention and “spiraling” of dissolved and particulate nutrients. We examined transient storage and channel morphology in small gravel and cobble-bedded upland streams (wetted width 2–5 m; slopes 2.6–8.3%) representing a wide range of flow stages, LWD loading, and channel complexity, including measurements before and after LWD was added to enhance fish habitat. While transient storage volume fraction decreased as flow stage increased in simple channels, those with complex morphology and well-developed riparian vegetation maintained high transient storage fractions even during storm flows. LWD additions increased transient storage and channel complexity over the 2 years of post-treatment measurements. We predict with considerable precision two different formulations of transient hydraulic storage fraction using single-variable linear regressions on residual depth (R 2 = 0.61–0.89), thalweg depth variance (R 2 = 0.64–0.91), or large woody debris volume (R 2 = 0.48–0.74). Demonstration of these likely causal associations contributes to understanding the process of transient storage and redefines the use of thalweg profile metrics as a new approach to quantifying morphologic and hydraulic complexity in streams.

Transient erosion in the Valencia Trough turbidite systems, NW Mediterranean Basin

Submarine canyons can efficiently drain sediments from continental margins just as river systems do in subaerial catchments. Like in river systems, submarine canyons are often arranged as complex drainage networks that evolve from patterns of erosion and deposition. In the present paper we use a morphometric analysis of submarine canyon-channel long-profiles to study the recent sedimentary history of the Valencia Trough turbidite system (VTTS) in the NW Mediterranean Sea. The VTTS is unique in that it drains sediment from margins with contrasting morphologies through a single “trunk” conduit, the Valencia Channel. The Valencia Channel has been active since the late Miocene, evolving in response to Plio-Quaternary episodes of erosion and deposition. The integrated analysis of long-profiles obtained from high-resolution bathymetric data across the entire turbidite system shows evidence for transient canyon incision in the form of knickpoints and hanging tributaries. Multiple factors appear to have triggered these periods of incision. These include a large debris flow at 11,500yrBP that disrupted the upper reaches of the VTTS and glacio-eustatic lowstands that forced shifting of sediment input to the VTTS. Based on these inferences, long-term time-averaged incision rates for the Valencia Channel have been estimated. The evidence we present strongly suggests that Foix Canyon has played a key role in the drainage dynamics of the VTTS in the past.This study builds conceptually on a recent modeling study that provides a morphodynamic explanation for the long-term evolution of submarine canyon thalweg profiles. The procedure and results from this work are of potential application to other submarine sediment drainage systems, past and present, including those containing mid-ocean type valleys like the Valencia Channel.

Efectos de la tectonica y el clima en la configuracion morfologica del relieve costero del norte de Chile

This work analyzes the morphological features of the western margin of the northern Chile Coastal Cordillera, characterized by the presence of a major Coastal Cliff. The main purpose of this research is to better understand the tectonic and climatic controls on the topography. The methodology used consists basically of a detailed study of the topography with quantitative geomorphic indexes such as incision grade, hypsometric curves and integrals, thalweg profiles and sinuosity index. These are applied to digital elevation models and satellite images. The main results show that the topography of the western margin of the Coastal Cordillera, exhibits an increasing intensity of the erosion with latitude. This increment is non uniform. The tectonic processes that produced coastal subsidence/uplift and fault activity, and the erosion of the paleotopography of Coastal Cordillera due to processes related with the construction of Coastal Cliff and increasing rainfall, modify the morphology of the coastal border of north Chile. The existence and conservation of the Coastal Cliff result from a non-equilibrium between the uplift and erosion rates. Both rates have one-two order magnitude difference, controlled by the extreme hyperarid climate of the Atacama Desert. These conditions prevailed at least for the last 2 m.yrs. The variations in the intensity of the erosion from north to south are interpreted as the result of the southward increase in precipitacion by one order of magnitude.

Efectos de la tectónica y el clima en la configuración morfológica del relieve costero del norte de Chile

Tectonic and climatic effects in the morphologic configuration of the coastal relief of northern Chile. This work analyzes the morphological features of the western margin of the northern Chile Coastal Cordillera, character- ized by the presence of a major Coastal Cliff. The main purpose of this research is to better understand the tectonic and climatic controls on the topography. The methodology used consists basically of a detailed study of the topography with quantitative geomorphic indexes such as incision grade, hypsometric curves and integrals, thalweg profiles and sinuosity index. These are applied to digital elevation models and satellite images. The main results show that the topography of the western margin of the Coastal Cordillera, exhibits an increasing intensity of the erosion with latitude. This increment is non uniform. The tectonic processes that produced coastal subsidence/uplift and fault activity, and the erosion of the paleotopography of Coastal Cordillera due to processes related with the construction of the Coastal Cliff and increasing rainfall, modify the morphology of the coastal border of northern Chile. The existence and conservation of the Coastal Cliff results from the existance of non-equilibrium between the uplift and erosion rates. Both rates have one-two order magnitude difference, controlled by the extreme hyperarid climate of the Atacama Desert. These conditions prevailed at least for the last 2 m.yrs. The variations in the intensity of the erosion from north to south are interpreted as the result of the southward increase in precipitacion by one order of magnitude.

Geologia e hipsometria de bacias de drenagem do centro-norte catarinense (Brasil): implicações para a zona costeira

A região centro-norte do Estado de Santa Catarina/Brasil é composta basicamente pelas bacias hidrográficas dos rios Itajaí-Açu, Itapocu e Tijucas, drenando diretamente para o Atlântico Sul. O objetivo do presente trabalho é analisar e correlacionar a hipsometria dessas bacias para melhor entender a evolução holocênica e a dinâmica dos estuários e planícies costeiras associadas àqueles rios. Foram compilados dados gerais da cobertura geológico-sedimentar, topografia, dimensões e clima de cada bacia hidrográfica. Esses dados foram analisados e interpretados no contexto geomorfológico. Dados de hidrodinâmica e sedimentologia dos estuários e estratigrafia das planícies costeiras foram também considerados. Os resultados apontam diferenças importantes na geologia, topografia, perfis de talvegue e hipsometria das bacias estudadas. Foram também identificadas diferenças significativas na morfologia, dinâmica e estratigrafia das respectivas regiões estuarinas e planícies costeiras, que foram diretamente correlacionadas com as características das bacias de drenagem.

Physical integrity: the missing link in biological monitoring and TMDLs

The Clean Water Act mandates that the chemical, physical, and biological integrity of our nation's waters be maintained and restored. Physical integrity has often been defined as physical habitat integrity, and as such, data collected during biological monitoring programs focus primarily on habitat quality. However, we argue that channel stability is a more appropriate measure of physical integrity and that channel stability is a foundational element of physical habitat integrity in low-gradient alluvial streams. We highlight assessment tools that could supplement stream assessments and the Total Maximum Daily Load stressor identification process: field surveys of bankfull cross-sections; longitudinal thalweg profiles; particle size distribution; and regionally calibrated, visual, stream stability assessments. Benefits of measuring channel stability include a more informed selection of reference or best attainable stream condition for an Index of Biotic Integrity, establishment of a baseline for monitoring changes in present and future condition, and indication of channel stability for investigations of chemical and biological impairments associated with sediment discontinuity and loss of habitat quality.