Meander Cutoffs Research Articles

Causes for the decline of suspended‐sediment discharge in the Mississippi River system, 1940–2007

AbstractBefore 1900, the Missouri–Mississippi River system transported an estimated 400 million metric tons per year of sediment from the interior of the United States to coastal Louisiana. During the last two decades (1987–2006), this transport has averaged 145 million metric tons per year. The cause for this substantial decrease in sediment has been attributed to the trapping characteristics of dams constructed on the muddy part of the Missouri River during the 1950s. However, reexamination of more than 60 years of water‐ and sediment‐discharge data indicates that the dams alone are not the sole cause. These dams trap about 100–150 million metric tons per year, which represent about half the decrease in sediment discharge near the mouth of the Mississippi. Changes in relations between water discharge and suspended‐sediment concentration suggest that the Missouri–Mississippi has been transformed from a transport‐limited to a supply‐limited system. Thus, other engineering activities such as meander cutoffs, river‐training structures, and bank revetments as well as soil erosion controls have trapped sediment, eliminated sediment sources, or protected sediment that was once available for transport episodically throughout the year. Removing major engineering structures such as dams probably would not restore sediment discharges to pre‐1900 state, mainly because of the numerous smaller engineering structures and other soil‐retention works throughout the Missouri–Mississippi system. Published in 2009 by John Wiley & Sons, Ltd.

Cumulative impacts of river engineering, Mississippi and Lower Missouri rivers

AbstractThe goal of this study was to construct a large, data‐rich model to test hydrological responses to engineering modifications on over 3200 km of the Mississippi and Lower Missouri Rivers. We compiled model explanatory variables from a geospatial database quantifying construction of all bridges, wing dikes, bendway weirs, levees, artificial meander cutoffs, channel constriction and navigational dams over the past 100–150 years. Response variables were derived from 68 rated and un‐rated hydrologic stations in the study area, with responses analysed across a range of discharges from within‐channel flows up to moderate floods. Correlation analysis, multiple linear regression and stepwise regression analyses document strong and consistent responses to construction history, both in individual reach‐scale models and systemwide. Meander cutoffs are associated with degradation and acceleration of flow that has reduced stages across the full discharge range. Navigational dams on the Upper Mississippi River increased low‐flow stages and flood levels to a lesser extent, with little or no post‐dam change. One of the strongest signals was the hydrologic response to wing‐dike construction, which resulted in large back‐water increases in stage upstream of wing dikes and mixed effects downstream, including the overlapping effects of incision and velocity losses. Levees were associated with local flow concentration, overbank storage loss and floodplain conveyance loss depending on reach‐scale conditions. The results presented here (1) quantify incremental and cumulative hydrologic responses to a range of engineering activities and (2) provide an empirical tool for verifying and assessing hydraulic and other models of river‐system change. Copyright © 2009 John Wiley & Sons, Ltd.

Flow and sediment processes in a cutoff meander of the Danube Delta during episodic flooding

This article analyzes the water and suspended solid fluxes through a straightened meander of the southern branch of the Danube Delta (the St. George branch) during episodic flooding. The Mahmudia study site corresponds to a vast natural meander which was cut off in 1984–1988 by an artificial canal opened to shipping. The meander correction accelerated fluxes through the artificial canal and dramatically enhanced deposition in the former meander. After his formation, the cutoff meander acted as sediment storage locations, essentially removing channel and point bar sediments from the active sediment budget of the main channel. Increases in slope and stream power in reaches upstream and downstream have also occurred, but to a lesser degree. During the one-hundred-year recurrent flood in April 2006, bathymetry, flow velocity and discharge data were acquired across several sections of both natural and artificial channels with an acoustic Doppler current profiler (aDcp Workhorse Sentinel 600 kHz, Teledyne RDI) in order to investigate the distribution of the flow and sediment and his impact on sedimentation in a channelized reach and its adjacent cutoff. The contrasting hydro-sedimentary processes at work in both channels and bifurcation/confluence nodal points are analyzed from the measured flux distribution, morphological profiles and velocity and concentration patterns. In the cutoff, a diminishing of the intensity of the flow velocity (c. 50%) and of the SSC was observed correlated with the aggradation of the river bed. In the bifurcation/confluence nodal points and in the artificial canal were observed the most intensive hydrodynamic activity (high flow velocity, SSC concentration, degradation of the river bad). Both the event-scale and long-term morphological trends of the alluvial system are discussed analyzing the boundary shear stress and SSC variability. Excess boundary shear stress in the sub-reaches directly affected by cutoffs resulted in scour that increased downstream bed material load. These high sediment loads play a key role in driving morphological adjustments towards equilibrium in the cutoff channel. The approach followed in this paper combines detailed episodic in-situ aDcp measurements and robust numerical 1D modeling in order to provide a practical comprehension of the relevant morphodynamical processes. The 1D model reproduces robustly the continuity of hydrodynamical variables along the streamwise axes of the two-channel network. The simulated are used in the paper for highlighting reach-scale morphological processes, at both event and long-term scales.

Hydrologic history of the Mississippi and Lower Missouri Rivers based upon a refined specific‐gauge approach

AbstractA refined specific‐gauge approach was developed to quantify changes over time in hydrological response on 3260 km of the Mississippi River system using long‐term data observed at 67 hydrologic measurement stations. Of these stations, 49 were unrated (stage‐only) stations, for which over 2 000 000 ‘synthetic discharges’ were generated based on measured discharge values at nearby rated stations. The addition of these synthetic discharges nearly tripled the number of stations in the study area for which specific‐gauge analysis could be performed. In order to maintain spatial homogeneity across such a broad study area, discharges were normalized to multiples of mean daily flow (MDF). Specific‐gauge analysis calculates stage changes over time for invariant discharge conditions. Two discharges were analysed: low‐flow and flood conditions at each station. In order to avoid the large errors associated with extrapolation of annual rating curves, a new ‘enhanced interpolation’ technique was developed that calculates continuous specific‐stage time series, even for rare discharges. Thus enhanced, specific‐gauge analysis is a useful reconnaissance tool for detecting geomorphic and hydrologic trends over time. Results show that on the Middle Mississippi River and Lower Missouri River, flood stages increased at all stations in spite of widespread incision of the river bed. On the Lower Mississippi River, both low‐flow and flood stages decreased, mainly the result of artificial meander cutoffs in the late 1920s and 1930s, except downstream of Natchez, MS, where net aggradation was observed. On the Upper Mississippi River, the specific‐gauge trends were dominated by emplacement of navigational dams and impoundment of slackwater pools. On all four river reaches, these results document hydrologic responses to the different engineering toolkits used on the different portions of the Mississippi River system during the past 75–150 years. Copyright © 2008 John Wiley & Sons, Ltd.

The Impact of River Engineering Works on the Dyje River floodplain in the Czech Republic

Abstract The article deals with changes to the lower part of the Dyje River floodplain, Czech Republic, caused by river engineering works. It investigates channel changes as well as changes in the number of cut-off meanders (cut-offs), oxbow lakes (oxbows), and pools, focusing on the impact of river engineering works on land use and specific natural conditions of the floodplain. The author begins with a history of river engineering works in the study area and their impact from the perspective of natural conditions and human activities. She then divides the study area into five sections, according to the period when the works were carried out, and examines each one using graphical methods and old maps to determine their morphometric characteristics at different periods. Her research shows a significant shortening of the Dyje, a decrease in its sinuosity, and fluctuations in its surface area and in the number of cut-offs, oxbows, and pools. As regards the natural environment, the engineering works determined a lowering of the water table and some changes in plant species. From the perspective of human activities, in the form of land use changes, the study indicates that there was a significant reduction of meadow and pastureland, an increase in recreational areas, and a decrease in forestland during the period under investigation. Some of these changes – notably the increase of the overall water surface and the decrease of forestland – are a direct result of river engineering works, while others – the ploughing of the floodplain and the spread of recreational areas around reservoirs – are an indirect consequence thereof.

Use of multitemporal remote sensing data for mapping the Alfios River network changes from 1977 to 2000

Alfios is the biggest river of the Peloponnese and the ninth longest river in Greece. It drains an area of almost 2575 km2 in Western Peloponnese and discharges at Kiparissiakos Gulf. Due to its extent, the Alfios basin presents complex physiography and geomorphology. During the last 50 years, major direct human activities have had an important impact on the river network shape and on the valley floor morphology. The most important human activities were the channel straightening caused by meander cut-offs in 1950, the construction of two dams: the Ladonas dam in 1955 and the Flokas dam in 1968 and the illegal in-stream gravel extraction that started in the 1960s and continues until today. As a result, the Alfios riverbed has presented very rapid vertical and horizontal (lateral) erosion and significant changes to its network shape. The drainage network follows a straighter course, the number of meanders has been reduced and the seventh order branch is deeply incised. Besides the morphological impacts to the river network shape there are also serious economic damages caused by human activity. During the winter of 1999, the Flokas dam bridge was closed for a long period due to damages at its foundation caused by the heavy rain and illegal gravel extraction. Transportation between the villages in the area became extremely difficult and time consuming and the cost of the repairs was estimated at 500 000 euros. In this study, there is an effort to map the network shape transformation for the period 1977–2000 using multitemporal and multisensor satellite images. One Landsat MSS image, three Landsat TM images, two Landsat ETM images, and one Terra ASTER image have been orthorectified and processed in order to cover the specific period. Principal Component Analysis (PCA) method and Geographical Information System (GIS) techniques have been applied to map the changes in the Alfios River channel. The drainage network straightening and the cut-off of five big meanders were detected and mapped. These changes occurred between 1986 and 2000.

Embedding reach-scale fluvial dynamics within the CAESAR cellular automaton landscape evolution model

We introduce a new computational model designed to simulate and investigate reach-scale alluvial dynamics within a landscape evolution model. The model is based on the cellular automaton concept, whereby the continued iteration of a series of local process ‘rules’ governs the behaviour of the entire system. The model is a modified version of the CAESAR landscape evolution model, which applies a suite of physically based rules to simulate the entrainment, transport and deposition of sediments. The CAESAR model has been altered to improve the representation of hydraulic and geomorphic processes in an alluvial environment. In-channel and overbank flow, sediment entrainment and deposition, suspended load and bed load transport, lateral erosion and bank failure have all been represented as local cellular automaton rules. Although these rules are relatively simple and straightforward, their combined and repeatedly iterated effect is such that complex, non-linear geomorphological response can be simulated within the model. Examples of such larger-scale, emergent responses include channel incision and aggradation, terrace formation, channel migration and river meandering, formation of meander cutoffs, and transitions between braided and single-thread channel patterns. In the current study, the model is illustrated on a reach of the River Teifi, near Lampeter, Wales, UK.

Deposits and Cutoff Ages of Horseshoe and Marion Oxbow Lakes, Red River, Manitoba*

Horseshoe Lake and the Marion Lake scar, along the Red River, southern Manitoba, were cored to investigate the timing of the meander cutoffs and the sedimentology of the channel in-fill deposits. The Horseshoe Lake core, 10.75 m long, consists of 9.73 m of silt-rich deposits inferred to be lacustrine from 0 to 4 m deep, transitional from 4 to 5 m deep and alluvial below 5 m deep. Four wood and charcoal specimens sampled from the core yielded radiocarbon ages of 310 ± 40, 1730 ± 50, 2040 ± 50 and 2240 ± 50 BP. The Marion Lake core, 16.77 m long, consists of 14.73 m of silt-rich deposits inferred to be lacustrine from 0 to 5 m deep and alluvial below 8.5 m deep; the transition is indistinct and falls between 5 to 8.5 m deep. Four wood samples from the fluvial deposits yielded radiocarbon ages of 1600 ± 40, 1700 ± 40, 1660 ± 40 and 1620 ± 40 BP. The cutoffs that led to the formation of Horseshoe and Marion lakes are interpreted to have occurred at ~1990 and ~1520 cal BP or shortly thereafter, respectively. The silt-rich, alluvial-lacustrine deposits in the lakes lack structural and textural characteristics that can be readily recognized in core to distinguish the depositional environments. The absence of coarse sediments at the base of the fluvial units at both sites implies that minor to negligible amounts of sand were transported along the thalwegs of the channels prior to the meanders being cutoff. The dominance of silt within the oxbow deposits reflects sediment supply as the geomorphic setting of the river is within an extensive glaciolacustrine clay plain.

Nonlinear analysis of the geometry of meandering rivers

The geometry of four meandering rivers has been investigated to detect linear and nonlinear links in their planimetry. To this aim, some robust nonlinearity tests—DVS plots, reversibility tests, and the Kaplan test—have been applied to the spatial series of local curvatures. The results are coherent and show that no relevant nonlinearities seem to be detectable in the analyzed meandering river patterns. From a river dynamics point of view, this can be explained by the occurrence of meander cutoffs and external forcing which prevent nonlinear morphodynamic processes from significantly influencing the long‐term meandering dynamics and, consequently, the river geometry.

Cutoffs galore!: occurrence and causes of multiple cutoffs on a meandering river

The creation of cutoffs and of oxbow lakes is a well-known phenomenon of meandering rivers, but views on the extent to which they are inherent in meander behaviour have varied. Assumptions of meander behaviour have shifted from those of stability and equilibrium to recognition of gradual evolution and increased complexity of form. Alternative explanations of cutoff occurrence are discussed here in relation to a remarkable set of cutoffs that occurred in one reach of the River Bollin, UK, for which long-term historical evidence of meander evolution existed and which has been monitored for change and processes over the last 20 years. The cutoffs occurred during the high floods of winter 2000–2001. A series of hypotheses is examined, including the occurrence of floods and effects of hydrological changes. Although the flood events actually caused the cutoffs, the long-term pattern accords with ideas of chaotic behaviour and sinuosity of a river reaching a critical state at which clustering of meander cutoffs takes place. It is suggested that the occurrence of the cutoffs can be explained as inherent in meander behaviour.

COMPREHENSIVE ANALSYS OF WATER QUALITY IN AN ENCLOSED WATER BODY RECEIVING DRAINAGE FROM AN URBANIZED BASIN

The Barato River in northern Sapporo was formed from a cutoff meander of the Ishikari River. Since the river is in an urban area, it receives urban wastewater. As a largely enclosed water body withlittle exchange of water but with inflow load from the surrounding area, the Barato has undergone eutrophication, as evidenced by water bloom. We conducted a comprehensive study on the hydrological and material cycles of the Barato River basin. We clarified some causes of the water quality problem of the Barato River. One cause is rainfall runoff from the sewer system. Another is interaction between water and bottom deposits. The Barato River adjoins a backwater section of the Ishikari River, where flow alternates with backflow. Such a hydrological mechanism is considered to significantly affect the water and heat balances of the Barato River. In an effort to solve the eutrophication problem, we are conducting observations and simulations to identify additional causes, toward suggesting an effective water quality improvement method.

Middle Danube tributaries: Constraints and opportunities in lowland river restoration.

Modifications to large active rivers to aid navigation, reduce flood risk, improve land drainage or generate hydroelectric power have adversely affected river habitats and the hydrological connectivity between the river and its floodplain. Consideration of natural river processes helps to identify sustainable rehabilitation options within constraints imposed by the need to maintain required river functions and any restrictions imposed by space limitations due to floodplain development. A brief review of rehabilitation opportunities for active lowland rivers constrained by river functions is presented. This approach is used to identify suitable rehabilitation measures for two lowland meandering Slovak rivers, the Morava and Hron, which have been significantly influenced by engineering works. Both rivers differ in hydrological, morphological and ecological characteristics. Previous rehabilitation efforts based on locally targeted ecological and landscape restoration measures have actually exacerbated ecological degradation due to their failure to consider the abiotic processes which control river morphology. For rehabilitating cut-off meanders, full meander integration is recommended as the best solution for lowland rivers, such as the Morava, which actively transport bed material load. On the Hron River, an alternative method for improving the water regime and ecological quality of cut-off meanders is required due to engineering constraints. The study demonstrates the need for an integrated interdisciplinary approach to river restoration.

A Geomorphic Explanation for a Meander Cutoff Following Channel Relocation of a Coarse-Bedded River

The Veteran's Fishing section of the Blackledge River in central Connecticut was relocated in the late 1950s. The relocation resulted in an unstable channel despite extensive efforts to prevent erosion. Overbank erosion and meander cutoffs were investigated using detailed survey data, characterizations of sediment deposits, flow modeling, and a moment-stability analysis. Limited reworking of revetment boulders indicate that riprap bank material was immobile during a 1979 flood event responsible for the formation of the cutoff channel. A moment-stability analysis factor-of-safety value of 1.1 supports the conclusion that riprap was not directly eroded from the banks. Alluvial particles with d(95) values ranging up to 120 mm were deposited along a bar downstream from the cutoff channel at flows estimated to be below a 1.5-year recurrence interval flow. Development of the bar deposit resulted in locally elevated water surfaces at high flow. The resulting overbank flow across the meander neck to the adjacent downstream bend led to the creation of an upstream migrating knickpoint, the erosion of approximately 16,000-year-old sediments, and the subsequent meander cutoff. The results of the study indicate that traditional erosion-control measures cannot prevent extreme channel adjustments if the geomorphic processes that control sediment continuity also are not considered.

Flood plain and channel dynamics of the Quinault and Queets Rivers, Washington, USA

Comparison of historic channel migration rates, modern planform conditions, and overall sediment, wood, and flow conditions and interactions for the Quinault River and Queets River in the western Olympic Peninsula, Washington, reveals decadal- to century-scale interactions between gravel-bed channels and forested flood plains in temperate maritime environments. The downstream alluvial portions of these two rivers can be divided into three reaches of different slope, flow, sediment, and wood regimes: (i) the upper Quinault River is aggrading behind Lake Quinault, a natural lake that traps most sediment and wood transported from the Olympic Mountain headwaters. (ii) The lower Quinault River, downstream of Lake Quinault, transports only sediment and wood derived from reworking of flood-plain deposits and contributed from valley margins. (iii) The Queets River has unimpeded movement of sediment and water from the mountainous headwaters to the Pacific Ocean. Measurements of channel planform characteristics and historic migration rates and patterns show that these three reaches have correspondingly distinct channel and flood-plain morphologies and dynamics. The aggrading and sediment-rich upper Quinault River has the widest flood plain, widest active channel, greatest number of low-flow channels and flanking gravel bars, and an average channel migration rate of 12.7±3.3 m/year between 1900 and 1994. The comparatively sediment-poor lower Quinault River has the narrowest flood plain, narrowest active channel, and lowest channel migration rate (4.0±1.2 m/year); and most flow is through a single channel with few adjacent gravel bars. The Queets River has attributes intermediate between the lower and upper Quinault Rivers, including an average channel migration rate of 7.5±2.9 m/year. Flood-plain turnover rates are similar for all three reaches, with channels eroding the flood plain at the rate of about 0.2% of the flood-plain area per year, and with corresponding flood-plain half-lives of 300 to 500 years. Observations from this study and previous studies on the Queets River show that channel and flood-plain dynamics and morphology are affected by interactions between flow, sediment, and standing and entrained wood, some of which likely involve time frames similar to 200–500-year flood-plain half-lives. On the upper Quinault River and Queets River, log jams promote bar growth and consequent channel shifting, short-distance avulsions, and meander cutoffs, resulting in mobile and wide active channels. On the lower Quinault River, large portions of the channel are stable and flow within vegetated flood plains. However, locally, channel-spanning log jams have caused channel avulsions within reaches that have been subsequently mobile for several decades. In all three reaches, log jams appear to be areas of conifer germination and growth that may later further influence channel and flood-plain conditions on long time scales by forming flood-plain areas resistant to channel migration and by providing key members of future log jams. Appreciation of these processes and dynamics and associated temporal and spatial scales is necessary to formulate effective long-term approaches to managing fluvial ecosystems in forested environments.

Plant regenerative strategies after a major disturbance: The case of a riverine wetland restoration

In aquatic habitats, both vegetative and sexual reproduction are involved in vegetation maintenance. Restoration projects carried out in such habitats rarely consider the role of both sexual reproduction (seeds) and vegetative (vegetative propagules or clonal fragments) components of propagule banks in vegetation regeneration. Moreover, changes in the seed bank due to the restoration itself have rarely been studied. The aim of this paper is to answer the following questions. Does restoration consisting of sediment removal in a cut-off meander lead to a change in the propagule bank (both sexual and vegetative propagules), and does the vegetation regeneration involve vegetative propagules created by plant breakage during the disturbance? Vegetation was visually surveyed three times during a 15-year period before the restoration and two years afterwards in both the restored zone and an upstream undisturbed zone. The propagule bank was characterized just before and one year after the restoration in the restored zone by sampling of sediment cores and propagule germination in a greenhouse. The impact of restoration on the propagule bank was very pronounced. Sprouting propagules were three times more abundant in post-restoration samples than in prerestoration samples. The ratio of seeds / vegetative propagules in the bank was lower in the year following restoration. Links between propagule bank and established vegetation were assessed through calculation of correlation and similarity indices. We observed a large difference between established vegetation and the propagule bank before restoration, suggesting that recruitment from the propagule bank was low before restoration. Restoration enhanced the recruitment from rhizomes and vegetative fragments, which were highly correlated with the established vegetation. the pre-eminent role of some vegetative propagules in the recolonization and the increasing abundance of an invasive species are related to the season (June) in which restoration was carried out. Impact of restoration on both the propagule bank composition and environmental conditions (especially those promoting or inhibiting germination or sprouting) thus has to be considered in further vegetation management work.

Mediated equilibrium: the influence of riparian vegetation and wood on the long‐term evolution and behaviour of a near‐pristine river

AbstractAnalysis of the contemporary geomorphology, vegetation and in‐channel wood within the relatively pristine Thurra River in southeastern Australia provides insight into river behaviour prior to European disturbance. This sand‐bed river has an extremely low channel capacity with a pronounced pool–riffle morphology. Lateral migration rates are low (11–24 mm a−1), as are floodplain aggradation rates (average = 0·27 mm a−1). Sedimentological evidence is used to place contemporary channel dynamics within a 16 ka evolutionary framework. The floodplain has continuously aggraded over this interval, despite a number of avulsions and numerous meander cutoffs. Avulsions occur on a timeframe of once in 5 ka or more, while cutoffs occur around once in 1 ka. The morphology and evolution of the Thurra River are appraised in terms of a mediated equilibrium condition, in which channel capacity, hydraulics, bedload transport rates, bank erosion rates and in‐channel deposition are substantially influenced by vegetation and wood within the channel and on the floodplain. Copyright © 2002 John Wiley & Sons, Ltd.

The formation of natural levees as a disturbance process significant to the conservation of riverine pastures

Disturbances and patch dynamics are inherent to many ecosystems of the world, especially in the riparian zone. This paper describes the influence of natural levee overbank deposition on riverine grasslands along the meandering River Dinkel (The Netherlands). Here, the rare vegetation type Diantho-Armerietum, characterised by Dianthus deltoides, Thymus pulegioides, Pimpinella saxifraga and Galium verum, has been identified as important to nature conservation. Diantho-Armerietum shows a strong preference for dry, nutrient poor, sandy and relatively young soils, with an elevation approximately 30–50 cm above bankfull discharge level, corresponding to a flooding frequency of 2–3 times per year. The lower zones are strongly influenced by nutrient-rich water, whereas the higher zones are vulnerable to soil acidification. In the intermediate zone, soil development may be reset due to the supply of calcium, adsorbed to recently deposited levee sands. Since deposition rates will decrease with increasing levee heights, new levees are regularly needed to stop the decline of this floriferous vegetation type. The formation of new natural levees is favoured by the occurrence of meander cutoffs, causing a cyclic succession of landforms along the river. Therefore a conservation strategy for this vegetation type needs to aim at the rehabilitation of the natural levee disturbance process, in conjunction with encouraging the meandering of the river.

Stream Erosion and Densities of Etheostoma rubrum (Percidae) and Associated Riffle-Inhabiting Fishes: Biotic Stability in a Variable Habitat

The Bayou Pierre system in western Mississippi is currently experiencing extensive erosion, with pulses of erosion moving from lower to higher stream reaches (e.g., headcutting). This erosion has caused substantial changes to the system, including channel widening and deepening, general loss of downstream riffle habitats, and the creation of new riffle habitats in more upstream locations. We address the impact of the rapid and ongoing geomorphic changes on Etheostoma rubrum [bayou darter; a federally listed species (Threatened) endemic to Bayou Pierre] and associated, riffle-inhabiting species, including Etheostoma lynceum (brighteye darter), Noturus hildebrandi (least madtom), Cyprinella camura (bluntface shiner), and Etheostoma whipplei (redfin darter). We first characterize stream reaches based on geologic and geomorphic data and then estimate population densities of riffle-inhabiting fishes in each geomorphically homogeneous stream reach. To assess trends in relative abundances of all riffle-inhabiting fishes and population sizes of numerically dominant riffle-inhabiting fishes, we compare relative abundances and densities from 1993–1994 with those from 1986–1988. There is an overall trend for recent erosion to occur in the upper reaches of the Bayou Pierre system, with lower reaches characterized by later, recovery, stages. Between 1940 and 1994, the point of active headcutting moved over 7 km upstream at rates of 48–750 m/yr. The relative abundances of E. rubrum and associated riffle-inhabiting fishes have not changed significantly from 1986–1994. Combined densities of all riffle-inhabiting fishes, as well as densities of three of the four most abundant species (E. rubrum, E. lynceum, N. hildebrandi), have also remained temporally stable. Overall, densities of riffle-inhabiting fishes averaged 6.7 ± 0.2/m2. The density of all riffle-inhabiting fishes varied across geomorphic stages, as did the densities of E. rubrum, E. lynceum, and N. hildebrandi, with higher fish numbers in areas of active or recent erosion. This has resulted in a general pattern of greater fish densities in more upstream areas. Ultimate factors responsible for the rapid headcutting are located downstream of currently affected reaches in Bayou Pierre and the Mississippi River. These potentially include natural meander cut-offs, channel avulsion, channelization, and instream gravel mining. Thus, river-management decisions impacting areas spatially distant from the study area appear to have resulted in major local changes.

Evolution of undercut slopes on abandoned incised meanders in the Eastern Highland Rim of Tennessee, USA

Field surveys, location-for-time reasoning, and computer modeling were used to study the evolution of slopes on valley walls of abandoned bedrock meanders on the Eastern Highland Rim, Tennessee. Hillslopes on the undercut slopes of cutoff incised meanders were ordered as to relative age by the height of their meander floors above the modern stream level. The assumption is that the undercut slope is actively eroded by the stream until abandonment of the meander, at which time the slope begins to evolve to a different form. More-advanced stages of evolution occur on walls of higher meanders that were abandoned earlier. The most rapid change in this initial form is the elimination of a free face, which occurs soon after the meander is abandoned. In addition, the hillslopes associated with even the lowest (youngest) cutoff meanders show somewhat gentler overall gradients than the actively undercut slopes. Hillslopes associated with meanders 3 to about 20 m above modern stream level maintain straight segments with angles showing only a slight decrease from the 36–38° associated with the lowest cutoffs; overall angles decrease, however, as the straight segment becomes shorter. The oldest slopes, those on cutoffs 30 m or greater above modern stream level, have developed into convex–concave slopes with maximum slopes of 15°. A hillslope evolution model based on previously published algorithms was used to simulate the transition of actively undercut hillslopes into hillslopes on abandoned meanders. Hillslope modeling is particularly useful in this setting. If the valley incision rate is known, an age can be estimated for the cutoff and hence for the hillslope. Alternatively, if hillslope process rates are known, a model age obtained for the hillslope can be used to estimate an incision rate. Even where both incision rates and hillslope process rates are poorly constrained, as in the present setting, modeling allows assumptions about specific rates to be evaluated by determining their implications for other rates. For example, for three cutoff meanders along one stream, best-fit criteria were used to select process rates for the model. Model ages of hillslopes were then obtained and compared with those calculated from a valley-incision rate measured elsewhere in the same physiographic province. For two of the hillslopes, model ages were found to be much younger than those calculated from the incision rate. In order to make the two ages agree, unreasonably low process rates had to be used in the model, thus implying that the incision rate probably underestimates the actual incision rate in this valley. Experimentation with heights of initial profiles, again using best-fit criteria, suggests that since abandonment of the highest cutoff, the plateau has been downwasting at a rate about one-fourth that of the valley incision rate, a finding in agreement with published rates of chemical denudation in the area.

Vegetation controls on channel stability in the Bell River, Eastern Cape, South Africa

Channel instability has occurred in the Bell River in the form of meander cutoffs, a number of which have occurred since 1952. Increased sediment loading from widespread gully erosion in the catchment has been proposed as the trigger for this instability. Willow species of the Salix family, in particular S. caprea, have been planted along the banks in an effort to prevent further channel shifting. This study reports the results of an investigation into the effect of vegetation on channel form and stability over a 17 km stretch of channel. Results indicate that riparian vegetation has significant effects on channel form which have implications for channel stability. Riparian vegetation increases bank stability and reduces channel cross-sectional area, thereby inducing stability at flows less than bankfull. Evidence indicates that narrow stable stretches are associated with relatively high levels of riparian vegetation. Wider, unstable channels are associated with relatively less riparian vegetation. The effectiveness of riparian vegetation relative to bank sediments was investigated. A dense growth of willows was found to have an equivalent effect to banks with a silt-clay ratio of about 70 per cent. The channel narrowing induced by vegetation may contribute to channel shifting at high flows. The reduced channel capacity is thought to result in more frequent overbank flooding which may ultimately lead to channel avulsion. Thus where increased sediment loading is pushing the channel towards instability, vegetation may be effective in imparting local stability, but it is unable to prevent long-term channel shifts, and may rather help to push the system towards more frequent avulsions. Copyright © 1999 John Wiley & Sons, Ltd.