Elsevier

Geomorphology

Volume 130, Issues 3–4, 15 July 2011, Pages 197-207
Geomorphology

Meander dynamics in a changing river corridor

https://doi.org/10.1016/j.geomorph.2011.03.016Get rights and content

Abstract

In the first decades of the twentieth century, the Ebro River was the Iberian channel with the most active fluvial dynamics and the most remarkable spatial–temporal evolution. Its meandering typology, the dimensions of its floodplain (with an average width > 3.0 km), and the singularities of its flow regime produced a especially interesting set of river functions from the perspective of the fluvial geomorphology of the largest Mediterranean channels.

The largest dynamics of the Ebro River are concentrated along the meandering profile of the central sector. During the twentieth century, this sector experienced a large alteration of its geomorphological structure. We present here an analysis of this evolution through the cartographic study of a long segment of the river (~ 250 km) in 1927, 1956, and 2003. The study is focused on a wide set of geomorphic parameters and indicators that represent the forms of the meander belt, its lateral dynamics, and the overall mobility of the river corridor. The results of the analysis show a large transformation of the meander dynamics, as well as a massive loss of the river lateral activity, most of which occurred in the second half of the twentieth century.

This intense geomorphological transformation becomes visible in (i) the large reduction of the bankfull width and the active channel area; (ii) the decrease in the rate of lateral channel migration; (iii) the loss of channel activity; and (iv) the large reduction of coincidence of the active channel areas. However, the most traditional form parameters (i.e., wavelength, amplitude, radius of curvature, and meander length) do not show significant differences throughout the time interval analysed. The study reinforces the necessity of integrating a wide range of dynamic indicators, which may complement the classical form parameters and represent the real functioning of the river corridor, in the geomorphological analyses of meander dynamics.

This work also shows the most important procedures for the recuperation of the ecomorphological processes of the meander belt. It highlights the most urgent measurement for ecological recovery and illustrates the management scenarios that have led to the present-day situation of the river system. This work further highlights the management scenarios that could be most important for the continued good status of the meander dynamics in this changing river corridor.

Research Highlights

► We analyze the overall meander dynamics of the middle Ebro River. ► The study reach shows a large alteration of the ecomorphological dynamics. ► A static equilibrium has been found in the meander corridor. ► Indicators of channel activity are necessary to characterize the meander dynamics. ► It is most necessary to study, simultaneously, the spatial and temporal dynamics.

Introduction

Until 1970, the research on meandering channels was based primarily on the analysis of the meander morphology and its relationship with primary control factors, such as flow regimes (Schumm, 1960). Based on these studies, we generally assumed that meanders tend to present equilibrium, which was understood as the symmetry of forms in natural conditions. This form should be characterised through standardised parameters.

The rate of channel lateral migration depends on the resistance to erosion of the concave bank (Nanson and Hickin, 1986), the duration and magnitude of flows (Odgaard, 1987), the radius of curvature of the channel (Nanson and Hickin, 1983, Nanson and Hickin, 1986, Odgaard, 1987) and the capacity of the flow to convey sediments (Neill, 1987, Nanson and Hickin, 1986, Po-Hung et al., 2009). Channel migration is a non-continuous process, considering its association to certain hydrologic events (Brice, 1977, Nanson and Hickin, 1983). In earlier stages, channel bends migrate transversely to the main axis of the valley, but later on, bends also advance in the valley direction (Brice, 1977, Knighton, 1984, Leeder and Bridge, 1975, Nanson and Hickin, 1983, Nanson and Hickin, 1986).

Large channel lateral migrations have been documented in some meanders, including the lower reach of the Mississippi River (20 m/y). However, the rates of lateral migration most frequently measured are around 1 m/y or less (Lutgens and Tarbuck, 1995). Meanders also migrate downstream, which enhances large changes in their overall morphology. In most channels, the majority of meander activity occurs in periods when liquid and solid flows and bed erosion are much larger than usual. Because of incision processes, water conveys a higher amount of sediments in the inner bank of the bend. This process must have been known for centuries, since the antique Mediterranean civilisations connected their channels with the exterior convex zones of bends (Edwards and Smith, 2000).

Previous studies distinguish five approaches for the analysis of the evolution of meandering channels. Three of these are referred to as techniques focusing on the meander morphology. They are based on the comparison of the channel over a certain time period. However, the last two approaches focus directly on the changes suffered by the river planform (Hooke, 1984). The five approaches are the following, ordered from simpler to more complex procedures:

  • (i)

    Graphic comparison of channels: direct assessment of changes. It is usually conducted in association with the analysis of the bend parameters. The simplest procedure comprises the visual presentation of superimposed channels or the utilisation of sequences of maps or photographs (Gurnell et al., 1994).

  • (ii)

    Bend parameters: measurement of the specific dimensions of each bend. It usually comprises form-size parameters, such as wavelength, amplitude, meander width or radius of curvature (Hooke, 2007).

  • (iii)

    Bend adjustment: based on the adjustment of meander forms to preknown function. They can be adjusted visually or mathematically, partially or completely (Lagasse et al., 2004). Some visual techniques include the adjustment to circles, measuring their attributes such as length of strings, axes, and radii.

  • (iv)

    Spectral and spatial series analysis: study of the direction or curvature of data generated after the digitalisation of points along a channel. The spectral analysis of a data series is based on the ability of the spectrum power to show the fluctuations and variance of the different frequencies. The form of the spectrum may indicate the character of the meandering scheme (Addink and Kleinhans, 2008).

  • (v)

    Models and change classifications: graphic models capable of showing the overall changes of meander forms (Howard, 1992). The frequency of the different types may be measured, and models may be used to classify the bends afterward. This is, however, a very subjective procedure.

An exact prediction or knowledge of the changes suffered by a channel is not possible considering the wide range of environmental and historical processes in the basin that contribute to its evolution. Diverse works have indicated that traditional hydraulic focuses (such as regime theory or other geometric geomorphic approaches) are inadequate for the prediction of the effects of environmental changes in river systems. Questions such as the working scale, the magnitude of changes, or the spatial distribution and propagation indicate that the response of river systems is not (or at least is not necessarily) dependent on extrinsic factors. On the contrary, it depends on the system configuration as a result of its geomorphic history, thus showing a lack of linearity in the response of river systems to the modification of extrinsic conditions (Downs and Gregory, 2004).

The data required for the utilisation of any of the aforementioned techniques may be obtained from the analysis of the river planform in different time periods (Downward et al., 1994). This information could be derived from historical maps (e.g., Hooke, 1977); aerial photographs or other historical documents (Hooke and Kain, 1982); empiric proofs of the dimensions of meanders, vegetation, or palaeochannels (e.g., Hickin, 1974, Hickin and Nanson, 1984); or direct observations (e.g., Hooke, 1980).

The application of aerial photographs in the assessment of channel and floodplain changes developed quickly in the 1970s (Lewin and Manton, 1975, Lewin and Wier, 1977). However, the first widespread application of photogrammetry to the analysis of a river system took place in the 1940s on the Mississippi River. Fisk (1944) used maps, aerial photographs and field surveys to explain the changes suffered by the lower reach of that river. Later, the same author would examine the effects of the alluvial deposits of fine materials on the current mobility of the river channel.

These works quantified the geomorphic changes on an intermediate scale (hundreds of metres). However, they did not reach the level of spatial accuracy required for the quantification of small-scale changes (tens of metres), which is typical in the local modifications of geomorphic parameters. Burkham (1972) made use of field surveys, maps, and photographs to study the morphology modification of the Gila River (Arizona, USA), whereas Ruhe (1975) used maps from the period 1852–1970 and aerial photographs from 1925 to 1966 to document the changes of a specific meander of the Missouri River. Williams (1978) used photographs of the Platte River in Nebraska (USA) to evaluate the channel width decrease caused by river regulations. Hooke, 1984, Hooke, 1995 used historical maps and aerial photographs to survey the lateral mobility of river channels in Devon (England) over a 50-year period. In more recent times, the improvement of vertical and oblique aerial photography has increased the ability to more accurately analyse the minor changes (Lane et al., 1994, Lane et al., 1998, Barker et al., 1997, Butler et al., 1998).

In other relevant work, Brice (1977) developed a system of classification of alluvial channels through the analysis of the planform attributes of 200 river reaches in topographic maps and aerial photographs. On the basis of these data, he correlated different aspects of river behaviour with river typology, such as lateral erosion or erosive depth. Afterwards, he evaluated meander migration and river stability based on a comparative assessment of aerial photographs, maps, and field surveys in the channels (Brice, 1982). Also WET (1990) used historical maps and aerial photographs to conduct a geomorphic analysis of more than 100 miles of the Sacramento River in California. Previous works have also developed meander evolution models to identify critical sections where river defences should be constructed or where a higher probability for channel cutoffs could be expected.

The evolution and functioning of river channels has also been analysed, in the last decade, from an ecomorphologic focus. This view can be applied at a variety of ecologically relevant scales and consists of an investigation of how geomorphology provides a structural template that shapes, and is shaped by ecological processes. This functional ecomorphology approach has been used by Fisher et al., 2007, Corenblit et al., 2008. Other authors, such as Murray et al. (2008) use the term “biomorphodynamics” to characterise a subset of ecogeomorphologic studies that investigate not only the effects of organisms on physical processes and morphology but also how the biological processes depend on morphology and physical forcing.

This paper attempts to complement all these contributions in the analysis of the following questions:

  • (i)

    What was the spatial and temporal evolution of the main geomorphic parameters of the Ebro River in its central free-meandering reach (250 km) between 1927 and 2003?

  • (ii)

    Which parameters best describe the ecomorphological dynamics of a meandering channel that suffers high human pressure?

  • (iii)

    What is the relative behaviour of the morphological evolution in the most and least dynamic subreaches in the study area?

  • (iv)

    What ecological implications can be derived from the morphological changes of the Ebro system, and what are the consequent recommendations for its integral restoration?

Section snippets

Study area

The river area analysed in this paper comprises the floodplain of the free meandering reach of the Ebro River (NE Spain), between Rincón de Soto (La Rioja) and the small dam of Alforque (La Zaida, Zaragoza). This reach includes the majority of the active meandering area of the channel. Upstream of the upper end, a short meandering subreach may still be found. However, no historical cartographic materials are available for it, and it was therefore not used in the analysis. The chosen reach is

Results

The measurement of the eight parameters introduced resulted in the values shown in Table 2. Consistent with the average value for any parameter at the different times recorded, additional statistical indicators (the standard deviation and the variation coefficient) are included for the wavelength, amplitude, and radius of curvature to make later interpretation easier.

The values of the form parameters (wavelength, amplitude, and radius of curvature) show minor differences in the three records.

Discussion

In the last eight decades, the central sector of the Ebro River has experienced a remarkable evolution of its geomorphological functioning. In this study, the results of spatial and temporal analysis of the morphological parameters and indicators allow for a detailed discussion of the main items involved in this evolution.

The parameters that are frequently measured in analogous studies (wavelength—P1, amplitude—P2, and radius of curvature—P3) changed slightly during the time interval evaluated

Conclusions

  • i.

    The central sector of the Ebro River has suffered, during the last eight decades, a progressive geomorphological transformation. In the first half of the 20th century, it showed active ecomorphological dynamics. The river system has been impacted by human activities for centuries. This conditioned the river evolution in certain subreaches, but it did not impede the overall mobility of the river system. The channel showed a remarkable meandering behaviour characterised by a large active area,

Acknowledgements

The authors acknowledge the Confederación Hidrográfica del Ebro and CEDEX for access to the cartographic data. Violeta Roch helped with graphics and layouts. Roberto Martínez at CEDEX and Alfredo Ollero at the University of Zaragoza undertook a first review of the manuscript.

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