Fluvial geomorphological analysis of the recruitment of large woody debris in the Yalobusha River network, Central Mississippi, USA
Introduction
Large woody debris (LWD) resulting from tree-fall into rivers is a natural occurrence in wooded river systems. LWD can impact the hydrology and hydraulics of flows, the transport and storage of sediments, solutes and other organic matter, and the spacing and variance of fluvial geomorphology features (Gurnell and Sweet, 1998). This normally leads to far greater physical habitat diversity in river reaches with LWD rather than without. Conversely, river managers concerned with flood defence often view accumulated LWD as an obstruction to the passage of flood flows. These obstructions form generally at channel constrictions, such as under bridges, or in shallow channel sections where flow is divergent (Diehl, 1997) and may cause localised flooding and erosion where flow is deflected towards channel banks. LWD may even contribute to bridge failures by causing deflection of flows towards piers and abutments. Striking a balance between the environmental benefit of LWD and its possible economic consequences is a testing objective for contemporary river management. An analytical assessment of this issue is critically dependent on the rate of LWD recruitment into the river system relative to the rate of tree removal. Ideally, this understanding would result in a LWD ‘budget’ (Keller and Tally, 1979) that has parallels in geomorphology with a sediment budget.
Unless trees are removed by management action to de-snag channels, the removal rate of in-channel LWD will be some function of the combined rate of wood decay and the occurrence of large floods sufficient to float the trunks. Assuming climatic stationarity, the rate of removal may be imagined as a constant over the long-term, leaving the LWD ‘budget’ dependent primarily on the rate of tree input, or recruitment, into the river system. Therefore, for channels with a wooded riparian zone but not subject to commercial forestry operations, the recruitment rate is related to the geomorphological processes in the river channel, to mass slope-failure delivering trees directly to headwater channels and to other factors. Developing this theme, three typical recruitment scenarios can be envisaged.
(1) Rivers where the channel morphology is essentially static and LWD enters the channel as a function of dead trees toppling into the channel, of wind-thrown trees downed in storm events and trees contributed by fires, floods, landslides, ice storms and beaver activity, depending on the environment.
(2) Dynamically-stable or unstable meandering or braided rivers shifting across their floodplain in which recruitment is a function of the rate of outer bank migration (meandering rivers) or the more general rate of lateral erosion (braided rivers) in addition to the functions named in (1).
(3) Dynamically-unstable rivers where the channel width is increasing either according to progressive alterations in the hydrological regime or due to rapid base-level change that destabilises the banks, in addition to the functions named in (1).
The first scenario is largely a stochastic process of recruitment in natural or semi-natural rivers. The second scenario may also occur in response to natural channel mobility but could relate to cases where the rate of channel adjustment has been accelerated by upstream flow alterations caused by urban development or channelisation (Brookes, 1987a). The third scenario, of channel widening or cross-sectional enlargement, may occur in response to natural climatic variations or may be due to intrinsic material properties of a complex floodplain stratigraphy that provides the context for channel change Brown, 1995, Brown, 1996. Alternatively, it may result from changes in hydrology or sediment transport downstream of human activity such as urbanisation (e.g. Neller, 1989, Roberts, 1989, Gregory et al., 1992) or channelisation (Brookes, 1987b). However, the most dramatic example of the third scenario is likely upstream of straightened channels. Here, the increased channel gradient in the straightened reach creates upstream-migrating knickpoints (a ‘knickzone’) that cause rapid base-level lowering and consequent channel widening Parker and Andres, 1976, Simon, 1994, Simon and Hupp, 1986. Rapid base-level lowering may also occur downstream of river impoundments Petts, 1984, Williams and Wolman, 1984. In straightened channels, the degree of channel incision varies according to the magnitude and distance from the imposed disturbance (Simon, 1994) and the character of the bed material, leading to models of straightened channel evolution that predict a sequence of accelerated bed and bank erosion rates Schumm et al., 1984, Simon, 1989. Accelerated channel erosion leads to accelerated LWD recruitment rates, thus shifting the balance between LWD recruitment and removal in the river system. Potentially, this LWD build-up may be unacceptable from a flood management standpoint even though morphologically, LWD-associated sediment storage may be critical in stabilising the channel Keller and MacDonald, 1995, Wallerstein, 1999.
Where rapid base level lowering exists, LWD management should be based on a quantitative assessment of the severity of the problem. This requires an understanding of the degree to which recruitment rates have been accelerated and knowledge of locations that are critical from a management perspective. There are however, few quantitative assessments of LWD recruitment (Lassettre, 1999) because of the complexity involved in combining the inter-related dynamics of fluvial geomorphology (particularly bank instability) and riparian vegetation. The assessments that do exist are based on the character of riparian vegetation and morphology and a probabilistic function of tree fall (e.g. Robison and Beschta, 1990a, Van Sickle and Gregory, 1990, Hairston-Strang and Adams, 1998). Fluvial geomorphological processes are not considered. This can be a serious omission, especially for incising rivers in which recruitment rate is primarily a function of the channel width increase caused by mass wasting of the banks. In this regard, we advance a method for predicting accelerated LWD input deterministically as the basis for judging the severity of the LWD management ‘problem’, at critical locations. The method requires data about channel morphology, the rate of knickpoint migration and characteristics of the riparian vegetation in conjunction with recent advances in bank instability modelling (e.g. Simon et al., 1999) to allow channel width increases to be predicted. The current study is applied to the Yalobusha River catchment, Central Mississippi, USA, and focuses on reaches that are the most critical from a LWD management perspective. An example management application is provided whereby the calculated LWD recruitment for each reach is used to predict the potential significance of the LWD recruitment in terms of debris dam formation. From this basis, rational management decisions may be taken. The overall accuracy of the model is largely dependent on three factors, namely: bank-stability analysis, knickpoint dimensions and migration rate, and characterisation of the riparian tree stand.
Section snippets
Channel evolution in the study area
A large number of rivers in the midwestern United States are subject to substantial changes in morphology following the migration of knickpoints promoted by channel straightening for flood defence and land drainage during the early 1900s, and again in the 1950s and 1960s Speer et al., 1965, Simon, 1994, Simon and Rinaldi, 2000. In certain areas, such as those typified by highly erodible loess silts of the ‘bluffline streams’ of Mississippi, adjustment processes have been intense, with
Method and data
A comprehensive geomorphological evaluation (Simon, 1998) has demonstrated that the processes of erosion in the Yalobusha River System conform well to a model of post-straightening channel-evolutionary adjustment noted previously by several authors (e.g. Schumm et al., 1984, Simon, 1989). The Simon model is illustrated in Fig. 4.
Within the channel evolution sequence, accelerated LWD recruitment is achieved during the period of greatest bank erosion as riparian areas succumb to channel widening.
Analysis—shear strength and channel-bank stability
Accurately estimating LWD recruitment in incising channels is determined largely by the accuracy in estimating the extent of channel widening that occurs after the banks are destabilised by the retreating knickpoint. Widening occurs until the bank reaches a stable angle and is a function of the shear strength of the bank material relative to the height of the knickpoint. Therefore, there is no simple empirical relation of channel widening to discharge or drainage area, and the river length
Results
Having collected the necessary input data and performed the bank stability analysis, the estimation of LWD recruitment at each site is performed through a sequence of calculations as to
- 1.
identify channel dimensions at critical locations upstream of major knickpoints and add knickpoint/knickzone height to existing bank heights;
- 2.
apply bank-stability analysis to determine the stable angle under future conditions;
- 3.
estimate future top width of the channel at the critical locations;
- 4.
calculate remaining
Application—LWD recruitment and significance for debris dam formation
In the Yalobusha River catchment, the obvious final resting place for LWD that does not decay during its passage along the river network is the major debris dam at the transition from the channelised to non-channelised section downstream of the Topashaw Creek confluence (see Fig. 1, Fig. 3). According to our analyses, the ‘accelerated’ rate of LWD recruitment is approximately 28 m3 of wood per year, comprising about 100 trees (22 ‘large’ trees) from the eleven major knickpoints. Assuming this
Conclusions
This paper examines LWD recruitment and significance from a fluvial geomorphology perspective. Unlike analysis of LWD in non-incising streams where recruitment is driven primarily by tree death, wind-throw and/or channel shifting and must be examined stochastically (or retrospectively, Piégay et al., 1999), short-term LWD recruitment in incising channels can be examined deterministically in terms of riparian vegetation characteristics and bank stability. The significance of the recruited LWD in
Acknowledgements
We would like to acknowledge Geoff Waite and Mark Griffith for fieldwork assistance, Nick Wallerstein for reviewing an earlier version of this paper and Elaine Watts of the Cartographic Unit, School of Geography, University of Nottingham for drafting some of the diagrams. Thanks also to the anonymous referees for their constructive review of the submitted manuscript. This research was funded by the US Army Corps of Engineers, Vicksberg District, but the views expressed are those of the authors
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