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Hydro-ecolog}': Linking and Agnatic Ecology* (Proceedings of Workshop MW2 held at Birmingham, UK. July 1999). IAHS Publ. no. 266, 2001. 113

In- metamorphosis in a semiarid, mixed bedrock/alluvial system: implications for Instream Flow Requirements

M. W. ROUNTREE Centre for Water in the Environment, Department of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg 2050, South Africa e-mail: markr@,gecko.biol.wits.ae.za

G. L. HERITAGE Telford Institute of Environmental Systems, Department of Geography, University of Sal ford, Manchester M5 4WT, UK

K. H. ROGERS Centre for Water in the Environment, Depart ment of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg 2050, South Africa

Abstract Semiarid river systems have been shown to experience phases of interspersed with related stripping. In 1996 a high magnitude flood in the Olifants River, South Africa, snipped most of the vegetation and sediment from the channel, exposing extensive areas of the underlying bedrock template. A 50-year aerial photographic record of the stripping and accumulation phases was examined to elucidate pathways of river landscape change in the 100 km study reach. Thirteen channel type states were identified from a study of the 57 channel segments. The pattern of change between the various states was used to develop a conceptual model of in-channel metamorphosis. Preferential pathways of change suggest that the underlying bedrock template plays an important role in predetermining the pathway of change. However, the multiple pathways of change identified show that this template does not exclusively control the change pathway. The mixed (alluvium and bedrock) anastomosing channel type was shown to have a highly stable planform, but in most in-channel metamorphosis appears to be the rule rather than the exception. This is particularly apparent in the mixed pool-rapid and mixed braided channel types where switching between these two states was common and frequent. These changes have implications for the selection of sites for Instream Flow Requirement determination and long-tenn monitoring sites. The desired future states for the management of South African do not explicitly allow for the inherent channel changes identified in this study. Therefore the definition of desirable geomorphological states and change need to be reassessed.

Key words bedrock influence; geomorphic change; instream flow requirements; semiarid river

INTRODUCTION

In arid and semiarid areas the variation in between rare and more frequent floods (such as the 2-year event) is much higher than in temperate climates and change in seldom conforms to the Wolman & Miller (1960) 114 M. W. Rountree et al. uniformitarian principle (Baker, 1977). Instead, episodic stripping of has been used to explain fluvial changes in several semiarid rivers of Australia, India and North America (Womack & Schumm, 1977; Baker, 1977; Nanson, 1986, Kochel, 1988; Gupta et al, 1999; Bourke & Pickup, 1999). Episodic stripping occurs when gradual aggrada­ tion () is set back by extreme stripping (erosional) events (Nanson, 1986). Episodic stripping is believed to occur also in the mixed bedrock/alluvial influenced semiarid river systems of the Kruger National Park, South Africa. In 1996 extreme high magnitude floods occurred throughout the region and exposed the bedrock template underlying the various channel types of the Olifants River. The floods left the river in the most bedrock-influenced state on record. Under the episodic stripping scenario, extreme floods would periodically scour the river to a condition similar to that of 1996 (i.e. down to the bedrock template). Previously studies of episodically stripped rivers have tended to focus on changes resulting from the stripping event. The prolonged accumulation periods between stripping events have seldom been considered or explicitly studied. Since channel changes during the prolonged accumulation periods are not well documented, we know very little about the long-term stability of channel types or planforms. This information

® Witbank

Cape Town'

Fig. 1 Location of the study area. In-channel metamorphosis in a semiarid, mixed bedrock/alluvial river system 115 is crucial for the determination of the Instream Flow Requirement (IFR) of rivers, and for monitoring the efficacy of the recommended IFR. In South Africa the Instream Flow Requirement is determined using the Building Block Methodology (King & Louw, 1998). To determine the IFR, cross-sections of the channel are needed to estimate the discharges required to inundate different . These selected sites would also be monitored later to validate and monitor the delivery of the IFR. Sites that offer stable cross-sectional profiles, with a range of habitats, should therefore be chosen. In mixed bedrock/alluvial river systems, this is often the pool-rapid channel type because it presents the bedrock as a control feature and intuition would suggest that bedrock sections are less likely to change than alluvial sections in the same river. The pool-rapid channel type would therefore provide good future monitoring sites, and the stage-discharge relationship is relatively easier to model in this channel type. This study was initiated to determine medium- to long-term channel type metamoiphosis in the Olifants River, a semiarid, mixed bedrock/alluvial river system, and assess the implications of the results for IFR site selection and river monitoring.

STUDY SITE

The Olifants is one of South Africa's largest rivers—740 1cm long with a catchment of 54 800 km2 (Carter & Rogers, 1995). The section of the river examined in this study is in the lower reaches as it runs through the Kruger National Park (KNP) (Fig. 1). There have been no previous studies on the geomorphology of this section, but considerable geomorphological research has been conducted on a morphologically similar section of the nearby Sabie River (Heritage et al, 1999). Both rivers have highly variable flow regimes and channel-in-channel physiographies with the inner channel carrying flow most of the time (Fig. 2). The larger "macro-channel" acts as a restrictive flood plain, outside of which floods have very infrequent and limited influence. In addition to the variable flow regimes and relatively high sediment loads, these rivers are also characterized by a high degree of bedrock influence. The macro-channel floor has generally eroded down to the bedrock template, although the bedrock can be concealed by deep alluvial deposits. The variable underlying geology results in rapid changes in slope and associated sediment deposition patterns. A geomorphological classification developed for the Kruger National Park rivers (van Niekerk et al, 1995) identified four basic channel types: single-thread, braided, pool-rapid and anastomosing. These four basic types can be further classified according to the proportion of bedrock and alluvial influence in the channel segment. Therefore channel types may be either fully alluvial, of mixed influence or fully bedrock controlled channels with very little alluvial influence. In the Sabie River four dominant channel types were found (Heritage et al, 1996): alluvial braided, mixed pool-rapid, mixed anastomosing and bedrock anastomosing (Table 1).

METHODS

Aerial photographs from 1944, 1965, 1974, 1986 and (post-flood) 1996 were used to analyse geomorphological change at the channel type scale. The 1996 aerial 116 M. W. Rottntree et al.

Anastomosing channel type

Macro-channel

Active channels /l L \

Single thread channel type

Macro-channel —— Active channel I

Braided channel type

_ . <| Macro-channel -

\ Active channels

WÊÊÈlMmÊmMlmËÊÈÊÉlÊËËÈlF. •••

j ! Bedrock /ium

Fig. 2 The macro-channel and active channels which flow within it.

photographs were used as the initial condition of the river. The accumulation phase is thus represented by the 1996/44/65/74/86 aerial photograph sequence (Fig. 3), showing the development of more alluvial channel types upon an initially highly bedrock influenced template.

Table 1 Characteristics of the four dominant channel types in the Sabie River, Kruger National Park (from Heritage et al, 1999).

Channel type Geomorphological characteristics Average low flow water (after Heritage et al., 1999) surface slope (after Birkheade/a/.,2000) Alluvial Confined braiding within the macro-channel. No bedrock 0.0020 braided influence due to very deep alluvial deposits. Narrow macro- channel. Mixed pool- A series of deep, mixed bedrock and alluvial influenced pools 0.0029 rapid separated by usually free of sediment. Narrow macro- channel. Bedrock Multiple channels in the macro-channel flowing over resis­ 0.0089 anastomosing tant bedrock. These steep segments were expected to inhibit sediment deposition and make this channel type highly stable. Macro-channel typically 3-4 times the average width. Mixed Multiple bedrock, mixed and alluvial channels in the macro- 0.0023 anastomosing channel separated by long, wide, largely alluvial bars colonized by reeds (Phragmites mauritanus). Macro-channel the widest in this channel type. In-channel metamorphosis in a semiarid, mixed bedrock/alluvial river system 117

Time • Fig. 3 Dates of aerial photographs represent the condition of the Olifants River during an accumulation phase and a stripping event.

The channel type classification system developed for the Sabie River (van Niekerk et al., 1995) was applied to the Olifants River. The delimitation and classification of the channel segments (channel type units) were determined independently by two of the authors for each set of aerial photographs. The independent classifications were correlated revealing 57 channel segments in the 100 km length of the KNP section of the Olifants River. The results from the five sets of aerial photographs were then used to examine channel state changes during the accumulation phase.

RESULTS AND DISCUSSION

Nine channel types and four transitional channel type states (transitional between two channel types) were identified in the KNP section of the Olifants River. The transitional states could not be reliably classified as a specific channel type because of the unusual mix and proportion of their morphological units, upon which the channel type classifi­ cation is based. Four channel types—the mixed braided, mixed pool-rapid, mixed anastomosing and bedrock anastomosing state—were dominant (representing 42 of the 57 segments in 1996). This paper focuses on the results from these four channel types.

Mixed braided

The mixed braided state is an alluvial dominated channel type similar to the alluvial braided state described for the Sabie River, but has minor exposures of the underlying bedrock template. There were two dominant patterns of change during the hypothesized accumulation period. The first was from the mixed bedrock/alluvial toward the fully alluvial braided state (Fig. 4). The initial condition (1996) represented the scoured state of these segments. During the accumulation phase, progressive 118 M. W. Rountree et al

Channel Types

Bedrock influence < • Alluvial influence

BA BST MA BA/PR MA/BR MA/SI BI'R MPR MPR/BR MBR ABR AST MST

Fig. 4 Classification changes through the accumulation phase of the 14 segments initially classified as mixed braided. Each single thin line represents a single segment. The channel type states are: BA bedrock anastomosing BST bedrock single thread MA mixed anastomosing BPR bedrock pool-rapid MPR mixed pool-rapid MBR mixed braided ABR alluvial braided AST alluvial single thread MST mixed single thread MPR/BR transitional MPR/braided BA/PR transitional BA/pool-rapid MA/BR transitional MA/braided MA/ST transitional MA/single thread sedimentation resulted in a total loss of the bedrock influence and the development of the alluvial braided state. The second pattern of change observed was a strong trend towards the mixed anastomosing state, with the development of stable reed covered bars. The assumption that the initial (1996) condition of the river represents the most scoured condition of the river is not true in some isolated segments. Womack & Schumm (1977) showed that whilst episodic may be the normal course of events in some rivers, it was difficult to identify periods of cutting which had affected the entire river at one time, possibly due to local gradient or influences. In the Olifants River, large volumes of sediment deposited during the flood in former mixed anastomosing segments resulted in the development of highly active braiding segments, thus accounting for the observed trend.

Mixed pool-rapid

Frequent switching between mixed pool-rapid and more alluvial braided states (Fig. 5) was the dominant partem of change in this channel type. These changes did not occur in phase—between 1944 and 1965 change to both more and less alluviated states occurred. The frequent state changes suggest that the mixed pool-rapid channel type is inherently unstable, responding to very small local changes in the hydraulic and sediment regime of the river. In-channel metamorphosis in a semiarid, mixed bedrock/alluvial river system 119

Bedrock anastomosing

Most bedrock anastomosing segments changed to the more alluviated mixed anastomosing state as the accumulation phase progressed (Fig. 6). Changes to highly alluviated channel states, resulting in a total loss of bedrock influence, also occurred. Despite the steep gradients and high energy slopes, this channel type was not resistant to alluviation during the accumulation phase.

Channel Types

Bedrock influence < • Alluvial influence

BA BST MA BA/PR MA/BR MA/ST BPR M PR MPR/BR MBR A BR AST MST

Fig. 5 Classification changes through the accumulation phase of the eight segments initially classified as mixed pool-rapid. Each single line represents a single segment. Refer to Fig. 4 for the channel type abbreviations.

Fig. 6 Classification changes through the accumulation phase of the 11 segments initially classified as bedrock anastomosing. Each thin line represents a single segment. Refer to Fig. 4 for the list of channel types and abbreviations. 120 M. W. Rountree et al.

Mixed anastomosing

Only one of the seven segments classified at the beginning of the accumulation period as mixed anastomosing changed, and then only to a transitional mixed anastomosing/ braided state before changing back to its original state (Fig. 7). The mixed anastomosing channel type was therefore the most stable channel type during the accumulation phase. This was partly due to its unique character, and partly to the manner in which "change" has been assessed. Field evidence strongly suggests that bars in mixed anastomosing channel type segments grow by vertical accretion. Flood plains formed in an episodic or cyclical process of vertical accretion and catastrophic erosion have been described before (Schumm & Litchy, 1963; Burkham, 1972; Nanson, 1986). Vertical accretion would be aided by the extensive reed growth on the anastomosing bars in this channel type, since reeds promote sediment deposition (Tsujimoto et al., 1996). Alluvium storage changes can only be inferred from area changes when using aerial photographs, hence increased sediment storage by vertical accretion cannot be detected. The confinement of low and medium flows to the numerous would maintain these channels and the exposed bedrock within them. During extreme high flow events, the stage of the flood would increase above the capacity of the channels, spreading out and over the extensive reed beds of the wide macro-channel. With the flow depth reduced and roughness greatly increased by the reeds, sediment deposition would result in the vertical accretion of the anastomosing bars. Thus despite their stable planform, sediment storage is also increasing in this channel type during the accumulation phase. Not all segments of the mixed anastomosing channel type began the accumulation phase in the mixed anastomosing state. After the stripping event some segments were in the mixed braided state as a result of large sediment deposits in these segments. It would therefore appear that channel types are not totally constrained by their underlying bedrock templates.

Channel Types

Bedrock influence Alluvial influence

BA BST MA BA/PR MA/BR MA/ST BPR M PR MPR/BR M BR ABR AST MST

o o 0 O o o o o o o © nitial ( 1 ')%) i 1944 o o 0 O G o o © o o o o

1965 o o o j>o 0 o 0 O 0 o o o

1974 0 o / o 0 o o 0 O 0 0 0 o

1986 o 0 • 0 o o o o o o o o o

Fig. 7 Classification changes through the accumulation phase of the seven segments initially classified as mixed anastomosing. Each thin line represents a single segment. Refer to Fig. 4 for the list of channel types and abbreviations. In-channel metamorphosis in a semiarid, mixed bedrock/alluvial river system 121

Continuum of channel types

Specific bedrock channel type templates were exposed by the stripping event but exami­ nation of the patterns of change during the accumulation phase showed that channel type metamorphosis was not exclusively predetermined by this underlying template. The mixed braided state developed in segments that were initially mixed pool-rapid, mixed anastomosing or even bedrock anastomosing. Similarly, the mixed anastomosing state developed in segments that were initially mixed braided or mixed pool-rapid, although most developed from bedrock anastomosing states. Thus, although some pathways of change are more probable, any channel state can potentially develop from any other, depending on local hydraulic and sediment delivery controls.

< Gradient • Fig. 8 Multiple pathways of change identified during the analysis of high probability transitions between channel type states. See Fig. 4 for the list of channel types and their abbreviations.

A conceptual model of in-channel metamorphosis (Fig. 8) has been constructed showing the likely changes between channel states using the high probability changes observed on the Olifants River and the morphological characteristics and slope data of the channel types found on the Sabie River (Birkhead et ai, 2000). During a stripping event channel states rapidly change to steeper, more bedrock influenced states as sediment is scoured out and the underlying bedrock exposed. Sedimentation during the prolonged accumulation period results in channel states gradually changing towards the more alluvial channel types as sediment is deposited on bedrock features.

IMPLICATIONS FOR IFR DETERMINATION AND RIVER MONITORING IN SEMIARID RIVERS

Much of the classic unifonnitarian theory has become entrenched in both the conceptualization and analysis of river geomorphological change. Therefore the 122 M. W. Rountree et al. recognition of a continuum of channel types, where there is continuous grading between different states, as opposed to discrete channel types, will require a fundamental shift in thinking and monitoring of semiarid river change. Insneam Flow Requirements in South Africa are determined using the Building Block Methodology (King & Louw, 1998). The Building Block Methodology (BBM) determines the timing and magnitude of flows required to "aid maintenance of the natural channel structure" and maintain the river "in some pre-defined desired state" (King & Louw, 1998 page 112). Sensitive areas likely to change with future flow manipulation are identified as part of this methodology (Rowntree & Wadeson, 1998). In a field where managers seldom have geomorphological expertise, these statements could be taken to imply that channel change is inherently undesirable and this could entrench the common assumption (Ferguson, 1987) that an equilibrium state exists. This study has shown that no such equilibrium state exists. The pool-rapid channel type, frequently chosen as study sites in IFR determinations, is highly susceptible to rapid, frequent state changes. The frequent alternations between the mixed pool-rapid and mixed braided states, with state "switching" occurring in 10 years or less, suggest that there is no major threshold change between these two channel types. Similar changes were recorded between single thread and braided channel types in the Sabie River (Heritage et al, 1999). The recognition of these highly dynamic channel complexes as opposed to two separate channel types has important implications for river management. Frequent state changes may not be catered for in the desired state of the river if an equilibrium philosophy has been adopted and monitoring might thus perceive changes in the river morphology as undesirable. The switching pool-rapid/braided state would also have implications for the flow determination. Under the scenario, two very different IFRs would be developed for the same segment depending on whether it was in a braided or pool-rapid state. A case in point is the Sabie River IFR, where three different IFRs resulted from the simultaneous but independent analyses of three different channel types (Tharme, 1997). A study of a flood-driven system in India (Gupta et al, 1999) revealed similar results. At the scale of the alluvial and bedrock channel types, specific flow volumes affected different channel types. We could thus expect particular channel types to respond to particular flows, especially in semiarid areas where the flow variability and channel type diversity are both high. This study demonstrates that channel metamorphosis could be the rule rather than the exception in semiarid rivers. The results also suggest that channel type complexes may be more appropriate than the current concept of distinctive channel types in the classification, monitoring and management of semiarid rivers. The BBM process is open to subjective bias because of the reliance on professional judgement and specialist experience of the river system (Tharme, 1996). Therefore greater understanding of spatial and temporal channel type change in rivers with highly variable discharges is urgently required if management of such rivers is to be successful.

Acknowledgements The authors wish to thank the South African National Parks Board personnel for logistical support, and in particular Holger Eckhardt for assistance In-channel metamorphosis in a semiarid, mixed bedrock/alluvial river system 123 with aerial photograph acquisitions. Financial support from the Andrew Mellon Foundation, who provided the funding for this research and a grant to Mark Rountree to study in the UK for four months, is gratefully acknowledged. Comments of earlier drafts from Martin Thorns and Chris James were extremely helpful.

REFERENCES

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