Many rivers in the world have undergone human-induced changes, often negatively impacting freshwater ecosystems. |
Expanding human populations and consequent land-use change places freshwater ecosystems under increasing pressure due to human demands for water.
In South Africa, current and future development needs will place even more pressure on scarce water resources.
The ReserveEncouragingly, the South African national water policy (Act No 36 of 1998) emphasises equity and the sustainable use/protection of water resources (including rivers, wetlands and underground water), encompassing a far-reaching vision, with the core concept: ‘Some, for all, for ever, together’. Notably, the water act stipulates only one right to water, that being the ‘reserve’.
The reserve has two components, namely: water for basic human needs, and the ecological reserve – a certain quantity and quality of water flow to maintain and sustain freshwater ecosystems. Ideally, after water needs of the reserve have been met, only then will other water allocations (or re-allocations be considered, for uses such as agriculture, mining and industry. However, the ecological reserve concept is often misinterpreted as protection of ‘fish in the river’ over development needs.Additionally, perceptions of water use comprise mostly ‘out-of-river’ uses where water is extracted away from the river, e.g. for mining and irrigation purposes.
Although less obvious, freshwater ecosystems do also provide essential ‘in-river’ goods and services, including fuel wood production, grazing, food, recreation and tourism, flood control, wetlands that act as natural filters of pollutants - but all this relies on healthy functioning ecosystems.
It is these benefits to society, and the associated need to protect entire freshwater ecosystems, that makes the ecological reserve concept an important component of the national water act.
What Affects Kruger's Rivers?Five major river systems traverse the Kruger National Park, including: the Luvuvhu River in the north, the Crocodile River forming the southern boundary, and the Letaba, Olifants and Sabie rivers in-between. Although South Africa’s water policy is highly progressive, encompassing issues of development, sustainability and freshwater protection, these rivers continue to be negatively impacted, with further degradation occurring.
There are two major contributing factors. Firstly, the geographic location of the Kruger National Park, running in a north-south direction, contrasts with the west-east orientation of the major river systems.
Consequently, the head-waters (where rainfall is captured in the catchment) of the rivers are located some 100 km or more outside, upstream of the park to the west. Thus, before entering the park, rivers cross areas not under active conservation, comprising many different land-use types.Impacts along the way include: more silt and other sediment eroding into the river from cultivated lands, reduced river flows through water abstraction, flow regulation via dams, and pollution. Second, although the department of water affairs and forestry (Dwaf) is working determinedly, to date implementation of the national water policy has been slow.
Institutional and logistical reasons are factors, but also a general lack of compliance with the water act. This is of concern to Kruger’s managers because they do not have direct control over the above mentioned impacts. Ongoing declines in river flows and water quality, with loss of river biodiversity, is a major problem.
The Ecological Importance of Krugers RiversBesides providing critical habitat/water sources for animals during the drier winter season, and a favourable tourist attraction, the river ecosystems of Kruger are, in their natural state, highly bio-diverse corridors within the surrounding landscape. The flow pattern has a lot to do with this, by distributing essential moisture through different parts of the area between the tops of the river banks (the macro-channel) over time.
The flow pattern includes moving and depositing various amounts of sediment over a complex range of differing rock types. Consider how the river landscape of some of Kruger’s rivers altered after the 2000 floods.
The semi-arid climate of the region, with sporadic rainfall events mainly during summer months, generates a highly variable flow pattern, both within and between years.
Consequently, flooding events are highly unpredictable, e.g. the one in one hundred year flood of February 2000 could reoccur anytime soon (although unlikely). It is this variable flooding pattern that creates, modifies and affects a variety of habitat features along the rivers, generating much habitat variability for plants and animals.All major rivers flowing through Kruger have a steep macro-channel bank either side of a lower macro-channel flood area, thus there is no true floodplain area (except for some stretches along the Luvuvhu River). This influences plant diversity occurring within the broader riparian area (the fringes of rivers and streams).
The much higher macro-channel bank is only inundated during larger, but less frequent flooding events. The lower macro-channel floor area receives more flood disturbances making this area more prone to inundation stresses and habitat change because flood waters move and deposit sediment here.
Vegetation In The River ChannelGenerally, plant species occurring along the macro-channel floor have adapted to higher flooding disturbance frequencies. Some of these adaptations include: tolerating being submersed by water for periods of time, flexible stems for bending to withstand physical forces of flooding, and timing of seeding for water-dependant seed dispersal. These plant species have a high water demand, therefore require easier access to water, and mostly exist lower down.Plants occurring along the higher macro-channel bank are generally less tolerant of flooding, but have developed ways to conserve water, and therefore can exist higher up. Occasional flooding and access to the water table is sufficient to satisfy their water needs for reproduction, growth and survival.
Plant species composition has been found to change along the macro-channel bank based on how many floods occur, dependant on height above the low-flow stream. This possibly also relates to soil moisture and soil nutrient content, and differences in the ability for different species to access water from the water table.
The Influence of the Underlying RocksAdditionally, the major rivers of Kruger flow across many different types of geology between the escarpment and Mozambique, influencing the rivers’ nature and shape as they flow downstream. For example, the Luvuvhu River flows through a gorge, spreading into a wide floodplain area before meeting the Limpopo River.
The Olifants and Letaba rivers have fairly wide macro-channels but at their confluence are constricted in a gorge as they flow through the Lebombo Mountains in the east.
Within Kruger, rivers traverse mainly granite in the west, Karoo sediments, basalt and rhyolite in the east, and also cross intrusive dykes of dolerite, sections of gabbro, and minor faults in places. These geological differences account for changes in downstream slope and macro-channel configuration, with changes occurring rather abruptly in some places.
Along the macro-channel floor area patterns of sediment erosion and deposition interact with the slope and different geological formations, resulting in many different patterns of water, sand and rock occurring. These represent different types of habitats or niches allowing many plant and animal species to co-exist.
Water Speed : Deposition Versus Transportation of SandOver steeper river sections, flood waters generate a lot of energy causing the river to carry more sediment than it deposits. Here, a greater proportion of exposed bedrock occurs, having important implications for in-stream habitats and related fauna.
For example, obstruction and constriction of water flows through steeper outcropping bedrock sections (rapids) reduces the speed of the water immediately upstream of these obstructions.
Pools develop here with some deposition of sedimentary features (layers of deposited sand and stone) because of the reduced speed of flow. Alternation of these pool and rapid sections along the river provides important in-stream habitat variation for numerous fish and invertebrate communities, pools for crocodile nesting sites and hippo. The exposed bedrock also plays an important role in what plant communities occur within the riparian area.For example, seedlings of some tree species (e.g. matumi) have adapted to germinating in the cracks of exposed bedrock. The bedrock provides suitable anchorage for roots during flooding events, without which these seedlings would be washed away.
Once established, these trees trap sediment, and this further build up of sediment facilitates colonisation by other tree species, such as water berry and water pear. Smaller flood energies are generated in gently sloping river sections and allow for more sediment deposition, with these sections having little or no exposed bedrock.
Sand bars form low down close to the low-flow stream, and are often colonised by reeds which can withstand a lot of water inundation stresses compared to other plant species. Over longer time periods as more sediment is deposited, trapped by the reeds, these sand bars grow higher above the low-flow stream, becoming less inundated by smaller flooding events. Consequently, this generates more suitable habitat for colonisation by shrubs and then trees. This vegetation succession leads to patches of forest in the river, including species such as bush willow, common cluster fig and water elder. The establishment of reeds and different trees and shrubs in and along the rivers generates important habitat niches for many bird species, and a browsing source for herbivores.
However, larger flood events, e.g. that of 2000, potentially resets these patterns in some sections of the river by stripping away much sediment and individual plants. Overall, this variability in habitats and resultant plant/animal species composition (including changes over time) generates much biodiversity along the rivers of Kruger.
Managing for this BiodiversityIt is currently accepted within ecology that ecosystems are not static, but highly variable. Essentially, this means that different types of habitat occur naturally across landscapes, but the type of habitat at a specific location may also change over time. It is believed that maintaining these differences across space and through time in the landscape promotes overallecosystem biodiversity.
Within the river landscapes of Kruger there is a great deal of this diversity, as explained above. This has important implications for ecosystem management within the Kruger National Park. The park’s Strategic Adaptive Management (SAM) system encompasses a vision statement along with an agreed-upon desired ecosystem state that the park staff strive to manage for. Among others, maintaining biodiversity in all its forms is an important component of Kruger’s vision statement.
The Effects of Human Interference with Water FlowsIt is the natural flooding pattern within the rivers that generates a great deal of riparian habitat diversity, important for many animals and plants. Interception of flows during floods and augmentation of water shortages in droughts (mostly through the management of dams) upstream of the park alters the river’s natural flow variability.
This decreases the frequency of floods that act to flush out sediment, important for maintaining exposed bedrock in the rivers, and associated habitat diversity. Reducing various flooding frequencies will also allow encroachment of terrestrial plant species (not adapted to flooding) into the riparian area, thus reducing plant diversity. Furthermore, less water coming down the rivers from upstream due to water extraction/damming upstream reduces low-flows during winter months. This increases sediment deposition and silting up of the active streams, thus affecting quality of fish habitat and impeding fish movement along the rivers. Diminished low-flows may also lower water table levels, limiting water access for many riparian tree and shrub species.
Alteration of the natural flow pattern is, therefore, in direct conflict with Kruger’s management goal for preserving habitat variability and overall biodiversity. This has implications for delivery of many in-river goods and services, such as ecotourism within the park, but also water, food and wood fuel provision for communities further upstream of Kruger.
Strategic Adaptive ManagementManaging for habitat variability is, however, difficult because these ecosystems are highly complex, and complete understanding about how these systems work is lacking. To deal with all this uncertainty better, SAM consists of a number of objectives (goals) that branch downward from the value-laden vision statement, culminating in more technically stated operational goals at the bottom of these objectives.
Within SAM, these technical goals are known as Thresholds of Potential Concern (TPCs), defined as upper and lower levels along a continuum of change in selected environmental indicators. The TPC process involves research,anagement and monitoring, thus integrating these components in the park, critical for making management strategic rather than reactive to problems as they arise.The role of research is to generate understanding about major ecosystem processes that impact the system, and to determine associated environmental indicators to monitor. The upper and lower levels of change in selected indicators are also determined based on best available knowledge and aim to allow for natural changes within the ecosystem (according to the desired state).
Appropriate, feasible and cost effective monitoring programmes work to assess if change in these indicators is within the specified TPC levels. If not, decisions are then made about potential causes of the extent of change and if appropriate management intervention is required.
Sometimes more research is needed, and/or the actual TPCs need adjustment. Very broadly, this is what SAM is about, adapting as new understanding is generated through research/monitoring/management, thereby improving management of complex ecosystems.
Monitoring Kruger's RiversMuch of the ecological understanding surrounding the Kruger’s rivers came out of the Kruger Rivers Research Programme of the 1990s. A major focus concerned understanding and predicting impacts of catchment level changes on the state of Kruger’s rivers.
In fact, it was during this programme that the SAM concept was developed, which actually co-evolved with ideas which influenced South Africa’s national water act. During this programme major management problems were identified, namely: altered flow patterns, increased sedimentation in the rivers due to upstream influences, and water quality. Also, TPCs were developed to assess and adaptively manage rivers in Kruger.Two major TPC categories exist:
1) Flow requirements – recommended flow pattern to maintain a healthy, most natural river ecosystem state, and water quality guideline levels for a set of important physical variables and chemical constituents that should not be exceeded, related to organisms in the river (e.g. pH, nitrate, total dissolved solids);
2) Biodiversity related TPCs, including a) Geomorphology (the shape of the land and what causes it) - in particular the proportion of exposed bedrock and related habitat in the rivers,
b) Tree/shrub responses to changing flow and sedimentation along the rivers, c) Various fish, invertebrate and bird monitoring initiatives. To date, Kruger has actively monitored river flows and water quality as part of a national river monitoring programme coordinated by Dwaf. A major concern for managers in the park is the amount of water in the rivers, often being below the recommended flow levels to sustain the river ecosystem (with the Olifants River drying up for 78 days in 2005!).
This may also negatively affect water quality levels in the rivers. As part of the national River Health Programme, Kruger has also been monitoring fish and invertebrate populations in the rivers, although no specific TPCs have been used so far. The geomorphology and tree/shrub monitoring has not been implemented to date. One reason is the effect of the large 2000 floods, initially considered to have removed a large amount of sediment from the Kruger’s rivers. This would have alleviated those problems related to loss of exposed bedrock through sedimentation. However, it was soon apparent that sediment was mainly redistributed, not flushed out of the river systems.
After the 2000 floods, it was realised that improved implementation of adaptive management for rivers and a revamp of river TPCs was required. A major focus of a workshop held in February 2007 in Skukuza was to rethink and consolidate river TPCs and broader issues surrounding adaptive management of Kruger’s rivers.
Coordination And Cooperation - Upstream And DownstreamRealistically, development activities occur upstream of the Kruger National Park within the catchment areas of which Kruger is a part, providing important economic and social benefits. Rivers supply valuable water resources for domestic water supply, irrigation farming, industrial and mining developments.
Due to exploitation of this water resource in the past, it is recognised that the main rivers of Kruger are not entirely unmodified and over several decades have been impacted to various degrees. Returning the rivers to their natural state may not be feasible.However, Kruger’s river objectives specify the need to maintain, and where appropriate, restore river ecosystem health and related biodiversity. Kruger will continue to monitor river flows and water quality, but a plan to refine, test and implement the biodiversity related TPCs is under way. This will provide an important learning opportunity about impacts of river flows and water quality on biodiversity.
Along with this, it is recognised that the rivers of Kruger cannot be managed in isolation from the rest of the catchment area upstream of the park, but will need to align with integrated catchment management processes.
Future implementation of the national water policy, particularly the ecological reserve, will be an important step towards protection of rivers in Kruger and wider within the catchments. However, full realisation of the ecological reserve is a complicated process, dependant on various components being put in place. Among others, this includes categorising different rivers for various management options and setting up relevant institutions to oversee water resources within the 19 water management areas throughout the country. Kruger straddles three water management areas.
In accordance with the national water act, Catchment Management Strategies need to be drawn up for each water management area, with the arduous task of balancing water resource protection with development needs.
However, preliminary environmental flow requirements and water quality levels have been established for the rivers of Kruger (some culminating in the ecological reserve). However, joint cooperation between many stakeholders within the relevant water management areas is crucial for achieving relevant flow and water quality targets within Kruger and the broader catchment area.