Freshwater Ecosystems and the Importance of Flow and Sediments

Freshwater Ecosystems and the Importance of Flow and Sediments (Taken directly from: BROWN, C.A. AND KING, J.M. 2010. Chapter 8: Environmental Flows in Shared Watercourses: Review of Assessment Methods and Relevance in the Transboundary Setting  In Transboundary Water Management: Principles and Practice (Eds A. Earle, A. Jägerskog and J. Öjendal) pp 107 – 128 (Earthscan, London).

Freshwater ecosystems are the foundation of every country’s social, cultural and economic well-being.
Healthy freshwater ecosystems – rivers, lakes, floodplains, wetlands and estuaries – provide clean water, food, fibre, energy and many other benefits that support economies and livelihoods around the world. They are essential to human health and well-being.

Freshwater ecosystems have evolved in response to the natural geology, topography, climate and vegetation of their basins, and the volume and pattern of water draining them. Together, these create the habitats that support a wide variety of animals and plants, and provide a host of services to people. The suite of ecological services provided by inland water ecosystems is valued at about US$6 trillion per annum (Postel and Richter, 2003), and they have among the highest value per unit area of any of the world’s ecosystems (Costanza et al, 1997, cited in Postel and Richter, 2003). The services they provide include:

  • edible plants and animals;
  • water;
  • raw materials – wood, rocks and sand
  • firewood, genetic resources and medicines;
  • ornamental products;
  • groundwater recharge;
  • flood attenuation;
  • water purification);
  • spiritual; religious; aesthetic appeal;
  • nutrient cycling;
  • carbon sequestration);
  • breeding areas for marine fishes, and more (King, 2009).

Different flows are important for maintaining ecosystems (e.g. Carter et al, 1979; Poff and Ward, 1990; Richter et al, 1997; Bunn and Arthington,2002). In rivers, floods and the sediments they carry shape the channel, creating a diverse array of habitats for animals and plants (Richter et al, 1997). Floods also inundate floodplains, trigger fish migrations and breeding, and maintain the vegetation on floodplains and riverbanks that, in turn, protect the riverbank from erosion and act as a buffer against chemicals and sediments flowing off the catchment (Tabacchi et al, 1998).

The low flows maintain the basic ephemeral, seasonal or perennial nature of the river, and determine the animals and plants that can survive there (Brown and King, 2003). The different magnitudes of low flow in the dry and wet seasons create more or less wetted habitat and different hydraulic and chemical conditions, which directly influence the balance of species. The timing of these different flows is as critical as their size, as the reproductive and other behaviours of plants and animals are attuned to, and dependent on, the seasonal fluctuations of flow and temperature. Flow variability, on a daily, seasonal or annual basis, maintains biological diversity through increased heterogeneity of physical habitats. Variability also means that conditions are optimal for different species at different times, which ensures that no one species proliferates to ‘pest’ proportions (e.g. Zakhary, 1997). In wetlands and lakes the influence of flow is less apparent but no less important. The timing and magnitude of seasonal fluctuations in inundated area and depth drive the chemical and thermal characteristics and set in motion the breeding and growing cycles of plants, fish and other animals (e.g. Welcomme, 1979; Karenge and Kolding, 1994). Seasonally flooded areas provide grazing for migratory species such as antelope (e.g.Mendelsohn and el Obied, 2004), breeding grounds for fish and birds (e.g. Welcomme, 2001;Kamweneshe and Beilfuss, 2002), and fertile land for recession agriculture (e.g. Heeg and Breen, 1982).

Similarly, freshwater inflow to estuaries is a fundamental part of estuarine chemistry, morphology and biology. The mixing of fresh and salt-water creates and sustains a unique type of environment that is among the most productive of any on earth (e.g. Nixon et al, 2004). The amounts, duration and intensity of flow events influence estuarine geomorphology, water temperature, salinity, pH, turbidity, nutrient status, organic inputs, dissolved oxygen concentrations, olfactory cues, mouth status, tidal prism, habitat diversity, primary and secondary productivity, fish recruitment, food availability and competition
(Whitfield and Wooldridge, 1994).

Because they are the drainage points of whole basins, these ecosystems are highly vulnerable to changes in their basins (e.g. Baron et al, 2002).  Land-use changes, water-resource developments, point- and diffuse-source pollution, and other interventions all impact on the freshwater environment and the services they afford people, sometimes profoundly and possibly for hundreds of kilometres downstream. This is particularly true for changes in the flow of water and sediment, the influences of which tend to track farthest (and fastest) down the system. Nor are the impacts always unidirectional. The effects of developments such as dams can also be felt upstream if, for instance, in-channel dams act as barriers to fish migrating upstream to breeding and feeding grounds (King, 2009). Hence, for sustainable development to occur, these ecosystems need to be managed holistically within their whole drainage basin (King and Brown, 2009a).

Removal of water from freshwater ecosystems and/or manipulation of their flow regimes will always be a trade-off between loss of ecosystem function and resilience, on the one hand, and benefit derived from the use of that water elsewhere, on the other. The greater the divergence from a natural flow regime (in terms of both the volume and the timing of different magnitude flows), the more the ecosystem could be expected to change. Furthermore, if the water-resource infrastructure changes the natural sediment supply, these changes will be even more pronounced (Petts and Gurnell, 2005).  This presents particular challenges for sustainable management in transboundary river basins, where political borders often separate development pressures and the need for ecological protection (Fox and Sneddon, 2007). The solution lies in countries recognizing the critical importance of the hydrological and sediment regimes as the primary drivers of ecological processes in river–floodplain systems (Carter et al, 1979; Richter et al, 1997; Tharme, 2003) and engaging in cross-border collaboration to assess the impacts of water-resource developments on the integrity of downstream ecosystems, and on the suite of services offered to people by those ecosystems. Specifically, they need to reach agreement on future condition of those systems, on which of the valued services will be maintained and the volume and timing of the flows required to sustain them  – the so-called EFs.

EFs refer to ‘the quality, quantity, and timing of water set aside to maintain the components, functions, processes, and resilience of aquatic ecosystems that provide goods and services to people’ (after Hirji and Davis, 2009). They arose as an explicit response to the need for sustainable development of the world’s water resources (Gleick, 2002; United Nations, 2007) and as such do not focus on conservation or protection of nature and the environment, although they can be used as a tool for both. Rather, they reflect the recognition that water-resource development does and should occur for the betterment of people, but that this will affect the ecological condition of the targeted system. Aquifers, rivers, wetlands, lakes and estuaries can be managed to be at different levels of condition (health), from pristine, when they provide a range of natural services of benefit to humans; through various stages of change from natural, when the original services disappear and others appear; to serious degradation, when many of the natural services are lost (King and Brown, 2009a). The finally agreed condition should be a societal choice based on the consideration of all options. For instance, in the case of a large dam development, the decision may be to release a portion of the inflows for maintenance of the downstream river, which would reduce the yield of the dam, but retain valued ecosystem attributes in the reaches downstream. In Lesotho, 10–14 per cent of the mean annual runoff (MAR) of the rivers is released from the Lesotho Highlands Water Project dams, which will maintain the rivers in a condition that is reduced from natural but that will still support some of the fish and woody vegetation that the riparian people use (LHDA, 2003).

The science of EFs has five main objectives (after Brown and King, 2006):

  1. to understand the nature and functioning of freshwater ecosystems;
  2. to be able to predict how these ecosystems will change with flow change;
  3. to understand how people use the freshwater ecosystem resources;
  4. to be able to predict how these uses will be affected by change in these ecosystems;
  5. to combine the predictions into scenarios reflecting the costs and benefits of a range of development options.

EF assessments are thus part of a comprehensive approach to water-resource management that can guide more sustainable use of aquatic ecosystems.  They provide hitherto absent information on the consequences of water-resource developments for downstream freshwater ecosystems and the people who depend on them, which can empower and inform decision making (Richter and Postel, 2004; Arthington et al, 2007; Brown and King, 2006). EFs can be incorporated into water-resource developments at virtually any stage (Postel and Richter, 2003), but consideration of them should preferably be done early in the planning process of water development, together with an analysis of the economic benefits of the proposed scheme, so that the agreed trade-off between development benefits and natural resource degradation can guide project design and operation (Watson, 2006).