No man ever steps in the same river twice, for it is not the same river and he is not the same man. What a piece of wisdom from the Greek Philosopher Heraclitus (535 – 475 BCE) on the facts of Nature, life and society - the reality of impermanence (Gautama Buddha, 563 – 483 BCE) that defines the ever changing fluxes of mind and matter and all things that surround us (impermanence does not connote, for example, that a relationship is not supposed to last, but that the nature of relationship does not remain the same over time). Apart from some obvious reasons – humans had been attracted to rivers since time immemorial, feeling and witnessing their vibrating energy and transformative changes – rich with the messages of the realities of life. This piece is not about this reality – but the reality of a different sort – the macro-scale processes that define a river. And understanding these processes equip us to better manage a river.
These are the aspects we all know about yet do not pay adequate attention to – we tend to think of those aspects so guaranteed that we often fail to take proper account of them while planning catchment and river-bank developments, flow-interventions, or while dumping materials of all sorts into them. These are the basin-wide factors that control the birth and flow of a river defining its characteristics, and the functions it shoulders on its journey. The factors and functions are unique to each river – and define the characteristic regime of a river system. A river is better understood as a system comprising of the tributaries that contribute to its load; the distributaries that carry part of the load; the floodplains that alleviate its burden by functioning as a storage basin, and slowly releasing the flood water; and the delta it builds as it debouches into open water.
But while one talks about the basin-wide factors – encompassing the catchment area from the headwaters to the base level at lake or sea – a river has also distinctive reach-wise factors of geology, geography, hydrology and hydraulics. People often see a river analogous to life – birth and infancy at the source, energetic youth at the steep terrain, broad and mature adult at the flat landscape, and demise at the base level. And since the geologic past, the river has been giving away the resources (hazardous when angry – flooding its basin) it carries – to quench thirst, to help cultivate lands, to build navigational trade networks, townships, cities and civilizations. No wonder – philosophy, poetry, fictions and religions are full of inspiring materials on river – with some attributing divinity to it.
With these paragraphs of introduction let me move on to the core materials of this piece. For simplicity and importance, I will primarily focus on alluvial rivers (a river that constantly changes and defines its own deformable granular bed and planform depending on the volume of water it carries and the magnitude and type of sediments it transports). The discussed aspects are based on my short river experience and three of my published papers:
What are the environmental controls of a river – that define its unique characteristics (for example, those responsible for distinguishing the Mississippi River from the Ganges-Brahmaputra system)? A river is the dynamic response of the earth’s surface to the draining of accumulated water mass. The accumulated water mass contributed by precipitation, snow and glacier melt is collected by numerous streams and gullies – all feeding to the development of a river. Rivers are also a showcase of annual hydrologic wave – the seasonal river discharge; but without the usual reversal of flow – rather with the wave-type fall-rise-fall-rise water level stages. As observed in the Bangladesh Reach of the Ganges River (my 1995 IEB paper), the river flows can be characterized in terms of a Seasonality Index (this parameter defines the river regime and is very useful to distinguish the hydrologic characteristics of one river from another) – indicating a rather steeper slope during the rising phase than the falling period. The implication of such an asymmetry lies in the existence of the so-called hysteresis or incoherence in the relationship between river stage and discharge – and between the discharge and sediment transport.
The dynamic behavior of a river represents an interactive process-response system driven by universal physical laws - but the actions and reactions take place within the boundary conditions or environmental controls where it belongs. What are these? The list is simple – (1) climate, (2) catchment basin size, (3) basin relief or slope, (4) type and extent of basin Natural vegetation, (5) basin lithology defining the soil material characteristics, (6) anthropogenic factors, (7) frequency and magnitude of episodic events, (8) base level or sea level changes, and (9) earth’s rotation. The last factor – the effect of earth’s rotation is important for large rivers.
While the list is simple, the controlling influences are not – because each of these factors has different variability and time-scales of change – the characteristic signature depending on the geographical location where the river belongs. And any change of these factors invokes a response from the river – the response depending on the magnitude, intensity and frequency of changes or modifications.
What are the responses? An alluvial river response to the above driving conditions can be thought of as comprising two categories. The first is the fluvial loading – water discharge, sediment transport, and sediment caliber (the sizes and types of sediments a river transports). The second is the fluvial geomorphology – width, depth, slope, and planform (such as meandering or braiding type). Both of these two categories are conditioned by environmental controls – with the fluvial loading acting as the imposed processes on the fluvial geomorphology. Together they represent an interactive process-response system that aims to attain dynamic equilibrium in time.
Discussing all the factors can be very elaborate, let me focus on two important ones:
At the outset it is important to make a distinction between resource and hazard. This distinction depends on defining a threshold for a particular phenomenon. The simplest of this is the flood level that one hears most often. The defined threshold helps planners and engineers to classify a river stage, whether or not it is at danger level or at tolerable level. When the threshold is exceeded, the water as a resource becomes hazardous. Another simple example is the much talked about proliferation of carp fish in the Mississippi River. It was a resource in small numbers, but as this invasive species overwhelmed others, the same fish has become hazardous.
The functions can be thought of as comprising four important broad groups: (1) biotic, (2) abiotic, (3) functional use, and (4) consumptive or diverted use. The biotic function is the sustenance of flora and fauna that thrive in a river basin. The most important biotic function is the fresh and brackish-water fish resources a river produces and sustains. Any change in the discharge volume, water temperature, salinity regime affects this important resource – most humans rely on for nutrition. A measure of the biotic function often relies on the level of dissolved oxygen (DO) and biological oxygen demand (BOD). Here again a threshold needs to be defined for a particular species – a higher BOD than the available DO can turn the same water from a resource to a hazardous one. In addition to dissolved pollutants that affect water oxygen level, floating non bio-degradable debris limits the recreational functions of a river.
The abiotic functions can be elaborated as: (a) drainage of surface run-off, (b) drainage of subsurface flows, (c) sustaining geomorphologic equilibrium, (d) land building in the deltaic reaches, and (e) preventing salt-water intrusion in estuaries. Functional use accounts for navigation, open-water fisheries and fish migration, and recreation. The biotic, abiotic and the functional use are essential river functions – let us say duties, a river is tasked to do. The extent of these tasks defines a river. Any unthoughtful development or intervention that does not give importance to its functions can jeopardize the long-term health and equilibrium processes of a river and its riparian areas.
Consumptive or diverted use – on the other hand accounts for net loss of river-water. They include: (1) domestic/municipal, (2) agricultural, (3) industrial – hydropower and cooling, (4) culture-fisheries, and (5) inter-basin water transfer. How much water a river afford to lose without compromising its essential functions? The question does not have easy answers – and definitely arises again and again when dams and inter-basin water transfer are planned. There exist many conflicts on the equitable sharing of waters by co-riparian states and countries. But when such plans are implemented, the old river ceases to exist – and its regime and basin characteristics are changed for ever. Most rivers around the world have lost their past regimes or characteristics because of extensive damming, dikes and urban developments.
Let me now attempt to elaborate the functions of a river – but for the sake of brevity focusing only on two of them:
Like the energy of a river, let the vitality of life flow into the New Year and to all years to come.
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- by Dr. Dilip K. Barua, 26 December 2018