Rivers are the not just the lifeline of our country, but life itself. We may revere them, even worship them but we continue to pollute, choke, and poison their waters. A river, its catchment area, and its floodplains have evolved over millions of years, and once damaged, may not be easy to reclaim.
An article titled ‘Environmental flow for the Yamuna river in Delhi as an example of monsoon rivers in India’, published in Current Science, discusses a case study of the river Yamuna in Delhi, to work out the minimum amount of water that will permit the river to perform all its natural functions. A brief outline below gives a picture of the present scenario and the way forward.
The river self cleanses during heavy rains or the monsoons when 80% of all rains fall each year. It is during this time that silt transportation occurs, groundwater is recharged, sub soil bacteria is revived and mangrove ecosystems thrive. If this flow is cut, these and many other functions will be disrupted. For a river to remain close its pristine state, the actual water use must not exceed 50% of the total annual flow of the river. Thus, at least half of the water that flows in a river in a year, must be allowed to flow, to avoid environmental stress.
River flow is not an exact science as the river quality and the water quantity needed at different times for varied purposes, fluctuate constantly. A fine balance has to thus be drawn between safeguarding the ecological integrity of the river and its practical usage. To understand the river dynamics, average parameters were used for calculations and it was found that:
Environmental flow performs various natural functions of the river that maintain its ecological integrity. Flooding deposits sand in the flood plains and flushes out the debris from the riverbeds avoiding silting of the channel. Some other functions that a river--not the Yamuna--performs include:
During the monsoon, 50% of the free flow is needed to avoid silting of the riverbed and during the non-monsoon period, 60% is needed to avoid algal choking. Thus for a genuine ecological flow for a river to be healthy and maintain all its associated functions, the free flow must be close to 50-60% of the total flow, all year round.
The long term consequences of overexploiting the river and cutting flows will terminally affect the river and its surroundings. In this context, the Yamuna is already overexploited. The paper concludes with a suggestion that since the lean season use is mainly for agriculture, restoration of flow in water short rivers can be accomplished by water harvesting, more efficient agricultural practices like drip irrigation and moving more water-intensive agriculture (for example for rice and sugarcane) to water surplus areas.