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.

Monsoon flow essential

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.

Present status of the Yamuna

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:

• average present mean annual flow is 4.4 TMCM (Thousand Million Cubic Meter), which is about 32% of the yearly flow in the river;
• present total non-monsoon flow is 0.44 TMCM, which is 16% of the non monsoon virgin flow of river Yamuna, and not the ideal non monsoon flow required for a river to perform its various functions.

One flow, many functions

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:

• Prevent algal choking: But the present non-monsoon flow in the Yamuna is less than 25% needed to avoid still water algal growth.
• Sediment transport: The river is already heavily silted and its depth is barely 0.6m in the summers. It needs atleast 50% of the monsoon flow as a flushing flow to be able to remove and transport river bed particles, which is not happening presently.
• Delta & sea interface: Where the rivers meet the sea, it is the transported soil particles that enhance the fertility of the delta, avoid ingress of the sea and help preserve mangrove interface. This enviromental requirement can only be met if the flushing flow in the monsoon months in the river is maintained.
• Floodplain recharging: Total width of the flood-plain recharged by the river during the monsoon is barely 1km, while the floodplain width stretches to at least 5 km. Thus the aquifers too are not getting recharged.
• Biodiversity maintenance: In the summer months, the water depth of the Yamuna before it enters Delhi is too meagre to support a healthy fish population. Downstream of Delhi, the river is more of a sewage conduit. At least 50-60% of the annual flow of the river must be maintained on average for its ecological requirement.
• Dilution flow: Delhi has a population close to 20 million, and the average non-monsoon season BOD (Biological Oxygen Demand) level, is close to eight times more than the desirable level. To counter this, a dilution of eight times is required to make the river water fall into safe health limits. Thus, the Yamuna needs a comprehensive cleansing of sewage to be safe.
• Minimum flow: As per a Memorandum of Understanding (MOU) between the basin states of the Yamuna, a minimum flow of 10 m3/s is required throughout the year for ecological purposes.This amounts to a minimum fresh water flow in the river of 0.86 MCM/day, whereas as per the paper findings, about 6.6 MCM/day is needed, just to avoid algal choking.

Conclusion

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.