10.3 THE ECOLOGICAL BASIS OF TRANSBOUNDARY IMPACTS

Are the impacts always negative?

How do transboundary impacts occur?--
transboundary environmental impacts in the Mekong River occur mainly because of the way in which the river environments are inter-connected (linked together). Water in rivers flows in a linear fashion, from uplands to lowlands to the sea. In addition, the flood-pulse causes exchanges between the main river channels and lateral aquatic environments (floodplains and lakes). There is lateral and longitudinal connectivity in the river system. This is why activities in one area can have an impact in another.

There are two major pathways through which transboundary environmental impacts occur:

· Changes in water quality, such as suspended solid loads, oxygen levels, pollution, etc.
  • Changes in water quality are transferred downstream, as the water flows through the river system
· Changes in water quantity flowing downstream, and the timing and duration of flooding events
  • Reducing water flows upstream (through building dams, removing water for irrigation, diverting it to other sub-catchments, etc.) results in impacts downstream.
   

o It is important to note that it is not only the quantity of water flowing downstream that is important, but also changes that occur in the timing and duration of flooding events (see Section 4). The animals and plants in the river are adapted to natural flow and flooding regimes, and may be significantly affected by changes in the flow regime (see Sections 6 and 12).

 
  o These changes can be significant and result in huge losses of aquatic habitat. They can also occur independently of changes in water quality. For example, through flood control/impoundments, water abstraction etc. most of the natural wetlands of the Mekong could in theory be lost – with no noticeable impact upon water quality in the main channels.
 

A major problem in rivers is that, generally, both of these effects (changes in water quality and quantity/flood timing etc.) are cumulative. That is, the effects of one small water diversion project may have limited impact on water quantity and quality, but if there are many small diversions, the impacts can be significant (See Section 13).

The basics of river ecology, form and function have been explained in detail in Sections 1 to 8. The ways in which human activities influence the Mekong River ecosystem are explained further in several other sections of the River Awareness Kit.


Are the impacts always negative?--
Unfortunately, most attention to transboundary issues is often directed towards negative impacts. It is important to note, however, that positive transboundary impacts can also occur. Measures undertaken to rehabilitate ecosystems (for example, restoring natural water flows in a dammed river in one country) can have significant benefits to the downstream countries. The potential for "positive" transboundary effects is often under-estimated in river basins. Impacts are not always negative!

In many highly degraded rivers in the world, rehabilitation measures (many of them transboundary in both intention and effect) are supplying major benefits to their riparian neighbour countries (see further reading under Section 11).

Other examples where positive transboundary impacts have occurred in the Mekong River include joint management and protection of fisheries resources and fish migration routes in the Lower Mekong Basin (see Section 12). Although these are modest initiatives at present, it signals willingness for riparian countries to co-operate on joint resources management, and provides room for optimism for the future of the Mekong River.

An important issue related to EIA is how transboundary impacts can be accounted for and incorporated into the various environmental impact processes. MRC is currently working to develop procedures and protocols that may be adopted by the four National Governments as mechanisms to incorporate transboundary impacts into their EIA procedures.