Report No 678-F-001 METSI CONSULTANTS: SUMMARY OF MAIN FINDINGS FOR PHASE 1 DEVELOPMENT 6
Report No 678-F-001 METSI CONSULTANTS: SUMMARY OF MAIN FINDINGS FOR PHASE 1 DEVELOPMENT SECTION 2. NATURE OF INSTREAM FLOW ASSESSMENTS 2.1. INSTREAM FLOW ASSESSMENTS AS A RESPONSE TO RIVER REGULATION An IFR is a description of a modified flow regime for a river, linked to a description of the condition or health of the river that this flow achieves. The flow regime of a river consists of several different kinds of flow, each of which contributes to the overall maintenance of the aquatic ecosystem. Low flows occur when the river is not in flood. They are larger and more varied in the wet season than in the dry, and define whether the river flows all year, only during the wet season or just after rains. They create different conditions in different seasons, dictating the occurrence and densities of various biotic species occurring at various times of the year. Large floods occur less than once a year. They trigger the same responses as do small ones, but also provide scouring flows that shape the channel. They move and cleanse cobbles and boulders on the riverbed, and deposit silt, nutrients, eggs and seeds on floodplains. They inundate backwaters, secondary channels and floodplains, and trigger bursts of growth in many species. They recharge soil moisture levels in the banks, thereby enabling seedlings of riparian trees to grow. Small floods occur several times within a year. They stimulate spawning in fish, flush out poor-quality water, cleanse the riverbed, and sort the river stones by size thereby creating different kinds of habitat. They trigger and synchronize various activities such as upstream migrations of fish and germination of seedlings on riverbanks. Flow variability, on a daily, seasonal or annual basis, acts as a form of natural disturbance. Fluctuations between low flows and small and large floods change conditions through each day and season, creating mosaics of areas inundated and exposed for different lengths of time. The more diverse the physical conditions, the higher the biodiversity and the greater the resilience of the ecosystem to disturbance. Manipulations of flow regimes represent unnatural disturbances to aquatic ecosystems. These disturbances increase in severity as flow regimes are altered from what lies within the realm of “normal” for the particular systems. Responses of the ecosystem become more extreme as disturbances increase, and can take many forms. For instance, hydrological cues that trigger fish spawning or seed germination may occur at the wrong time of the year or not at all, resulting in affected species perhaps failing to reproduce. Seasonal reversal of wet and dry season low flows could mean that hydraulic and thermal conditions become mismatched with life-cycle requirements, again causing species to decrease in numbers and abundance. Other species, including those regarded as pests, are able to take advantage of such environmental conditions, or the weakening of competition from the affected species, and increase in abundance. 2.2. PURPOSE OF IFR DETERMINATION IFRs are established to mitigate the potential impacts of river flow reductions on aquatic ecosystems in three ways. By reserving some water for ecosystem maintenance - in general, the closer to natural the desired condition, the greater the volume of the original flow regime required for the IFR. By ensuring that the reserved water is made available to the ecosystem at the times when it is most appropriate for river maintenance - for instance, if large floods are needed in a river to maintain 7