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Report No 678-F-001
METSI CONSULTANTS: SUMMARY OF MAIN FINDINGS FOR PHASE 1 DEVELOPMENT
floods, would pass through the weir gate. Matsoku dry-season low flows would not be reduced as much as in the
other rivers because of the 0.6 m 3 s -1 constant environmental release, however, the river would be affected by a
loss of variability in the flow regime.
6.2. BIOPHYSICAL CONSEQUENCES
The impacts of the Treaty flow regimes on the rivers would be the most severe of the scenarios and manifested
as strongly deteriorating physical and chemical conditions (relative to natural), particularly in the reaches
immediately downstream of the dams. Almost all aspects of geomorphology and water quality at three of the eight
IFR Reaches (2, 3 and 7). are predicted to show severe and/or critically severe changes. Because of
unregulated, incremental inflow from the surrounding catchments, the lower Senqu River (Reaches 5 and 6) is
expected to be the least affected, with low to moderate effects expected in the Senqu River at Reach 4, and the
Senqunyane River at Reach 8.
As a result of changing the constant releases through Matsoku Weir from 0.05 to 0.6 m 3 s -1 the severity of
biophysical impacts in Reach 1 are much reduced from that assessed earlier (LHDA 648-F-02).
Geomorphological impacts are judged to be reduced from critically severe to severe, water quality and
macroinvertebrates from critically severe to moderate, vegetation from severe to moderate, and amphibia,
mammals and birds from severe to low. The presence of the weir itself remains a major impact on fish
populations, including the highly endangered Maluti minnow.
Immediately downstream of the Katse and Mohale Dams, critically severe changes are predicted in most physical
and chemical aspects. Significant reductions in pool numbers, depths and sizes, are anticipated, with pools
disappearing almost completely in the higher reaches. The transport of sand, and shifting, scouring and sorting of
larger bed elements is expected to virtually cease, except during very rare large floods. The anticipated
consequences of this are that river beds would increasingly silt up; with all but the few remaining areas with
slightly faster flow gradually acquiring a relatively featureless, muddy bottom. Riffles would be largely or
completely lost in many reaches, and deep, soft deposits of mud would line river margins and fill backwaters.
There would be little of the habitat diversity and clean, scoured rock surfaces necessary for a diverse, healthy
aquatic community. Conditions would improve somewhat after large floods, but increasingly larger floods would
be necessary to reverse the deterioration as these changes became entrenched.
The reductions in low flows are expected to lead to corresponding reductions in temperature buffering capacity,
and to result in greater extremes of day and night temperatures. This would be followed by widespread
distribution of dense algal growths, particularly in IFR Reach 2. Daily ranges of water temperatures would
increase, as the small volumes of cold water released from the dams warmed up during the day but less so at
night. This effect could be mitigated in the Senqunyane River by mixing releases from different levels in Mohale
Dam wall to match as closely as possible the desired water temperature regime in the downstream river reaches.
In Reaches 2, 3 and 7, wetbank vegetation is expected to show moderate (20-40%) to severe (40-80%) changes
and the drybank vegetation severe to critically-severe changes, with an increased potential for encroachment of
exotic woody vegetation (such as Acacia dealbata).
Predicted shifts in invertebrate abundance would be mainly of concern in Reaches 2 and 7, and to a slightly
lesser extent in Reach 3. Here the shifts are expected to be in the moderate (20-40%) to severe (40-80%)
categories. There are as many increases in abundance predicted as decreases, and the overall abundance of
organisms would theoretically remain the same. However, many replacement species would live in muddy
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