River deposits Below the dam, and sometimes at several sites downstream, a deposit often covered the surface of the river bed sometimes up to 2cm thick, and would therefore have a blanketing and smothering effect on much of the invertebrate community particularly during periods of compensation level discharge only. It was noticeable that those organisms which normally occur on the clean surface of stones, e.g. Simuliids and Heptagenid mayfly were invariably absent at Yarrow, where the deposit was most prolific. It was also possible the high metal content of the deposit may be toxic. The metals originate from the anoxic hypolimnetic water at the bottom of the reservoir. It was shown that the metal levels in the streams feeding Kielder reservoir are generally higher than those found in the river system downstream.
Experiments show there was no toxic effect on fish development.
Temperature changes Results showed winter temperatures in the river immediately below the dam were significantly raised, e.g. 1.5o C above the dam at Butteryhaugh, compared with 4.5o C immediately below the dam at Yarrow and 2.5o C at Chollerford 35 km downstream. The warm winter temperatures below the dam were shown to cause an increase in the rate of salmonid egg development, this, when followed by cooler releases in early summer due to reservoir stratification, resulted in a considerable time-lag before the critical water temperature of 7o C was reached. It is at this temperature that young salmon begin to feed. This time-lag would cause newly emerged fry to starve. Trout on the other hand have a thermal cue of 4o C and since this was always achieved accounts for the dominance of Trout over salmon at sites close to the dam.