Just looked at the Brockville, Ont. weather report. It says the current temperature is 29C/84F. Hardly a heat wave IMO. But to your question.
How does that 100 gallon loss translate into inches or millimeters lost?
What is the total surface area of your pond? Have you added additional aquatic plants since the last photo you posted?
Here in the Florida panhandle, we have been running heat indices of 105+F most every day since mid-May. Actual daytime highs have been consistently in the 90F - 95F range for this same period, esxcept during the rare periods it has rained. My pond, as well as all the ponds that I service, have been running the usual evaporation rate for this time of year, usually less than .25 inch/day; rarely up to .50 inch/day.
The USDA research station at Geneva, NY reported an average Pan Evaporation rate of .24 inch/day for week ending July 30, 2011. This is basically just across the Lake from you, isn't it?
"Everybody is ignorant, only on different subjects." - Will Rogers