Being a circle or not doesn't have an influence on the average depth. The math is actually super easy if you use the metric system (which you all should btw ) :There are 1000 litres in a cubic metre, and a US gallon is 3.78541 litres.
So that is 17,699,527 US gallons, or 54.3 acre foot.
Can anyone do the math on what the average depth will be, assuming the lake is a perfect circle? Thats beond what I recall of volume etc.
2.87ha = 28.700m²
Depth = volume divided by surface
--> 67.000m³ / 28.700 m² = 2.33m average depth (7,6 ft)