Sealing and compacting clay

For the pond and dam builders:
I want to build a pond on sloped terrain by excavating into the high side and using the excavated material to build a wide dam on the low side. Soil is glacial till (grey coloured clay with gravel and rocks).
When compacting the clay that forms the dam, I have read that it should have "optimum moisture content". So what is "optimum moisture content" for clay? Is there a way to estimate "optimum moisture content" by soil texture?
At this time of year the excavated clay will be bone dry. How effective will my dam be if the clay is compacted dry?
If adding water is neccessary, how can this be done without making a sticky mess at the surface while still having dry clay underneath?