Generally dams are sealed by compacting them as they're being built.
http://www.aboutcivil.org/How-build-small-dams-Construction.html
If the pond leaks after being built some people seal them with a technique known as "gleying" in which animals are fed in the pond so their manure and trampling makes a biological seal.
Here's a thread about gleying with pigs:
https://permies.com/t/38201/ponds/Progress-Gleying-Pond-Pigs
One of Geoff Lawton's videos talks about gleying with ducks:
http://geofflawton.com/videos/fixing-a-leaky-pond-with-ducks/
Our upstream neighbor built a dam in the seasonal creek without compacting it properly and it completely washed out during the flood this Spring, sending all the dam material, including their old asphalt driveway chunks, onto our land. It isn't really something an amateur should attempt. Our neighbor across the road had a proper pond built and it took a guy with a bulldozer a few days to dig and compact it, and it holds water nicely. It is not on the creek channel, either.