There are a couple different ways to circulate the water in your pond or lake to prevent water stratification and turnovers. By far, one of the best ways is to install a diffuser system. Diffuser systems use an air compressor to pump air into the bottom of your pond, much like air stones in an aquarium. The diffuser breaks air into smaller bubbles which expand as they rise to the surface. The rising on the bubbles pushes water and forms a current that mixes the entire water column.
Other methods to circulate the water in your pond are pumps such as centrifugal irrigation pumps and sump pumps. Both of these water pumps push water but if not designed correctly they will not prevent stratification.
The trick is to make sure that these pumps are creating a current to mix the bottom layers of water with the surface layers. Irrigation pumps can also be installed on land as long as they draw water from the bottom of the pond. Water pumps are not self-cleaning so regular maintenance is important to keep them from getting clogged with debris and aquatic vegetation.