See the discussions on springs under Soil Questions.

Since you do not have a surface water source, you may have to rely on groundwater to fill your pond. It can be said that the groundwater surface generally mirrors topography. This means that the groundwater level should be higher in the hills than the valleys, which can in certain instances create artesian conditions. In other words, groundwater flow from the hills may create a high water table in the low areas. What I would do is take a post hole digger or hand auger and put as many test borings in the low areas as your back will allow. If you find a high water table you might be able to excavate a pond in the low areas. You will of course need enough water, which will be dependent upon many things including the size of the drainage basin, aquifer characteristics, confining unit characteristics, the evapotranspiration rate, and others.

Regarding siphoning, gravity flow from a well located on the hill is essentially the same thing as an artesian aquifer. In other words, if there is an aquifer on the hill that is of sufficient size, has a higher water level, and is in hydraulic connection to the lowland you won't need a well. Try to figure out which low area drains the largest upland area. This should be where you have the most water. Wetland vegetation such as cattails, boneset, marsh marigold, etc. are indicators of shallow water. The presence of wetlands, however, may provide regulatory obstacles.

I would only explore forming the pond solely by pumping well water after ruling out the above possibility.