most of the time when i see a spot like you are describing i believe whats happening is this. it rains and the hillsides have a lot of pervious soil in the top 2-4 ft. the hillsides soak up the dirt till the water hits the redox layer of soil.( redox = impervious or water wont travel through it) once the water hits the impervious layer of soil or rock etc. it slowly migrates downhill to this low spot you are describing but it takes the water a while to get there keeping the low spot wet for several days. if your hillside and low spot dried up quickly then there is not as much pervious soil in the top layers. usually you can see the redox layer when you go to digging its white and chalky looking and may only be 1-4 inches thick its where the water cant penetrate anymore and begins traveling to lower ground through the dirt.i learned this from a soil scientist during a recertification class for installing septic systems about 15 yrs ago. sometimes on a spot like you are describing i have to push a lot of pervious dirt behind the core before i can start building the pond i will use it in the dam but only behind the core. usually these types of sites are harder to build a pond in than usual because until you get to the impervious soil its usually muddy and hard to move unless your in a dry spell. but well worth it because the water is usually very clear from being filtered through the dirt. only thing that bothers me about a spot like this is just exactly what is that impervios layer rock,clay ,shale, etc. i'll take anything but rock. a lot of people call these spots wet weather springs. i hope this along with what the others have mentioned helps you a little. and if the silty dirt is deep extend that core up the hill until your above your desired water level of the pond or it will leak around your core. good luck