You may think you are blocking off the creek but it could flow up to 15 feet below grade you should keep the water flowing to provide water and oxygen also that open wter at the mouth lets gas escape.
As far as the frozen soil idea not much frost is found from the toe to the top deck in winter and none below the ice cap if snow is on the ground none at all . The water you have seeping out of the pond is around 48 degrees.
If you think you know where the leek is sink a dye tablet and flour mix to keep it on bottom and see what weeps out.
Good Luck Scott
