maybe Grassland/Freshwater Crayfish (Procambarus) or much less likely a Star-nosed Mole (Condylura cristata)

2010 by David in NC - What made the holes?

If Crayfish, see info:
http://en.wikipedia.org/wiki/Procambarus
a picture of grassland crayfish hole at this blog:
https://natureinquiries.wordpress.com/2015/04/21/mayslake-marsh-update-amphibian-traps/
also nice series of descriptive pictures of Crayfish and burrow:
http://hootowlkarma.blogspot.com/2014/08/cajun-crayfish-invade-nc-museum-of-art.html
This particular link seems to be more in keeping with the holes David found near water.

a picture of a similar hill made by a Crayfish see half way down the page:
http://www.georgialifetraces.com/tag/sapelo-island/
Now, at the same page, if you just keep reading from the Crayfish hill on down, you'll see reference to Star-nosed Moles being at least partially aquatic. The author says: " . . . star-nosed moles are also more comfortable next to water bodies and seek underwater prey".) I've read that in other places.

if Star-nosed Mole, see:
RANGE MAP: http://home.cc.umanitoba.ca/~campbelk/starnose.html

INSET: http://news.nationalgeographic.com/news/2005/02/0202_050202_mole.html

HILLS made by star-nosed: http://msue.anr.msu.edu/news/moles_in_the_lawn

HABITAT: https://bioweb.uwlax.edu/bio203/s2014/fisher_cody/habitat.htm
Can be found in a dry meadow occasionally, but prefers "land that lies alongside a body of water and has little drainage capabilities; this could be a river, lake, swamp, or any other presence of water."

NOSE: http://www.eurekalert.org/pub_releases/2005-02/vu-mmg013105.php
"The 22 appendages that ring its nose have a much larger surface area than the sensitive area of an ordinary mole nose. The flexible fingers also allow the star-nose to tap objects in its environment at a faster rate. These advantages mean that the star-nosed mole can find 14 times as many small prey animals in a given period of search time than its close cousin, the eastern mole, the researchers calculate.