EPDM is made out of a multiple polymer combination that lends to it's high durability and flexibility. EPDM liners are flexible and strong, but sometimes vulnerable to oils and solvents. However, a typicaly backyard pond won't need to worry too much about these factors.
EPDM's relative softness and flexibility make it ideal for oddly shaped ponds, bending around curves and corners with ease, unlike HDPE, which permanently folds when a crease is created.
