A seal of a pond or lake can be thought of as the foundation to a building; what happens at the liner affects everything above it. Skipping the addition of a liner may seem like a way to cut expenses initially, but leakage, erosion, and contamination problems may end up costing you far more than what you would initially save.
An unlined pond can leak several feet of water a year, making it difficult to keep your pond or lake full. Additionally, pond liners give shape and structure to your pond. As soil mixes with the water and the banks begin to erode, the pond may lose shape and depth if created without a liner. A liner also allows for the prevention of contaminants in your lake or pond in order to provide a healthy environment for plants or animals.