First things first, carefully excavate a hole somewhere in the basin to get through the accumulated muck and check for a liner that already exists. You don’t want to add new holes so be careful. A small pond like that is likely to have a liner that has failed due to age, and hidden due to thick muck. What that will tell you is if the soil has any chance of sealing or not, and how much work it will be to restore or improve it as the old liner will need removal.

IMHO a pond this size and smaller requires a liner unless you have a very high water table or natural water source that never quits. A nice heavy duty felt protection layer and thick UV protected liner is a worthwhile investment, especially if you are filling in a dry climate area. It may pay for itself over time in water cost differences. Even in our normally wet climate I must top off our lined pond a few times a year due to evaporation. Ours is 20x18 before lining with rocks and plants.

If you decide to line it, and have some elevation to your advantage, install a bottom drain. I wish I had as cleaning it out is a chore without one. What is nice with a liner is it does allow a partial drain and clean out periodically. Natural liners are not nearly as easy.

There are a lot of resources for designing and building lined garden ponds. I find the hobby incredibly rewarding, as I did my project myself with the help of a neighbor’s excavator.

Best of luck and please keep us posted on what you choose to do!