Beyond the superficial benefits of recreational use, retention ponds often serve a vital role for the surrounding landscape as reservoirs that help manage and disperse excess runoff from heavy rains. These ponds are designed to efficiently collect water from a wide swath of land and then, as the name suggests, retain it indefinitely. Building a successful retention pond is much the same as building any pond, but more care must be given to the feeder streams. Additionally, you'll want to work more closely with your county or city land management office to ensure you don't adversely affect the surrounding water table.
Position the retention pond at a strategic low point in the area it will draw from. Gravity will be your best ally for speeding away runoff during wet times, so you'll want to put the pond in a spot that receives the full measure of that benefit.
Dig the pond prior to digging the ditches. Otherwise, you may end up with a soggy workspace that is difficult to seal and prepare properly before the job is done.
Use the backhoe to remove dirt from the center of the pond and position it around the edges, forming the pond's banks. The depth is up to you, but you'll want to make sure the pond is capable of handling the amount of rain runoff that the surrounding land will produce. Additionally, you'll want a minimum depth of 12 to 15 feet for keeping fish, if you desire.
Determine a location for pond water to exit when water levels run high. For this purpose, you may want to build the banks up higher than you actually intend them to go, and then leave a gap where you plan for any excess water to escape. You'll need to create a path leading away from the pond toward a creek, stream or river. You'll also need the local government's permission to do so before you introduce a new tributary to the local water table.
Spread heavy clay soil throughout the pond bed so that you have at least a 1-foot thickness throughout. Use the heavy machinery to tamp down this clay layer so that it is for all practical purposes seamless. This will ensure that the pond is capable of retaining water indefinitely.
Dig your feeder ditches now that the pond is ready to hold runoff. Create two or three large troughs that run the length of the land, ending in one unified pond tributary. These troughs will be fed by smaller ditches that usher rainwater away from areas where it would otherwise collect and stagnate.