A drawdown is the lowering of pond water, or complete dewatering, for repairs, dredging, weed control, or fish or other crop harvests. A partial drawdown of several feet is a popular technique for control of unwanted shoreline vegetation and algae. Nuisance weeds can be manually removed when the water level is lowered, and the drying effects of sun and air help kill off submerged vegetation. In the North a drawdown that extends into winter months allows frost to penetrate the soil and kill pond weeds. Care should be taken not to expose piping to possible frost damage, or clay liners to dehydration, cracking, and leaks.
The benefits of a deliberate drawdown can sometimes be duplicated during droughty conditions, when pond water levels drop naturally, and weeds can be removed manually, and shore areas raked and cleaned. If possible, water inflows should be cut off during natural or deliberate drawdowns.
Drawdowns can be accomplished using drainpipes built into the pond structure. If there are no drains in a pond, pumping or siphoning may be necessary. Water should be released gradually to prevent downstream erosion. Drainpipes are usually about 4 inches in diameter, even when coupled to larger pipe spillways (risers for instance), to prevent downstream erosion during a drawdown, partial or complete. Some pond owners find that where the downstream grade is steep enough, siphoning with a flexible plastic pipe, allows them also to suck up accumulated sediment during the drawdown. A trash pump or other pump might accomplish the same objective. (Vacuum pumping up vegetation and sediment even in full ponds is gaining in popularity, because it may avoid the need for a drawdown.)
Drawdowns were originally used by farmers and aqua-culturists tending multiple ponds to harvest a crop and then let the pond lie fallow for a time, allowing the antiseptic effects of sun and air to rejuvenate the basin. Exposed to the air, previously anaerobic sediments could decompose, parasites (leeches for instance) die, and aquatic vegetation could be cleaned up and grass crops grown in the rich pond soil. Additionally, volunteer plants or cultivated crops produced seeds that attracted waterfowl for hunting when the pond was refilled. Pond rotation also allowed fish farmers to raise and harvest fish of one size or species in each pond, without mixing fry and mature grades, or different species.
The negative side of the drawdown is the loss of cultivated and natural life. Many pond owners who plan drawdowns as part of a cleanup regime are concerned about losing stocked and wild fish, frogs, salamanders, turtles, and other critters, as well as aquatic plants. Prior to a complete drawdown, fish should be caught out, or seined and transferred (unless, of course, they are being intentionally eliminated). I also recommend gradual drawdowns to allow pond critters to find another habitat. Keep in mind that even a partial drawdown may stress fish enough to kill them.
From Tim Matson's Earth Ponds A-Z, An Illustrated Encyclopedia. Click here to get your copy.
Topics vary from month to month and provide great information when researching where you want to build a pond, or how to keep it clean. Other areas of interest are pond use for more than swimming, and how to keep fish happy in your pond.