July 2018


Advancing the topic of improving pond water, let's consider the role pond shape plays in water quality. A key element to keep in mind: shallow ponds are likely to encourage conditions that led to unhealthy low dissolved oxygen levels, too warm water, algae, and unwanted pond weeds. Last month we covered water analysis and water conditioners. Now we'll look at overcoming problematic pond shape.

Most well functioning ponds are 8' deep or more. 12'-16' plus is good. Shallower depths will warm up more readily, which can be unhealthy for fish, encourage low oxygen levels due to excessive weed and plant growth, and allow more sunlight to reach the pondbed and stimulate photosynthesis and unwanted plant growth.

If you're stuck with shallow water, there are ways to overcome the negatives of shallow depths, mostly with mechanical aeration (usually diffusion is best), and/or infusions of cool water. If when adding water you design it to splash in, all the better for its oxygenating action.

But first things first. What causes a pond to be too shallow? First, it may have been built to less than best practice standards. Eight feet in deepest central area is cutting it close, and less is asking for trouble. Too warm water, low oxygen, and tendency to grow algae and weeds may be the result, if not immediately then not far down the road as inflowing sediment and decaying organic matter shallow out the pond.

Another design problem that can lead to poor water quality is shallow edges. At the shoreline, slopes into the water should be 3/1 or 2/1 if stable (well built and compacted). Swim entries and beaches can be flatter, 4/1 or so, but aren't usually very expansive, and may incorporate a geo textile foundation in addition to sand to help resist weed growth. But large areas of shallow edges will warm up faster than deep areas, encourage weeds and algae and invasive plants like cattails and rushes (sometimes welcome, sometimes not). Observant pond owners will note that as a pond deepens off shore, weeds and aquatic plants tend to diminish.

Another design situation that can cause poor water quality is an eroding inflow stream. If the pond is fed by a stream of significant size it will often bring in sediment, slowly over time, quickly in big rains and runoff situations. This adds to the pond's shallow area at the inflow and even further in, and can trigger turbidity throughout the entire pond during rain/runoff events.

Aside from routing the stream around the pond, sediment capture methods can be used to reduce sedimentation. Silt pools are often constructed in the stream and designed to capture sediment before it gets to the pond. These pools usually need a periodic cleanout because they can fill up pretty fast.

Problem shallows caused by poor construction and/or sedimentation are often fixed by excavation. The pond is usually dewatered and dug out, creating a deeper basin and steeper edges. But this can be an expensive big deal, especially in a big pond. Perhaps excavation can be limited to certain areas favored for swimming, and where the shallows are worst. One alternative to excavation is suction dredging.

Another way to improve water quality, as well as avoid expensive excavation, is aeration. There are several types of aerators: some that splash water in a concentrated area at the surface; fountains; even paddlewheels. But for most multi- purpose recreational ponds, diffusion aeration is generally considered most effective and economical.

Simply put, a compressor blows air through a flexible tube into the pond bottom, where it bubbles up, adding oxygen to the water, creating some circulation, and destratifying the water to help reduce high temperatures in the top layer of water. Fish like aeration (much like they do in a home aquarium with bubbler), and so do some types of bacteria which are energized to help decompose organic matter (sediment) on the bottom. Added oxygen also encourages decay of organic matter. This helps prevent shallowing out of the pond, and slowly over time can actually restore volume to a pond that has lost depth to sedimentation.

Diffusion aeration can be powered by grid electricity, solar power electric, and even wind turbines.

There are many companies offering various aeration systems such as Clear Pond, Outdoor Water Solutions, Kasco, Vertex, Clean-Flo and more (see links page).

In order to set up an effective aeration system, it's helpful to be able to describe the pond accurately to the aeration outfit. This may require some pond mapping. What's wanted is a drawing of the pond perimeter, including rough length and width, inflow(s) and outflow(s), and depths. Depths can be measured by going out on the pond in a boat, and using a measuring pole at regular intervals to take soundings. This can be done by drawing a line along the middle of the pond map, and then parallel lines half way between the mid line and shore. Using a measuring pole (1" pvc pipe works well, marked in one foot increments; put a block on end to prevent poking into mud) measure depths every 8-10 feet, and mark on your map. These soundings will help determine the size of compressor needed, and layout of underwater line(s) and diffuser(s) placement. In a pinch a long kayak paddle can be used for measurements.

For small ponds needing one diffuser such elaborate measuring may not be necessary; central depth, length and width may be enough. Bigger ponds may need the more extensive mapping.

Take a picnic and make an adventure out of your mapping day!

Read More Pondologies

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.  

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