Dams to Wetlands: Part 1 - What Makes a Good Dam?

Dams on farms have traditionally been used for farming practices - primarily stock watering and crop irrigation. As such, they were often placed on the land where these needs were best met or where they could easily be fed from a water source.

Nowadays, with a growing number of people looking for lifestyle improvement on their rural properties, dams have become a critical factor in both the aesthetics of the property and in supporting native flora and fauna. This means that health, position and appearance matter far more than in the past, but that traditional placement and management may be at odds with our new-found love for nature.

This is the first of many (I hope) blogs on the process of transforming dams to wetlands. In this edition, I want to cover some of the essential criteria of dams that make them well suited to being wetlands. However, I should point out that any dam can be a wetland if you want to make the effort!

Dam zones

A dam consists of several zones that each contribute differently to the ecology. Each zone requires a different approach to planting and maintenance with the goal of making the dam as healthy as possible.

Catchment zone

This is the surrounding paddock. The area where rain will fall and run in to the dam. This area needs to be grazed to a minimum, as the plant life and soil are critical for straining the run-off to keep chemical residue and sediment from running down in to the dam.

Buffer zone

This area is typically from the high-water mark out to 20 metres. From an ecology point of view, this is perhaps the most important area and needs to be fenced to exclude stock.

Once stock is excluded, this area will often naturally re-vegetate with plants that are well-suited, such as native trees, shrubs and grasses, however, some extra planting here can't hurt to give nature a helping hand.

One area of focus should be the north-west corner of the dam, which should be planted with groups of trees and shrubs to create both shade and a wind-break to protect the dam from evaporation on hot, summer days when the wind is blowing from the north west.

Another key function of this area is water filtration. Planting tussock-forming grasses in this area will help filter the sediment from the water, creating a cleaner dam, as well as stabilising the soil.

In this area, you can also add tree logs and other structural elements as habitat forming features that will help protect birds and and provide shelter for many animals.

Batter zone

This zone is between the low and high water marks. This area is nutrient rich and is best suited to planting native rushes, sedges and reeds. You will need to experiment with these plants as some might not survive water inundation when the dam is full, but this is dependent on your specific circumstances.

Permanent water zone

Fairly self-explanatory, this area is wet all year round.

Plants in this area can both filter the water and provide protection from wave action caused by the wind.

Location and permission

In Victoria, we can typically build dams and ponds up to 3 mega-litres without a permit, as long as they are not within a waterway or natural gully. A natural gully is not just a furrow on your land that feeds the dam, but a significant depression in the landscape that is part of the waterway system, even if it is dry most of the year. Check your local regulations.

The location for your dam depends on a number of factors determined by the use of the dam.

If your dam is for irrigation, then an elevated position is ideal as it allows gravity to feed the areas you want to irrigate, or at a minimum means minimal pumping heights.

You will need to consider how your dam is fed from the catchment zone. Some big dams are located in areas of minimal catchment and never fill, leaving a more unappealing view.

An alternative to a big dam might be a series of small, connected ponds. Tapping in to a catchment via a man-made "creek" can feed your ponds with the overflows connecting the chain. This can create a very aesthetically pleasing sequence of water-features that may be cheaper to create, fit the landscape better and provide a better habitat for the flora and fauna you are hoping to attract.

And again, this should be to be completed without a permit (but please get your own local advice on this).

Construction

One issue many landholders face is who will build their dam? Many contractors with diggers are great at digging holes, and may be able to construct the dam correctly in terms of sealing, overflow and overall soundness. However, not many will have the knowledge of where to place the dam and how that impacts on your usage, nor will they be able to make a proper dam, over just a hole in the ground

A key recommendation is to contact your local Landcare group, who should be able to put you in touch with an expert who can guide the process. They should be able to give advice on location, shape and size, based on your specific needs. They may also be able to connect you with an appropriate contractor that can do the job right. The consequences of a bad job are not worth considering.

The ideal dam

When it comes to the ideal dam for a wetland, we are looking for several key points.

The catchment, buffer and batter zones should have a gentle slope in to the water. This prevents rainfall from cascading down the hill at a high velocity, which in turn allows water penetration in to the soil and filtering to occur. This is, therefore, critical in preventing erosion and creating a healthy environment.

The gentle slope in to the dam creates the batter and buffer zones where the majority of wildlife will congregate, helping you achieve your goal of a wetland. Plants like rushes and sedges will grow in this area.

The dam should, in turn, not be a hole in the ground, like a pudding bowl, but be more like a... dam, with a wall on one side allowing water to naturally accumulate. Even dams in paddocks can be constructed with gentle sloping sides, even if they are on all sides.

While many dams are dug out like swimming pools to maximise water retention by minimising evaporation, many deep dams never fill as they are too big for the catchment area, leaving more of an eyesore than a wetland. Don't build your dam bigger than it needs to be - consider the cascading pond option instead.

If you want to swim in the dam, consider a floating pontoon to get out in to the water and avoid the shallow, muddy or planted areas.

Old dams and dam maintenance

If you have an old dam on your property that has been neglected, you might be faced with some issues that could cost you money in the future.

The number one issue is trees and shrubs growing on the dam wall. The roots of these trees can create air spaces around them, as they grow or die back, thereby creating escape paths for the water.

You can check if your dam is leaking by simply looking at the ground below the level of the dam. If it stays green during dry spells, you have a leak.

However, it is not advised to remove these trees as that will weaken the dam wall and may require completely rebuilding the dam.

To maintain the dam wall, simply pull out self-sown plants on the wall annually. There should be nothing more than ground cover on the wall.

There are some options to seal dams. New, high-tech, non-toxic polymers can be sprinkled on a full dam which then sink to the bottom and bind together. Products such as Watersave (I have no affiliation) are interesting options and there seem to be some success stories if you Google this.

Another issue is "guttering", where water is eroding the bare soil around the batter zone. Consider placing small rocks in the gutters that will help to minimise silt run-off while you get your plants established.


In the next blog, I'll talk about planting the dam and how we can get that wetland happening.