Mangroves encroach on historic wall

LINDSAY TITMARSH

BEING an historian and an amateur naturalist is a great hobby.

I concentrate on the history of the Mary and Susan Rivers, as well as anything to do with nature in that area.

It is very rewarding to visit a location, sometimes one that no one has been to for many years, and to get a story on something special.

I have set myself the following set of strict rules on recording history. Never exaggerate or write incorrect information in an attempt to make a story more interesting.

If not sure on something do not just have a ‘wild’ guess at it – check it out from records, or with someone who may know more than oneself.

Maryborough’s most experienced historian, Tony Clift, has been of great assistance to me on numerous occasions.

When travelling up a creek or river never presume that the stream banks are in the same location that they were many years ago. Do not hesitate to spend time walking to a location in the bush, or to get into river mud in the mangroves.

Recorded dates can sometimes be confusing, but locations and basic facts on a subject are nonnegotiable, they must be ‘spot on’.

The following story has been produced to correct inaccuracies and misinformation recently written about the Dundathu Wall.

By the end of the 1800’s, Maryborough was one of the major industrial and immigration ports on the east coast of Queensland.

The very survival of the new township depended on the Mary River being navigable to large vessels.

It was noticed that one section of the river was silting up, making navigation difficult.

This location was downstream of ‘Devil’s Elbow’ at Dundathu, which was then, and still is, the deepest section of the river between River Heads and Maryborough.

Below the big bend, the original river increased in width. This allowed floodwaters (and to some extent tidal flows as well) to flatten out and slow down, resulting in new deposits of silt, sand and gravel being dropped in the already shallow area.

It was suggested that a large rock wall positioned at the beginning of the widening section of river, would reduce its width, thereby increasing currents.

The wall did not run parallel to the riverbank – one such as that would do nothing to divert water flow. It may now appear to do so, but a walk in the mud through the mangroves will reveal that the wall originally started behind the mangroves at the original riverbank. It was constructed at a downstream angle, and when about 100 metres directly out from the shoreline, it then ran parallel.

The aerial view (pictured) shows how the wall replicates the riverbank on the opposite side of the river, thereby keeping the stream almost parallel.

After the wall was built there was almost no tidal current in the semienclosed impoundment behind it, resulting in much of that area quickly silting up. This condition is still ongoing today.

Since the aerial view was taken in 2006, most of the area has now filled up. In not many years from now the complete area behind the wall will fill in resulting in the healthy mangroves which are already enjoying the deep river mud, extending all the way out to the wall.

The ’Dundathu Training Wall’ as it is called, was constructed in 1902 by contractor Ned Armitage, who operated river punts (barges). His tugboat the ‘Geraldine’ was used to manoeuvre the barges on site.

A large amount of rock was required for the project. Almost everyone knows there is no very large rock quarry situated on the riverbank beside the Hervey Bay Road at Dundathu.

Some motorists may be unaware that the Mary River flows only about two hundred metres away from the road at that location.

The quarry is located three kilometres downstream from the road, on a farm owned by very good friend and neighbour, Gary Hansen. I am very familiar with the quarry, having been there numerous times.

The wall builders were fortunate in that the only place suitable rock was available on the complete length of the river between the township and river mouth, was on the high riverbank a few metres from the start of the wall. How’s that for luck?

The soft rock was blasted out, then knapped (hammered) down to man lifting size. A heavy wooden chute was constructed from the high quarry to the riverbank, a short distance below. It is suggested that rock was slipped down the chute to land on the riverbank below, from where it was loaded onto the flat-topped barges.

Another suggestion is that the chute overhung the steep riverbank, allowing barges to manoeuvre beneath, and so load rock directly onto them. The bottom end of the chute was supported by two large posts positioned near the steep riverbank - they are still there today.

I am not sure which method was used, but direct loading would have saved much hard labour. The deck of the barges had no sides, making it easy to just roll the rock off.

Prior to the start of construction, long posts were driven into the riverbed along the planned route of the wall. The spaced intervals of these allowed barges to be tied fore and aft, to prevent them from drifting around during unloading. A few of these still remain in place. Total length of the wall is approximately 800 metres, with 8-9000 cubic metres of rock used.

Although well over 100 years old, the Dundathu Wall is still serving the purpose for which it was built.

As is the case with most infrastructure built in marine areas, the wall has severely impacted on the riverbank bordering the Hansen farm.

Instead of having beautiful high rocky cliffs beside the Mary at that location, we now find them smothered by a massive mangrove scrub almost 100 metres deep.