North Australian FISH FINDER forums

The following is verbatim from the 156 page document from CDU.

Executive summary

Charles Darwin University study into the bank erosion of the lower Daly River. April 2016-May 2017

Recommendation 1: The lower Daly River should be recognised as an actively eroding river. Considerations should be given to the life time of expected river side infrastructure (e.g. Wooliana Rd.) and suitable buffers or management actions employed.

Recommendation 2: Whilst river bank erosion is greatest in the wet season, increased boat traffic and wash appears to be a significant contributor to bank erosion in this region in the dry season and should be considered in future management of the river.

Recommendation 3: Further regular monitoring of bank erosion measuring pins and boat traffic numbers should continue. This could be competently conducted by the Malak Malak rangers, who have undertaken training as part of this project. This could be supported by CDU or another party for analysis and interpretation if required.

There will be interesting debate and outcomes from this report in the future.

I think alot of the Erosion is Natural, not caused by Boats.
I have seen Big Chunks of the Banks fall into the River, that can not be caused by Boats going Past.
I was above the Crossing, one Day, and a Large Chunk of the Bank just Broke off, and fell into the River.

When a Boat goes Past, You see a Cloud of Muddy water on the Edge.

A Thousand Boats going Past, is not going to cause a Big Chunk of the Bank to Fall in to the River.

I think it has a lot more to do with the Ground being saturated during the Wet, then Drying Out, or Less Saturated after the Wet.

When the Land Dries Out, during the Dry, it leaves a Crack in the Ground, which fills with water during the next Wet, and causes a weak Point.
Sooner or later, that weak Point breaks off.

The most Erosion, happens when the River is in Flood, and you don't notice it, until the River drops.

I am guessing, because I have not taken that much notice, but I would say, the Chunks of Bank, that I have seen fall into the River, and there has been quite a few, would be in the first 2 - 3 Months after the Wet.

At School in Adelaide, we were taught it took hundreds of years, for Erosion to occure, but up here, You can watch it happen.

I know this is a sore Point, but maybe Daming the Daly, might be the solution.

I am definatly no Expert, but I have sat in a Boat for Hours, watching the waves from Boats, continually hitting the Banks. Usually all you see, is Cloudy water formed along the edge. Every now and then, (Rarely), you might see a couple of Inches of Bank eroded away.

But, I have never seen a Tree fall into the water, because a Couple of Boats went past.

Just put in a bed and bed claim for the entire Daly. That will solve the boat problem and save the river

There is no doubt that boat traffic causes errosion. Any wave action will have an effect. But the river compensates for this and has done for thousands of years. Its called evolution. The sand will move, the banks will move, the depths will change from year to year and in the salt tidal sections from day to day in some cases. Cant say that boat traffic caused the island opposite clear creek to blow open! Perhaps the millions of gigalitres of water from a 15 meters of flood had some affect! No matter what we do the river is gonna change. As it gets longer the mouth will get wider. As it gets wider the mouth will get shallower, the upper sections will scour out, and salt water will force further upstreamas the banks become flatter and the rain forrest trees get taken over by mangroves.

The lower Daly River should be recognised as an actively eroding river.

True, easy to get that one right.

Whilst river bank erosion is greatest in the wet season, increased boat traffic and wash appears to be a significant contributor to bank erosion in this region in the dry season

True perhaps, is it? but in context dry season erosion is very different to wet season erosion so to say that boating traffic contributes to dry season erosion could be right but when you compare the erosion of boating traffic to erosion across the whole year and I think it'd be a much different story. The question would need to be asked is there any greater impact of dry season erosion on bank stability etc. compared to wet season erosion regardless of amounts of sediment removed from the banks.

Further regular monitoring of bank erosion measuring pins and boat traffic numbers should continue. This could be competently conducted by the Malak Malak rangers, who have undertaken training as part of this project.

Jobs for the boys?

I know this is a sore Point, but maybe Damming the Daly, might be the solution.

If you go back to the previous thread on the Daly you'll note Wonderwobbler mentioned tidal pumping, the movement and deposition of silt into the estuary by tidal movement from offshore. A dam wont stop this from happening but it'll stop the wet season flush that pushes it all back offshore only to be pumped back in again and pushed out again etc. Some years the tide wins the battle, some years the flush wins. As an example this year was only an average wet for Kununurra but below average up catchment. As a result Echo Rock is silted up, last year it was about 3 to 5 metres deep after the wet. next year the channel next to the rocks could be deep again if we have a good wet.

PS caught my first fish on a lure using a 3/8 oz wonder wobbler, it was gar fish and one treble was hooked right in the mouth, very few catches have rivalled that one for oddness.

Have a look at the attached and you'd think " Why WOULDN'T the Daly banks be eroded"?

Where the Daly widens (near the mouth) thus spreading out and reducing the flood flow, the river banks has an elevation of roughly 7m above mean sea level (yellow printing).


As you go up the river you gradually get to about 70m above flood level (white markers). Pretty gentle drop.

But look at where the short tributories rise to the sides. Short tributaries but high catchments of 200m or so. The tributary waters travel at very high velocity with such big drops in elevation over short distances.

Go further up and the catchment elevations rise further. eg Katherine Gorge area and its tribuaries rise at 300m elevations.

Considering the amount of rainfall there are tremendous amounts of water at very high velocity being injected into the Daly.

ALL of its tributaries contribute in the same way.

With a drop in elecvation of 70m upstream to 7m downstream, that's enough to maintain the velocities being pumped down the Daly even though its relatively gentle.

Upstream in the tributaries, iots rocky country and the banks don't get eroded.

Howeber, downstream where there'smuch less rocky country, the banks do indeed get eroded.

Look at the banks downstream of the Daly River bridge. At every slight bend, the outside bank is undercut and everything falls into the river. There's very little rocky country to prevent it.


So the river seems to meander by itself. But there's no meandering involved. That happens down near the mouth as the velocity drops and the water spreads out.

Wash from boats causing it? No.

Wash from boats contributing to it? About as much as Aus contributes CO2 to the great climate change rort.

Jobs for somebody to monitor it? Yep and about as useful as somebody watching grass grow or Nissans rust.


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Some good points there Ronje. On the topic of wash, who are the goons who slow down to just below planing speed to pass you? That's when the worst wash happens