Sand mining: What a scientist says

A long-time advocate of applying science to the management of sand mining around Alice Springs, Rod Cramer, recommends as a guide the World Wide Fund for Nature report by Lois Koehnken, Impacts of Sand Mining on Ecosystem Structure, Process & Biodiversity in Rivers.

Mining of Roe Creek and Todd River has recently resurfaced with a clash between ecologists and the industry’s lobby.

The report says sand mining deepens river channels and the resulting depression can locally increase river slope. This will promote localised bed erosion due to increased river velocities.

Sand mining impacts on ecosystem structure, process and biodiversity in rivers [affecting the] relationship between sediment load, sediment size, stream slope and discharge.

Changes to any of these factors will cause a river channel to either erode or aggrade (experience deposition) to establish an equilibrium with the new conditions.

Each of these changes promotes local erosion that can propagate both upstream and downstream.

The upstream propagation of erosion from sand mining sites occurs due to the creation of a nick point in the river bed river: Turbulence created as water flows over this point causes erosion of the river bed, with the nick point retreating in an upstream direction.

This increases the slope of the river, resulting in an increase in water velocity during high flow events, leading to increased erosion downstream.

This process of lowering the bed of the river causes channel incision. Once initiated, the deeper and steeper river bed will cause an increase in water velocity, which in turn will increase river energy and erosion, creating a positive feedback for incision to continue.

Channel incision is frequently accompanied by channel narrowing. Due to the deeper, higher slope the river channel is able to convey water at a faster rate.

Rivers narrowed through incision may also become disconnected from their floodplains [which] are important habitats and provide a range of ecosystem services. Floodplain maintenance is dependent on episodic inundation. The exchange of water, sediment and organisms during inundation contribute to both in-stream and floodplain productivity, while simultaneously allowing groundwater flows to recharge.

Floodplains allow the river to spread out during periods of high water, and slow and absorb high flows.

This reduces both flood intensity and magnitude and hence limits their impact on downstream riparian (on the banks of a river) habitats and infrastructure.

Sediment deposition provides an influx of nutrients which are exploited by ecosystems and agriculture.

In rivers with deeply incised channels, greater water volumes are required before rivers overtop their banks and hence floodplain inundation occurs less frequently.

As a result, floodplains are no longer able to fulfil their important ecological and social roles.

As the erosion reaches the river banks, the bank toes can become undercut and destabilised, leading to bank collapse and ultimately channel widening.

Whether a channel will narrow or widen when subjected to bed incision depends on factors including the composition of the river bed, sediment supply and the flow regime.

In the case of braided river systems (e.g. a river with multiple channels separated by mobile islands), incision can lead to a fundamental change in the nature of the river system, with flow confined to only one channel as the channel deepens.

This change allows afforestation to occur in past river channels and gravel bars, decreasing erosion, accelerating the formation of stable islands, and contributing to the stability of the new single channel system.

The impacts of incision may spread beyond a river’s banks. By deepening the base of the river the banks and surrounding permeable areas drain to this lowered level, hence the ground-water level can also decrease, affecting groundwater availability and recharge.

The removal of sand from sand bars, known as bar skimming or scalping, can lead to bar erosion, local channel widening, and downstream erosion. The removal of gravel or course sand from bars exposes the underlying finer-grained sediment to erosion by high flows, thus causing a loss of the bar and increasing the flow capacity of the channel.

Additional channel widening can occur if the extraction causes river bank instability and collapse.

Over time, the increased capacity of the channel can reduce local water velocities and locally increase sedimentation, which can exacerbate downstream erosion due to an additional reduction in transported sediment.

Dry mining on floodplains may also alter the course of rivers. Such mining often creates a series of deep pits near river courses. During flood events these pits may be inundated, erode and become linked, forming an alternative channel that the river continues to flow through following the recession of flood waters.

These sudden changes prevent further mining in these locations, create stagnant lakes, and can increase bank erosion.

Floodplain mining that intercepts the water table also has the potential to alter ground water levels, if pits are pumped dry to allow access to the sand resource and contaminate aquifers.

Other physical impacts of sand mining that impact the physical condition of rivers include the creation of sediment laden plumes (clouds of suspended particles in water) during mining that move downstream and deposit in undesirable locations, potentially coating substrates and making them unusable as habitat.

Sediment plumes will also reduce the depth of light penetration in rivers. This is accentuated by sand mining generally coinciding with periods of low river flow when water clarity is naturally high.

A reduction in light will affect algal and aquatic vegetation growth. At a large scale, sand mining reduces the volume of sediment moving through the river and delivered to the river delta and coastal zone (in our case, presumably, affecting to floodout into the Simpson Desert).

In the absence of continued sediment deposition, deltas can erode and subside, reducing their capacity as highly productive agricultural areas.

The paper also sounds warnings useful in the debate about measures to prevent the Todd River flooding Alice Springs.

Dams, weirs, flood mitigation infrastructure, groynes (structures perpendicular to the shore), other water uses and land use changes all have the potential to impact river flows [potentially altering] both the flow regime and sediment budget of a river.

Large dams can modulate high flows and increase low flows, thus changing the sediment transport characteristics of a river system.

Dams trap sediment, with larger sediment sizes preferentially captured by impoundments (confining water).

This reduces the overall volume of sediment available for transport, starves the downstream river from specific sediment sizes and alters the range of sediment sizes available for transport.

Downstream channel incision, river bank collapse and change to the nature of the river bed are common changes below dams. These processes are very similar to those associated with aggregate mining.

UPDATE 18/12/25

Mr Cramer provided this update:

The river narrowing or widening from incision stuff might confuse some folk. Narrowing occurs in harder i.e. clay soils, while widening happens in sand, as in the photo.

That dead tree lying up on the shelf far left (note its roots are downstream) is the same one in the first two pics in the previous article.

We used to easily drive between it and the creek chasing Mexican Poppy. The one still standing just right of centre will go the same way. Note how little "topsoil" is above the sand.

One thing I should have pointed out to Tim Burrows is that dead rivergums generally take years, if not decades to fall after dying. I look at one every day here that was well dead when we arrived in '87, and it's still standing. Most of everything else he said was wrong, typical "shoot the messenger" stuff.