3 Geomorphology: Arckaringa Creek
3.6 Processes: River Behaviour and Habitat
This section describes the river behaviour at various flow heights. It should be noted that detailed work on flow conditions and sediment transport of rivers in the Neales catchment remains to be done. The descriptions below are not design parameters for engineering works. Some hypotheses on habitats and ecosystems are proposed, but there has not been scope within this project to engage in cross-discipline discussion with ecologists.
No-Flow Stage
Free surface water will be retained in some channel segments which are relatively large and deep. However, there are few waterholes deep enough to retain water for long, and
significant fish populations are unlikely to establish.
The river valleys are the most important terrestrial habitat in the Arckaringa catchment;
hillslopes are sparsely vegetated to almost bare. The only non-fluvial habitats with the possibility of productivity are the gilgai swamps, which are mostly small and located along the catchment edges.
Hyporheic water supports riparian and floodplain trees and terrestrial habitat; however, the alluvium, where present, is usually thin. It is likely that subsurface reserves of soil moisture are only available for a short time, in comparison to the Finke River which will have deep groundwater reserves for trees to draw upon. This suggests that in Arckaringa and Lora Creeks, deep-rooted vegetation may not have much of an advantage over shallow rooted vegetation.
Localised high-intensity rainfall from a single convective thunderstorms is likely to play an important role in delivering sediment from the hillslopes into fluvial transport, and promoting scarp retreat and headwards drainage extension. It may deliver flashy floods into the main Arckaringa drainage axis, but is unlikely to generate flow in the main creek channels downvalley of the Arckaringa/Lora confluence.
Low-Flow Stage
Small flows are unlikely to achieve much direct geomorphic activity. In the anabranching reaches, low water levels (where water is confined to channels) will not have enough stream power to entrain and transport the coarse sediments. In the anastomosing reaches, some bank erosion and transport of silty sediments may occur. However, consolidated fine
sediments are difficult to entrain, and at low flows some degree of antecedent moisture may be required for bank erosion to occur. In the absence of sediment transport, flowing water is likely to be clear.
Low flows will refresh moisture levels in channels and their adjacent floodplains, supporting terrestrial habitats. This is not only important ecologically, it is an important
geomorphological process since floodplain vegetation is a key element in fluvial processes.
Across the Neales catchment, hillslopes of different runoff coefficients will deliver rainfall into the fluvial system with different degrees of speed and intensity. Where high-intensity local rainfall coincides with a flow event, the runoff coefficient of the hillslope that delivers the water to the creek will determine whether the pulse of water is flashy or sustained.
High-Flow Stage
At high flows, channels and floodplains will be inundated, and will be actively transmitting water at fairly high to very high energies. Many parts of Arckaringa and Lora Creeks will experience flashy flow: brief but intense flood peaks, with very rapid build-up towards the peak and very rapid waning flow thereafter. Water will infiltrate into floodplain sediments, benefiting the terrestrial ecosystems.
Sediment transport is episodic, and is likely to occur in two phases: silty sediments
transported during most of the flow event, and in-channel bedload transport during the flood peak. It is not clear from this investigation what size of flow event is required to mobilise most of the river's bedload, or the depth to which the bedload will be mobile.
Bedload sediments will mostly travel within channels, but may be swept up onto bars and floodplains in high-energy locations or during local turbulence. Scouring of the channel base by moving bedload is likely to occur in high-energy locations. During waning flow, pulses of bedload are stranded at various points down the drainage axis. Pulses of bedload moving down-channel may alter stream configuration, clearing a channel in one section while filling a valley downstream with gravel (forming a riffle reach).
Fine sand and silt will be transported as suspended load, to be deposited over the floodplain.
The two most common mechanisms for fine sediment deposition will be
• in the anastomosing reaches and in riparian zones, deposition as flow is slowed through dense vegetation;
• in reaches with wider floodplains, especially those downslope from valley constrictions, deposition as flow spreads out and becomes shallow. Pulses of stronger flow will transport small pebbles across the floodplain.
The small size and scarcity of "waterholes" in Arckaringa and Lora Creeks is due to the constant bedload transport down the channels. It is hypothesised here that the presence of bedload during flow events dampens turbulence, reducing the likelihood of the
macroturbulent scour which is responsible for waterhole formation. Few wide, deep channel segments will be created; where they do occur they will not be as big as those of the Neales River. Most importantly, bedload fills up the channels, so any waterhole-shaped channel segment is unable to maintain space for free water.
In the anastomosing reaches, incision and erosion may create small channels, and a combination of sediment deposition and locally erosive flows is likely to promote channel relocation. Abandoned channels will almost immediately be filled with transported sediments.
Sediment levels under transport in the water will probably be variable spatially and
temporally on a small scale, as small pulses of sediment released by erosion or avulsion are likely to be trapped by vegetation and redeposited nearby.
Very High to Extreme Flows
At very high to extreme flows (century to multi-century recurrence interval), bedload
transport volumes will be very high, and the bedload will be mobilised to a greater depth than in less extreme flows. The channel base is likely to be scoured along a great percentage of its length. Flow strength may be sufficient to generate macroturbulent scour despite the sediment load, initiating new waterhole segments. Channel avulsion may take place in the
anabranching reaches. Large-scale changes may occur in the anastomosing reaches, with many channels relocating or being created, and possibly including widespread destruction of the vegetation. Scarp retreat in the higher-order valleys, headwards drainage line extension, and gullying and widening of the lower-ordered valleys are likely to occur.