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Water Has Made This Landscape
One need only look at the geology to understand why a channel in Mid West WA will put in for the Indian Ocean or else peter out on a sand plain, salt flat or some wide inland depression. The rivers are what they are because of old faults, ancient rock, deep weathering and the likes of rainfall which can come on you in an instant and be gone for months. Groundwater has its part to play too.
Anna Marlowe has put together this guide to the Murchison, Wooramel, Greenough and Irwin Rivers and the principal geological settings of the region. There is a straightforward lesson to be had: do not mistake a dry bed for a river that has failed. More often than not it is an intermittent-flow system behaving in accordance with the climate and the rocks.
The Rock Story Of The Region
You will find two distinct geological provinces in the Mid West. On one side are the crystalline rocks of the Mid West Minerals Province, older stock; on the other the younger sedimentary varieties of the Northern Perth Basin. It is a contrast that produces varied erosion patterns, aquifers and river valleys in the space of a short drive.
It makes a difference when you are on the ground. Resistant basement rock will see a channel make a narrow, stepped valley of itself, whereas unconsolidated sediment or sandstone is likely to let a channel open up into a floodplain. The rocks are more than scenery, they dictate where water is to pool, sink or go. This same process helps explain ancient canyon formations.
Basement And Basin
In the Mid West Minerals Province the rocks are very old, having been subjected to folding, faulting and uplift and weathered to a depth over the course of eons. Granite, gneiss and greenstone are among the metamorphic and igneous units you might encounter, though it varies by district.
Then there are the sedimentary rocks to the north and west from the Northern Perth Basin, layered deposits of siltstone and sandstone. With their porosity and bedding they have a say in how the surface drains and what the groundwater holds. You get a patchwork of it all, not a neat and tidy model for the regional rivers.
A Question Of Drainage
There is exorheic and endorheic drainage in the rivers of Mid West Western Australia. The former will discharge to sea, the latter to some inland depression where the water evaporates or is left in a salt-affected landscape.
Some would have you believe a river has to run to the coast without let up. That is not so here. Local relief and seasonality are in control. A channel may be at one with the sea in time of flood but is a dry, disconnected corridor when the season turns.
Coastal Versus Inland
Take the Murchison River, an exorheic system that puts in to the Indian Ocean at Kalbarri. The Wooramel is of a mind to head for the coast, but smaller affairs can lose themselves in alluvial fans and sandy beds before any outlet presents itself.
There are old salt lake systems about that bear witness to what was, but do not think the present day rivers are treading the same path. Uplift, basin shape and erosion have seen to it that the direction and efficiency of drainage is altered time and again.
Three Catchments For Comparison
To get to know the geology of the Murchison, Wooramel and Greenough catchments, compare them. They each have their own way of balancing the rock, the gradient, the rain and the coastal connection.
My first impression from the maps of Western Australian rivers was that the big ones would be like the English waterways I am used to with their permanent flow and green banks. The Mid West was quick to put me right. The map can be quite liberal but the water is choosy.
| River System | Geological Character | Water Behaviour | Points of Interest |
|---|---|---|---|
| Murchison River | Deeply weathered surfaces and mixed terrain with sedimentary sections | Flow is highly variable, particularly after a good fall of rain | Gorge incision and the coastal outlet at Kalbarri, floodplain deposits |
| Wooramel River | Broad alluvial surfaces and basin sediments in a low-relief catchment | Intermittent, with extended dry spells | Sandy channels, wide corridors and the influence of groundwater |
| Greenough River | Weathered uplands and sedimentary country | Intense rainfall will produce short-lived flows | Erosion at crossings, tributary junctions and the floodplain |
| Irwin River | Younger sediments and weathered rock in a coastal catchment | Driven by events and the season | The effect of land use and low relief on the channel and its sediment |
On Catchment Boundaries
By definition a catchment is the territory from which surface water is sent to a common outlet or channel. Yet you can have neighbouring catchments divided by little more than a few feet in elevation on ground that is as flat as a pancake. One will not often find roads, rail or property lines conforming to the natural drainage boundaries.
When interpreting the field, higher ground divides, tributary junctions and variations in channel material as well as floodplain width are what one should be on the lookout for. A gravel bed giving way without warning to fine sand is no random patch of riverbed; it could be a sign that the source rock, slope or transport energy has changed.
Faults, Weathering And Channels
Water will make use of the weaknesses faults and fractures put in place. In doing so they can expose rock to erosion, put a valley in its course or turn a tributary aside. One may not see the fault control from the ground, but an uncharacteristically straight valley or a channel making an abrupt turn is reason enough to have a look at the regional geological map.
Then there is the matter of deep weathering. In past geological climates the warm and wet would have worked over the bedrock far under where the surface is now. Subsequent erosion has stripped away some of that profile so the softer stuff is left in wide valleys and the stronger rock stands as ridges.
From Bedrock To Floodplain
Sand, silt, clay and pieces of bedrock are all found in a river channel. Steeper parts of the river will put coarse gravel in motion when flooding, low-gradient areas will lay down fine sediment on the floodplain. It is possible for a single flood to do both, burying one reach and deepening another.
Sedimentology makes sense of it: rounded gravel in layers is evidence of energetic transport, laminated silts and clays of more placid backwater. There is recent erosion to be seen on the cut banks, but they do not tell the whole story of a valley’s geology.
Groundwater Beneath The Channels
The hydrogeology of Mid West WA has a way of tying aquifers and porous sedimentary formations to the rivers above. Water can be moving under a dry channel to come up at a spring or stay in the sediment long after any flow is gone.
Take the Yarragadee Formation in the Northern Perth Basin, a key regional unit. The sandstone aquifer zones there will store and convey groundwater, though how it behaves is dependent on local structure, confinement, recharge and depth.
What An Aquifer Will And Will Not Tell You
Do not think of an aquifer as some subterranean river running orderly through a cave. Groundwater in most sedimentary basins is to be found in the pore spaces of the sand and sandstone; in fractured rock it takes the path of joints and weathered zones.
It is not as if groundwater levels mirror the river with any precision. Recharge can be uneven or slow, or only come after an odd rainfall. Add in the effects of pumping, salt, vegetation and evaporation and the connection between the surface channel and the aquifer is altered.
For the definitive word on regional mapping and geological layers the GeoVIEW.WA resources of the Geological Survey of Western Australia are the ones to refer to.
Kalbarri Gorges In Profile
At Kalbarri National Park the geology of a river is plain to see. The Murchison River Gorge has been around longer than the current flow of the river and shows its layered rocks and structural flaws for what they are.
Geologically speaking the gorge is a product of what was there to begin with and of erosion that has not let up. The river did not just carve it out in one go; over time it has made use of the weaker beds and fractures and of floods, with uplift and drainage changes providing the necessary gradient for incision.
Ancient Canyons And Modern Access
Call them ancient canyon formations if you like, but the gorge is no static relic. Rockfalls alter the slopes and floods put sediment in motion, the weathering goes on as it always does.
With a day to spare it is better to make for an accessible viewpoint and a signed lookout on the gorge than to try and cover every track. Factor in time for the heat, for photography and for the odd uneven surface. A car is generally the sensible way to get about on your own, a guide will take the strain of navigation off you but you may not linger as much.
As for walking conditions and whether a track is open or where to park, those things are subject to change so see what the park has to say before you set out. Take more water than seems necessary and do not be fooled by a dry gorge into thinking there is little risk; steep edges, loose rock and the vagaries of the weather are a tough mix.
Flow, Floods And Landscape Change
You cannot put an ephemeral river in Western Australia on a calendar. What matters is the intensity of the rain. A few weeks of a light shower might not amount to anything in the channel whereas a storm of short duration can send a flood pulse along.
In the wet season a traveller is liable to encounter flooded crossings and tracks made treacherous by quickly changing circumstances. When it is dry the isolation and lack of services are the issues, as is the heat and the allure of the deceptively harmless riverbed to put in more miles than intended.
- A few things to put in order before you head out: any weather or park warnings, the state of the roads, whether there is fuel and mobile coverage to be had.
- Once on site one should have at hand a supply of food and drinking water, sun protection, a fully charged phone, something for navigation in case of failure and footwear to stand up to the terrain.
- Do not be tempted to camp or put down in a low channel, a hollow in the floodplain or a drainage crossing; it may appear bone dry but that is no reason to do so.
- If the trip is to be a long one in the remote, let someone who is not with you know your plans and when you will be back.
Expectation Versus Reality
Expectation: You might expect a river to present as an unbroken blue ribbon. Reality: In reality it can be a dry channel, a string of pools or a wide gravel corridor biding its time for some heavy rain.
Expectation: The gorge is more than a pretty cliff to look at. Reality: What you are seeing is a cross section of rock strength, bedding, uplift and erosion that can be read for what it is.
Expectation: And while a four-wheel-drive is no guarantee of an easy route. Reality: The vehicle’s clearance will not get round soft sand, heat, poor preparation or closures. Do not mistake mechanical confidence for common sense.
Maps to put you right
GeoVIEW.WA will serve you well to put geology, topography and mineral occurrences in perspective. Put the geological data over satellite imagery and drainage lines and see how the rock boundaries relate to the valley orientation and tributaries.
There is still a place for the printed map in the bush. You cannot rely on a screen that might crack or a battery that decides to sulk at an inopportune moment, nor reception. When doing fieldwork of any seriousness, have a regional map to hand and make sure you are not on pastoral, Aboriginal or conservation land without permission.
A checklist for the field
In reading a river landscape, go about it this way:
- Make note of the catchment and outlet.
- Is the drainage coastal, inland or disconnected?
- See if the channel runs in line with any mapped faults or geological boundaries.
- Note any variation in the width of the valley or in the bedrock, silt, sand and gravel.
- A dry channel needs to be put in context of recent rain, evaporation and groundwater.
Some answers to common questions
The following should clear up the kind of misconceptions that tend to come up in discussion of Mid West rivers. A short answer is all very well but the rocks demand more of your patience than a tourist would give.
Are they permanent?
For the most part not. Flow is a matter of rainfall, the size of the catchment, soil and groundwater, and the vagaries of the weather. Some will be dry for a good while.
Will every river make it to the sea?
Not necessarily. An exorheic river will drain to the coast but endorheic systems can end up in a salt lake or simply evaporate. Then there are channels that only reach the coast in a major flood.
What accounts for the depth of the Murchison Gorge?
Over time the Murchison has made use of structural pathways and weak rock. Add to that the effects of repeated flooding and weathering and you have the gorge as it is seen at Kalbarri.
The Yarragadee Formation a river?
No, it is a sedimentary unit. Under the right hydraulic conditions its sandstone layers act as an aquifer for the passage of groundwater.
Open to beginners?
They can visit provided they stick to the signed tracks and lookouts and are prepared for the isolation and the heat. But a casual visitor with no experience in emergency or vehicle planning would be ill advised to try the unsealed tracks and the gorges.
For the traveller
Rivers in the Mid West are as much a function of deep time as they are of rain. The Wooramel, Greenough, Irwin and Murchison show the combined effect of climate, fault lines, aquifers and the ancient basement terrain on Western Australia’s drainage.
Have your map and respect the weather. Allow for a track to be rough as guts and read the river for the geological record it is. Even if it is dry it has a story to tell.