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River gravel consists of stones and rock fragments transported by rivers and then sold commercially as a gravel in landscaping. River Gravel forms when water flows over rock, gradually breaking the rock into pieces of varying sizes. River gravel can consist of a range of stone types, including igneous rocks like basalt and granite, sedimentary rocks like sandstone, and metamorphic rocks like quartzite. The mixture of stone types in river gravel depends on the geology of the river catchment area.
In general, river gravel consists of rounded stones between 2 mm and 64 mm in diameter. The stones have been transported and tumbled along the riverbed, giving them a smooth and rounded appearance. Larger angular stones are typically excluded from river gravel.
The size of the gravel depends on factors like the velocity and volume of the river’s flow, the hardness of the source rock, and the distance the gravel has traveled downstream. In general, larger rock fragments become gravel closer to where the rock first enters the river, while smaller particles of gravel form farther downstream.
The rounded shape of river gravel is a distinguishing characteristic. As the rock fragments tumble along the riverbed, they collide with each other and with other objects in the river. This abrasive action grinds off sharp edges and corners, resulting in the smooth, rounded shapes we typically see with river gravel.
Commercially sold river gravel for landscaping in Australia typically consists of gravel that has been sourced from riverbeds and then screened to specific size grades. The geology of the river gravel depends on the underlying geology of the river catchment area. Some common features include:
• Basalt river gravel – Many rivers in Victoria drain volcanic basalt plains, resulting in river gravel dominated by rounded basalt stones. This type of gravel is dark grey to black in color and is popular for landscaping due to its uniform appearance.
• Granite river gravel – Rivers draining granite terrain, especially in Western Australia and South Australia, often produce river gravel dominated by rounded granite stones. This type of gravel is light to medium grey in color with visible quartz grains.
• Sandstone river gravel – Rivers flowing across sandstone formations can produce river gravel with a mixture of sandstone and quartz stones. The sandstone stones are often red to brown in color. This type of gravel is commonly used for a more textured, colorful landscape appearance.
• Mixed lithology gravel – Many rivers drain areas with a mixture of rock types, resulting in river gravel with a diverse range of stone lithologies. This type of mixed river gravel can provide an interesting multi-colored appearance for landscaping.
Regardless of the geology and lithology, commercially sold river gravel undergoes processing to remove dirt, clay and organic matter. The gravel is then screened into specific size grades to suit different landscape applications. River gravel is a versatile, cost-effective and natural material for enhancing the aesthetics of gardens and outdoor spaces.
BROWN RIVER GRAVEL
Common reasons for brown river gravel in Australia include:
• Iron oxide cements – Many sandstones form under oxidizing conditions where iron oxide minerals act as the cementing agent holding the sand grains together. The presence of iron oxides often imparts a reddish-brown to the sandstone, resulting in brown river gravel.
• Organic matter – The decomposition of plant material within pore spaces of sandstone can leave behind organic compounds that stain the rock brown. This is a common cause of brown river gravel derived from sandstone in coastal regions.
• Clay mineral cements – Clay minerals like kaolinite that form during cementation can impart a yellow-brown to reddish-brown color to sandstones. This is often the case for sandstones formed under more humid conditions.
BLUE RIVER GRAVEL
Blue river gravel in Australia is most commonly composed of quartzite or metamorphic rocks like schist and gneiss. These rocks develop their blue color from the presence of certain mineral constituents.
Quartzite is a metamorphosed form of sandstone where the quartz cement has recrystallized into interlocking quartz grains. Blue quartzite typically forms when the quartz cement incorporates trace amounts of minerals like iron oxides or titanium oxides during its formation. These minerals impart a blue-gray to blue color to the quartzite, which is then reflected in any river gravel derived from the rock.
Metamorphic rocks like schist and gneiss may also produce blue river gravel in Australia. In these rocks, the blue color often originates from the presence of the mineral kyanite within their composition. Kyanite is an aluminum silicate mineral that occurs in blue, blue-gray and indigo hues. When kyanite-bearing metamorphic rocks are eroded and processed into river gravel, they give the gravel an attractive blue color.
Blue river gravel and brown river gravel are two types of river gravel commonly available from landscape suppliers in Australia. There are also cowra pebbles.
Blue river gravel gets its name from the bluish-grey color of the stones. It is composed mainly of rounded quartz pebbles that have been weathered and worn by river action. The stones range in size from around 5 to 20 millimeters in diameter with the most commonly commercially sold size being 20mm Blue River Gravel. Some suppliers also sell 30-60mm blue river gravel.
This gravel is primarily used for decorative purposes in gardens, paths, and driveways due to its attractive coloration.
Brown river gravel consists of more sedimentary stone varieties like limestone and sandstone. The stones have a brown or tan color and also range from 5 to 20 millimeters across, with the most commonly sold size being 20mm brown river gravel. Some suppliers also sell it in 30-60mm brown river gravel and the larger 40-80mm brown river gravel.
Brown river gravel is slightly cheaper than blue river gravel due to the materials it is made from. It is also commonly used for landscaping purposes where its subtle brown tones blend well into the natural environment.
30/60mm river gravel contains a mix of stones from 20mm up to 60mm in size. The larger stone size gives a bolder, more rustic appearance. Common uses for 30/60mm river gravel include:
Large landscape features like patios and retaining walls
Drainage layers
Gravel beds around large, established trees
Cowra gold pebbles have a distinct yellow coloration compared to the uniform white of cowra white pebbles. The yellow hue comes from the presence of iron minerals within the quartz pebbles.
Cowra white pebbles are a type of natural stone aggregate made of white quartz pebbles. They are primarily used as a decorative material in landscaping applications. Cowra white pebbles are prized for their rounded shapes, smooth texture and uniform white coloring.
Blue river gravel typically has a smoother and rounder appearance compared to brown river gravel due to being transported over longer distances. The stones have been tumbled and polished through movement in rivers and oceans. As a result, blue river gravel often exhibits a more consistent blue-gray color with less variation.
Blue river gravel is generally considered to be more durable and long-lasting due to the rounding and polishing of the stones during transportation. The rounded shape of the particles allows them to interlock tightly and resist displacement. The consistent color and grain size also help blue river gravel withstand exposure to weather conditions better over time.
The particle size of blue river gravel tends to be larger than brown river gravel. Blue river gravel is commonly available with grain sizes in the 20mm to 60mm range, while brown river gravel typically ranges from 10mm to 20mm, although commercially you can buy either from between 20-80mm. The larger grain size of blue river gravel gives a more coarse-textured, rustic appearance.
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River gravel makes attractive walking paths due its rounded smooth stones and contrasting color. The stones interlock with each other to form a stable surface that is easy to walk on. Blue river gravel or brown river gravel paths require little maintenance and can last for many years with occasional re-topping.
River gravel can be used to create decorative borders around gardens, trees and flower beds. For Blue River Gravel, the blue color provides an eye-catching contrast that helps define the edges of planting areas. River gravel borders are long-lasting and can be installed at various depths depending on the desired look.
A layer of River Gravel spread over a base such as bedding sand or crusher dust acts as an attractive, low-maintenance groundcover for a boat or car to park on. The gravel suppresses weeds, retains moisture and insulates plant roots. River gravel groundcover requires no fertilizer or regular watering once established.
River gravel can be used to create decorative borders around gardens, trees and flower beds. The blue color provides an eye-catching contrast that helps define the edges of planting areas. River gravel borders are long-lasting and can be installed at various depths depending on the desired look.
The volume to weight conversion factor for river gravel depends mainly on the density and porosity of the gravel material. River gravel typically has a density between 1.8 and 2.0 tonnes per cubic metre, meaning 1 cubic metre of gravel weighs between 1.8 and 2 tonnes. This range accounts for the following:
• Particle size – Larger gravel sizes will have lower densities since there are more air pockets between the particles. Finer gravel sizes pack more densely.
• Particle shape – More angular, irregularly shaped gravel particles will pack less densely than rounded particles of the same size.
• Compaction – More compacted gravel will weigh more per cubic metre since there are fewer air voids.
The porosity, or amount of air space between gravel particles, affects the conversion factor. River gravel typically has a porosity of around 30% to 40%, meaning 30% to 40% of the volume is empty space filled with air. This leaves 60% to 70% of the volume occupied by the actual gravel particles.
For an area of 5 metres by 2 metres with a depth of 10cm, you will need approximately:
Length x Width x Depth x Conversion Factor = Volume in cubic metres
5m x 2m x 0.1m x 1 = 1 cubic metre
Since 1 cubic metre of river gravel weighs approximately 1.8 to 2 tonnes, for your area you will likely need:
1 cubic metre x 1.9 tonnes/cubic metre = 1.9 tonnes
The river gravel can be spread evenly over the prepared road base or bedding sand to a depth of approximately 10cm and compacted lightly with a tamper or plate compactor. This should provide a durable and attractive river gravel surface for your landscaping needs. Read more about that here
The process of getting River Gravel delivered to your project site involves careful consideration of various factors, including your delivery preferences and requirements.
RUBBL is your go-to river gravel delivery partner. With three effective delivery options to suit all your landscaping needs, vehicle tracking, paperless invoicing, ordering, and receiving river gravel is a breeze!
There are other considerations to consider when ordering River Gravel, and RUBBL has them all covered. Factors such as site access, timing, and site preparation should be taken into account to prevent delays and complications. With the RUBBL App, you can specify all this information with ease, making your delivery incredibly simple and hassle-free.
Brown river pebbles are typically cheaper to buy in Australia compared to blue river pebbles. Several factors contribute to this price difference:
Demand - Blue river pebbles with their bluish hue are in higher demand for landscaping projects due to their aesthetic appeal. This increased demand drives up the price. Brown river pebbles are more common and in lower demand, resulting in lower prices.
Abundance - Blue river pebbles formed from rocks like quartz, granite and gneiss are less abundant in nature compared to the sandstone, shale and basalt that make up brown river pebbles. The relative scarcity of blue river pebbles also contributes to their higher cost.
Processing - Blue river pebbles often require more processing to achieve their smooth, rounded shape and bluish color. This additional processing incurs higher costs that get passed on to consumers. Brown river pebbles need less processing to achieve their natural brown hue and shape.
For best results, it is recommended to use a road base or compactible material such as bedding sand under river gravel, especially for high traffic areas. Placing river gravel directly on soil can cause several issues:
• Uneven settling - Soil has varying density and composition, which can lead to uneven settling of the gravel over time. Using a road base or compactible material helps create an even, stable base for the gravel.
• Weed growth - Weeds can grow up through the soil and into the gravel, making removal difficult. A road base or compactible material acts as a barrier to weed growth.
• Movement - River gravel placed directly on soil can shift and move around more easily from foot traffic and weather effects. A compacted road base helps keep the gravel in place.
• Ponding - Without a proper base, river gravel on soil can form low spots where water ponds after rain. A road base helps create positive drainage away from buildings.
For best results, it is recommended to lay down at least 4 to 6 inches of road base or compactible material like bedding sand. Crusher can also be used, however it may sometimes have its drawback that you can read more about here. Once the bedding sand is down, compact it with a plate compactor before adding the river gravel on top. This creates a solid, stable base that will ensure the longevity and performance of your river gravel landscaping.
Both blue and brown river pebbles are made of durable stone materials and will break down slowly over time from weathering and erosion. However, there are a few factors that impact the rate of breakdown:
The composition of the stones themselves. Quartz, granite and gneiss - which make up blue river pebbles - are among the most resistant and durable types of rock. Sandstone, shale and basalt that comprise brown river pebbles are slightly softer and less resistant, so they may break down at a slightly faster rate.
The porosity of the stones. Blue river pebbles tend to be made from denser, non-porous types of rock like quartz and granite. This reduced porosity slows the penetration of water that can cause weathering and cracking. The more porous sandstone and shale in brown river pebbles allow for greater water penetration and faster deterioration.
Exposure to chemicals and freeze-thaw cycles. All pebbles exposed to de-icing salts, acid rain and repeated freeze-thaw cycles will deteriorate over time. However, the slightly softer and more porous brown river pebbles made of sandstone and shale may be more susceptible to damage from these environmental factors.
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