Alluvial vs hard rock workings: how each shows up on satellite imagery

A flagged clearing on a satellite pass marks two things: bare earth where vegetation used to be, and a pond that's gone turbid. It doesn't say whether a dredge crew is working a gravel bar or an excavator is cutting a bench into a ridge, and for a field officer deciding what to bring on a check, that distinction matters. The two operations leave very different footprints on imagery long before anyone drives out to confirm it.

How an alluvial mine signature reads

Alluvial workings follow water. Point bars, oxbow terraces, old floodplain gravel, the pits and trenches sit wherever the crew can reach loose sediment near a channel. On imagery that means clearings that curve with the river rather than sitting on a grid. A trench dug this month might be gone or shifted fifty metres downstream by the next pass, because the crew works the deposit out and moves on.

The giveaway is the pond. Washing and sluicing need water, so you'll almost always see one or more ponds strung along the cleared strip, and they read turbid, a brown or grey-green that's visibly different from the clear water upstream or downstream of the operation. A single pond near a river bend with a narrow bare strip trailing off it is close to a textbook case. If the imagery shows several small ponds spread along a kilometre of channel rather than one large one, that's often a sign of multiple small crews working independent claims rather than one organized operation.

How a hard rock clearing pattern reads

Hard rock sits wherever the ore body is, on a slope or ridge regardless of what the nearest drainage looks like. The footprint is blockier than alluvial work: a bench or pit cut into the hillside, a haul road switchbacking up to it, and a waste rock or overburden dump pushed to one side that's often a different shade than the surrounding soil, pale clay or grey broken rock against green bush or brown topsoil.

Ponds do appear, but they read differently. A hard rock processing pond is usually smaller, more contained, sometimes lined or terraced into a series of cells rather than one open turbid pool in a river channel. And the site doesn't move month to month the way an alluvial working does. Once a pit is opened it stays put and grows outward, so change detection on a hard rock site tends to show an expanding bare-earth boundary rather than a shifting one.

Why it changes what you send

The equipment scale usually tracks the pattern. Alluvial sites are often small crews with hand tools, sluice boxes, or a small pump, which means they can pack up and relocate before a slow response arrives. A hard rock cut with a haul road and an excavator represents a bigger investment and isn't going anywhere fast, which changes the urgency calculus but not necessarily the response itself. Knowing the pattern before the truck leaves the office means the team isn't improvising once they're standing at the coordinates.

It also helps you sort out the false positives. A fallow agricultural plot or a recently graded access road can look like bare earth from a distance, but neither follows a river meander or a ridge-line pit shape, and neither comes with a turbid pond attached. Reading the shape and the water color together narrows down what's worth a field check and what isn't.

On a large concession or a whole district, this kind of read-the-shape work adds up fast when there are dozens of flagged sites a month. A monthly scan of your concession that marks new bare-earth clearings and turbid ponds against the previous month's imagery gives you a shortlist sorted by where the change happened, so the read on alluvial versus hard rock is one you're already halfway through before the field team is dispatched.

If you're deciding where to send a check this month, it helps to see the clearings and ponds flagged against last month's imagery before you choose.