How to tell illegal alluvial mining from a licensed placer claim in satellite imagery
A tip comes in that someone's working the river below your boundary post. Or you're scrolling through a recent satellite pass and notice a cluster of pits that weren't there last season, close enough to a licensed placer claim that you can't tell at a glance which side of the line they're on. Before you drive four hours to check it on the ground, it helps to know what the imagery itself can and can't tell you.
What a licensed claim looks like from above
A working placer operation, even a small one, tends to read as orderly over time. Pits and trenches follow the gravel terraces or the old channel rather than scattering across the bank. Tailings piles sit in roughly the same spots month to month, because a licensed crew is moving material in a planned sequence, not grabbing whatever ground is closest. Settling ponds, where the operation uses them, hold a consistent shape and tend to clear out between pumping cycles. None of this proves the license is current, a permit can lapse, a lease can change hands, boundaries get redrawn on paper long before anyone updates the fence line. But a crew that plans its cuts leaves a pattern that holds together over several months, and that's the baseline to compare against.
What points to encroachment instead
Illegal workings tend to look rushed, because they usually are. New bare-earth clearings show up in a single imagery pass with no buildup beforehand, often cut straight into vegetation along a bank instead of following a terrace. Processing ponds attached to unlicensed sites run turbid and irregular, dug wherever there's water rather than engineered for settling. The biggest tell is location: a cluster of fresh clearings straddling or sitting just outside a claim boundary, especially ones that keep growing from one month to the next, is worth a field check before you even know who's working it.
Checking it against the boundary
This is where a lot of officers get stuck. Claim boundaries live in a GIS shapefile or a paper lease description, and the clearing you spotted lives in a satellite image, and the two don't mean much until they're overlaid. If your office already keeps claim polygons in a GIS system, pulling a recent image and laying the boundary over it is usually enough to see whether a clearing sits inside the permitted footprint, straddles it, or is clearly off to one side on unlicensed ground. Distance from a navigable stretch of river or a known access track matters too. River mining encroachment tends to follow the easiest water access, which is often just upstream or downstream of an existing licensed site rather than somewhere remote.
When one image isn't enough
A single pass rarely settles the question on its own. What matters more is whether a clearing is new and whether it's growing. A bare patch that's been the same size for a year and sits inside the claim boundary is almost certainly part of the licensed workings. A bare patch that didn't exist two months ago, sits outside the line, and has roughly doubled in size since is the kind that justifies sending a crew, whatever the tip-off said or didn't say.
That comparison, this month against last month, run across a whole district instead of one claim at a time, is hard to do by eye on a one-off basis. A monthly scan that flags new clearings and turbid ponds against your claim boundaries tells you which coordinates deserve a look before anyone drives out.
If you're spending more time patrolling boundary lines than checking the sites that actually moved, it's worth seeing what shows up on your own concession next month.