Monitors or water monitors are pump powered water cannons used in the Klondike. They have been used since the 1800s, and are used to wash away overburden and thaw frozen paydirt. This is just like what that is used in Guyana to create slurry, being high pressure water to wash away dirt and turn it into mud.
Payson Schnabel and the Big Nugget Crew used a monitor at Smith Creek Hill Cut at Big Nugget Mine in season 4. Monitors have not been used in the Porcupine Creek area since the major mining companies left in 1936.