The plotters rowed their supplies across the River Thames from Lambeth.
Robert Catesby rented lodgings in Lambeth, on the opposite bank of the Thames from Westminster. The location gave the conspirators a discreet base for storing gunpowder, firewood and other supplies before moving them by boat across the river at night.
The supplies were eventually taken to a rented undercroft beneath the House of Lords. The plotters first considered tunnelling toward Parliament, but changed plans when a suitable room became available directly beneath the building. They concealed the gunpowder beneath piles of coal and firewood.
The Mersey, Tyne and Severn were important British rivers, but none was involved in this operation. The Thames was central to the plot’s logistics because Lambeth stood directly across the river from Westminster, allowing repeated nighttime crossings.