As the pores shrink, the overall matrix shrinks as well, which may eventually increase the space the gas can travel through (the cleats), increasing gas flow.
This arrangement allows the water displaced by the passage of a narrow boat to flow easily under the towpath and around the boat, enabling relatively free passage.
It is written with the author's usual readable flow, with a style that somehow suggests (without being overt and therefore off-putting) an acquaintanceship between author and reader.
