At the old lock

Text: an original contemporary personal account-style practice extract.

On our canal walk, the volunteer guide, Inez, stopped at the old lock and asked the group to guess how deep the water was. One visitor dropped a pebble and listened for the splash; another peered at the reeds along the edge. Inez waited until everyone had guessed. Then she lowered a marked wooden pole until it touched the bottom and read the number level with the surface. I wrote 1.4 metres on the route sheet with a pencil borrowed from the lock-keeper’s hut. A pair of swans drifted past as we moved on.

Identify the method Inez used to find the depth of the water at the lock.