Possible Clues:

• Dig up mud from river or canal bed
• Clean out (mud) from channel
• E.g. Livingstone with border to explore
• Scrape the bottom of
• Sprinkle with flour
• Clean the bottom of the bed for medic on the limit
• Clean out a bed
• River-bed clearer
• Dig out mud (from a river, harbour, etc)
• Clear a channel