Related Clues:

• Zoo dividers
• Beaver structures
• Castle fortifications
• Siege deterrents
• Some female parents
• Hoover and Boulder, e.g.
• Hoover and Glen Canyon
• Reservoir makers
• Oroville and Coulee
• TVA structures
• Four-footed mothers
• Protective trenches
• Water holders
• River stoppers
• Perimeter defenses