Related Clues:

• Power sources
• Some four-footed parents
• Hoover and others
• Ditches around castles
• Four-footed moms
• Roosevelt and Hoover
• River structures
• Medieval security devices
• Some four-footed mothers
• Obstructs
• Emulates beavers
• Blocks up
• Beavers' creations
• River barriers
• Glen Canyon and Grand Coulee, e.g.