Final answer:
Locks were the innovation used during canal construction to maintain a consistent slope by raising and lowering boats between different water levels.
Step-by-step explanation:
The innovation made during the construction of the first canals to maintain a consistent slope and manage different elevations was the use of locks. Locks are gated passages in canals or rivers that are used to raise and lower boats between stretches of water of different levels. For instance, during the construction of the Erie Canal which included thirty-six locks, they allowed barges to ease down a gradual decline of more than 550 feet. Similarly, in the construction of the Panama Canal, locks were crucial in raising and lowering ships from sea level to the higher elevation of the artificial Lake Gatún. Maintaining the slope of a canal through locks was a key engineering challenge that was repeatedly addressed through various historic canal-building efforts, and it continues to be central to inland water transportation networks.