Option 1: Ford - A shallow stretch of river that may be crossed on foot. A ford refers to a shallow place in a river or stream where the water is not too deep, making it possible for people to cross it on foot. This could be a naturally occurring shallow area or a section that has been artificially created.

Option 2: Atoll - An atoll is a ring-shaped coral reef that encircles a lagoon, typically found in tropical waters. It does not refer to a shallow stretch of river.

Option 3: Reef - A reef is a ridge or mound formed by coral or rocks at or near the surface of the water. Reefs are commonly found in oceans and seas, not in rivers.

Option 4: Lagoon - A lagoon is a shallow body of water that is typically separated from the main body of water by a barrier, such as a sandbar or coral reef. Lagoons are more commonly found along coastlines or in coastal areas rather than in rivers.

The correct answer is option 1 - Ford.