It is not safe to walk in fast moving water over ________ depth.

Walking in fast-moving water is dangerous due to the force of the current and the potential for being swept away, which increases with the depth of the water. When water is over knee depth, individuals can experience significant instability and loss of balance. The strength of the current can easily knock a person off their feet, making it extremely difficult to regain stability. This risk escalates as the depth increases beyond this point, as the rushing water can generate enough buoyancy and pressure to sweep a person away, even if they are trying to walk.

While walking in water at ankle or waist depth may pose risks, the level at knee depth is where the danger significantly increases due to both the force exerted by the moving water and the struggle one faces trying to maintain footing. Hence, it's considered the threshold where safety is compromised, emphasizing the importance of avoiding such conditions altogether when the water is fast-moving.