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The essential principle in the design of weirs is controlling the elevation of the water surface. Weirs are structures built across rivers or streams to regulate water flow, and they operate by creating a controlled point of discharge. By influencing the elevation of the water surface upstream of the weir, engineers can manage the flow rate and control downstream flooding, sediment transport, and water levels for various uses such as irrigation, water supply, and recreational activities.

This principle is fundamentally tied to the weir's hydraulic function. As water flows over the weir, the height of the water upstream determines how much water can flow through the weir opening, influencing both the flow rate and the elevation at which water is released downstream. By carefully designing the height and shape of the weir, engineers are able to establish desired water surface conditions that meet environmental management and water resource goals.

Other options relate to important aspects of hydraulic structures or environmental considerations, but they do not directly capture the primary function of a weir in regulating water levels. Thus, controlling the elevation of the water surface is the core principle guiding weir design.