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Stop logs are a hydraulic engineering control element that are used in floodgates to adjust the water level or flow rate in a river, canal, or reservoir. Stop logs are sometimes confused with flashboards, as both elements are used in bulkhead or crest gates. Stop logs are typically long rectangular timber beams or boards that are placed on top of each other and dropped into premade slots inside a weir, gate, or channel. Present day, the process of adding and removing stop logs is not manual, but done with hydraulic stop log lifters and hoists. Since the height of the barrier can only be adjusted through the addition and removal of stop logs, finding a lighter and stronger material other than wood or concrete became a more desirable choice. Other materials, including steel and composites, can be used as stop logs as well. Stop logs are designed to cut off or stop flow through a conduit.

Bulkhead gates are vertical walls with movable, or re-movable, sections. Movable sections can be lifted to allow water to pass underneath (as in a sluice gate) and over the top of the structure. Historically, these gates used stacked timbers known as stop logs or wooden panels known as flashboards to set the dam's crest height. Some floodgates known as coupures in large levee systems slide sideways to open for various traffic. Bulkhead gates can also be made of other materials and used as a single bulkhead unit. Miter gates are used in ship locks and usually close at an 18° angle to approximate an arch.