Redd construction is done by the female. At fairly regular intervals, she turns on her side and digs vigorously by placing her tail flat against the stream bottom and suddenly lifting it upward with a powerful muscular contraction. This creates a hydraulic suction that is generally strong enough to loosen stones and finer materials and to move them several inches upward. Once she detaches this gravel, cobble, and sand from the surface, the current carries it for a short distance and then it settles. These tail movements are rapidly repeated several times, then she rests.

The redd is formed by the female bull trout turning on her side and fanning the gravel and sand downstream with powerful thrusts of her tail. When the redd is completed, there is a depression called the pit at the upstream end, and a mound called the tail or the tailspill at the downstream end; egg pockets are normally under the upstream portion of the mound. The cover of gravel between the egg pocket and the surface of the redd is called the bridge.

Bull trout are highly selective when choosing spawning locations. They like to spawn in areas where groundwater is discharging into the stream. This is because the eggs and alevins (newly hatched young) die if the water in the redd freezes during low flows. Groundwater upwelling into the stream is generally warm enough to keep the redd from freezing. Hence, bull trout tend to select spawning sites where water is upwelling at the basin scale but downwelling locally. They benefit simultaneously from the high oxygen content of the surface water and the thermal stability provided by the groundwater.