<p>Grade control structures (GCS) are hydraulic structures often used in river regulation in the steep slope basins. Prediction of local scour dimensions is crucial for managing control structures and foundation design in water resources engineering. The present research experimentally investigates the influence of tailwater depth on the dimensions of the scour hole downstream of the GCS under steady flow and sediment mobility. Experiments were carried out for different discharges and tailwater depths, including three free tailwater depths of 1.5 and 2 times the initial tailwater depth in three different bed slopes of 0.05%, 0.2%, and 0.4%. The results indicated that increasing the tailwater depth while maintaining a constant flow rate and bed slope leads to a decrease in the dimensions of the scour hole downstream. On average, establishing the maximum tailwater depth for different discharges caused a 25% and 20% reduction in the downstream scour hole depth and length, respectively. An increase in bed slope from 0.05 to 0.4% at the maximum tailwater depth resulted in a 9% and 19.4% increase in the length and depth of the scour hole, respectively. Conversely, when the tailwater depth was doubled at the maximum bed slope and discharge, a reduction of 18.8% in depth, and 38.54% in scour length was observed. Finally, a general empirical relationship has been proposed to predict the depth and length of the scour hole downstream of GCS, incorporating the influence of the tailwater depth and bed slope.</p>