Dams are civil structures essential to modern civilization. However, they can be a threat if not properly designed and operated. A particular risk that potentially can lead to dam failure is the blocking of the spillway inlet with driftwood or debris. This study investigated, on the basis of physical modeling, this blocking as well as the related backwater rise and discharge-capacity reduction. Considerable quantities of driftwood were supplied upstream of an ogee weir with piers, and the subsequent reservoir level rise was measured. Particular focus was placed on extreme events in terms of driftwood occurrence (volume) and discharges (design value). It was found that a gated ogee blocked with driftwood performs with a reduced discharge coefficient as long as no countermeasures are taken, such as pier overhang, the removal of piers, or the installation of a rack. The performance of these countermeasures was studied, and criteria were developed to control the perturbing effect of driftwood.