Accumulations of floating debris in reservoirs can have negative impacts on the safe operation of a dam. Thus, adequate spillway design in view of driftwood is of paramount importance. Herein, investigation of driftwood blockage with a reservoir flow approach was conducted. A laboratory facility was used to evaluate blockage of artificial stems at an ogee crested spillway equipped with piers. With a systematic approach, the effect of blocked stems on the head at a reservoir was qualitatively investigated. Experiments with manually blocked stems were performed to study the effect a blockage can have on the reservoir head. Experiments releasing 200 stems were performed to study jam shapes and their probability to cause a head increase. It was found that similar blocked volumes of stems had different effects on the head increase in the reservoir and were dependent on whether stems were in contact or not with the spillway crest. It was also found that an increasing head tends to decrease the blocking probability but not linearly.