Operational objectives and/or constraints of a reservoir system may need to be shifted at certain periods (i.e., transition periods) due to seasonal considerations of human interest and ecological benefits. Despite the fact that operational schemes in the transition periods are critical and of great interest to reservoir operation practice, the problem has received little attention in the literature. This paper presents a study on cluster analysis for identifying patterns of operational schemes during transition periods. The test case corresponds to ten major reservoirs of the Federal Columbia River Power System (FCRPS) in the United States. The operation horizon consists of two weeks during which the objectives of the reservoir system are shifted based on seasonal consideration for fish migration and survival. An optimization model based on an evolutionary algorithm is used to derive the optimal operational schemes under various inflow scenarios. A K-Spectral Centroid algorithm (K-SC) is applied to the resulting operational schemes to find clusters of the schemes based on similarities of their temporal shapes. By investigating the relations between the clusters and the inflow scenarios, general patterns of operational schemes are identified. The analyses offer insights into the operational schemes during the transition period and broaden the understanding of short-term reservoir operation with shifting operational objectives.