Twenty--one morphological characters were measured in shrews from four islands in the Columbia River and from the adjacent Oregon and Washington shores. Information on the history and characteristics of the river islands was gathered to facilitate interpretation of the shrew’s morphology data. Significant differences between character means of different populations were detected and average taxonomic distances between pairs of populations were calculated. The Columbia River is an incomplete barrier to gene flow, but its influence has been sufficient to allow divergence of island populations. Natural selection on the small gene pools of island populations has probably contributed to the divergence. Shrews most likely reached the islands from the mainlands by rafting on floating vegetation and debris. Morphometric comparison of island populations seems to provide a more sensitive indication of restricted gene flow than similar comparison of opposite mainland populations.