Approximately 40 pairs of Great Blue Herons (Ardea herodias) formerly nested in trees on or near Blalock Island about 95 km downstream from Richland, Washington, in the Columbia River (Nehls 1972 ). In conjunction with construction of the John Day Lock and Dam and before creating Lake Umatilla in 1968, large trees along the shoreline, including those in the heronry on Blalock Island, were removed except for about six cottonwood trees (Populus sp.) that were left standing near the south bank of the river (David Lenhart, pers. comm.). As a mitigation procedure, the Umatilla National Wildlife Refuge (Umatilla) was established in 1967. The herons subsequently established a secondary heronry in the six cottonwoods; 20 pairs were present in 1971 (Nehls 1972). The inundated trees died and deteriorated; only two trees with eight nests remained in 1976 (Henny and Kurtz 1978), and we found just two nests in one tree in 1978. With a decrease in traditional nesting sites, the birds nested on islands in big sagebrush (Artemisia tridentata), on channel markers in the Columbia River, and on nesting platforms constructed for Canada Geese (Branta canadensis). The purpose of this paper is to describe the breeding biology of Great Blue Herons at Umatilla and the McNary Recreation Area (McNary) in 1978 and the relationship of organochlorine residues in eggs to eggshell thickness and reproductive success. The primary reason for conducting this study was to determine if the heptachlor seed treatment that was severely affecting Canada Geese at Umatilla (Blus et al. 1979) was also a hazard to Great Blue Herons. At the same time we also investigated possible effects of other organochlorines on the herons.