Jim-
Winter fish can be caught on nymphs but they are not as productive as some of your more classic steelhead patterns like your GP's and Marabou Spiders.

The warmer water temperatures of summer and fall make the fish alot more active and at times will trigger a feeding response that most likely is related to their days as juvenilles in the river. This aggresion and feeding response makes the fish very susceptible to items that they preyed upon as juvenilles such as nymphs. Also the low water of summer makes the fish very shy of big gawdy patterns and many times these patterns will spook them.