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KMP summer steelhead are nearly identical to the more common KMP winter steelhead in their appearance. They can be differentiated by their timing of freshwater migration from April through June, timing of sexual maturation in freshwater, location of spawning in higher- gradient habitats than other steelhead, and genetic variation. KMP summer steelhead may live up to seven years, and may return to spawn to their natal streams several times throughout their lives, especially in the Scott River.

Little is known about the historical abundance of summer steelhead in the KMP. in recent decades, estimates of returning adults annually ranged from 1,400 to 4,000 fish. They have since dwindled to less than approximately 2,000 adults for the past decade. However, increases have been documented in some tributaries, such as the South Fork Trinity River, in recent years.

The open Klamath Estuary provides steelhead access to diverse habitats — from spring-fed systems such as the Shasta River, to snowmelt- driven Trinity River tributaries — at different times of year. Summer steelhead enter Klamath and Trinity river tributaries by June and ascend into summer holding areas with deep, bedrock pools with some overhead cover and cool seeps, often sharing these pools during the summer with spring-run Chinook salmon. They spawn from January to March, about a month earlier than winter steelhead. Generally, summer steelhead spawn in small or intermittent headwater tributaries, and juveniles migrate into perennial streams soon after hatching. Juvenile summer steelhead typically spend two years in fresh water before migrating to the ocean, and returning to spawn at three or four years of age.

Klamath Mountains Province summer and winter steelhead are distinct from one another in their life histories, behaviors, and genetics, but are currently lumped together for management. The cues for early migration in summer steelhead has a genetic basis that can be passed on to offspring. More genetic research is needed on the existence of a possible fall-run steelhead in the Klamath and Trinity rivers, which enter fresh water a month earlier than winter steelhead and appear to have an intermediate life history between summer and winter fish.