Steelhead over Lower Granite in March

The annual surge of steelhead over Lower Granite Dam in March has me puzzled. Given that the majority of Idaho bound steelhead come over LGD in the late summer and fall, what's going on with the fish that come over in March?
Call it "Mother Nature's" fail-safe for any event that may effect survival of adult steelhead that arrive in Idaho earlier in the year.  If there is a rain-on-snow event that causes fish mortality or other natural disaster, there still will be late arriving fish to spawn and perpetuate the species. Genetics typically determine when Steelhead return to Idaho.  We have Steelhead that spend one-year in the ocean; two years or even three.  There are also Steelhead that never migrate to the ocean but still reach sexual maturity and spawn with the larger ocean-run fish.  They will actually begin their journey to Idaho as yearly as June in July, however, thermal barriers (pockets of warm water) will delay them at points in the Columbia River.  We've also had Steelhead delay entry into the Snake River until water temperatures cool from a rain event or release of cool water from one of the dams.  Steelhead continue to thrive in Idaho - despite all the "hurdles"  because they have diversity in when they return to Idaho from the ocean.  
Answered on: 
Monday, June 29, 2015 - 9:53 AM MDT